Compost a Gem

(210-VI-NEH, February 2000)

United StatesDepartment ofAgriculture

NaturalResourcesConservationService

Part 637 Environmental EngineeringNational Engineering Handbook

Chapter 2 Composting

Part 637National Engineering Handbook

CompostingChapter 2

(210-VI-NEH, February 2000)

Issued February 2000

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2–i(210-VI-NEH, February 2000)


CompostingChapter 2 Acknowledgments

Chapter 2, Composting, was prepared by Robert E. Graves, professor,Agricultural and Biological Engineering Department, The PennsylvaniaState University, and Gwendolyn M. Hattemer, engineering project asso-ciate, Agricultural and Biological Engineering Department, The Pennsylva-nia State University. Donald Stettler, environmental engineer, NaturalResources Conservation Service, National Water and Climate Center, Port-land, Oregon, coordinated the project. This project was originally conceivedby James N. Krider, national environmental engineer, NRCS, Washington,DC (retired) working with Dana Chapman of The Pennsylvania StateUniversity and NRCS (retired).

The major NRCS participates in the review of this document from its incep-tion were Barry Kitzner, national enviornmental engineer, Washington,DC, David C. Moffitt, water quality specialist, Fort Worth, Texas; Victor

W.E. Payne (retired), and Frank Geter (retired).

The final edit, graphic production, and publication formatting were pro-vided by Mary R. Mattinson, editor; Wendy Pierce, illustrator; and Suzi

Self, editorial assistant, NRCS National Produdction Services, Fort Worth,Texas.


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CompostingChapter 2

Chapter 2 Composting

Contents: 637.0200 Introduction 2–1

(a) Definition of composting ............................................................................. 2–1

(b) Composting in the United States ................................................................ 2–1

(c) General procedure ........................................................................................ 2–1

637.0201 Principles of composting 2–2

(a) General background ..................................................................................... 2–2

(b) Composting process ..................................................................................... 2–2

(c) Microbiology .................................................................................................. 2–4

(d) Chemical transformations ........................................................................... 2–6

637.0202 Design of compost mixtures 2–11

(a) Components of compost mix .................................................................... 2–11

(b) Typical raw material ................................................................................... 2–12

(c) Determination of the compost recipe ...................................................... 2–13

637.0203 Monitoring and parameter adjustment 2–17

(a) Temperature ................................................................................................ 2–17

(b) Odor management ...................................................................................... 2–18

(c) Moisture ....................................................................................................... 2–18

(d) Oxygen and carbon dioxide ....................................................................... 2–19

(e) Monitoring equipment ................................................................................ 2–19

637.0204 Odor generation 2–20

637.0205 Additives, inoculums, starters 2–21

637.0206 Pathogens 2–22

637.0207—Health risks of a composting operation 2–24

(a) Bioaerosols .................................................................................................. 2–24

637.0208 Aeration requirements 2–25


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637.0209—Analysis of raw materials and compost 2–28

(a) Determining moisture content .................................................................. 2–29

(b) Bulk density ................................................................................................. 2–30

(c) pH and soluble salts .................................................................................... 2–30

(d) Particle size distribution ............................................................................ 2–31

(e) Organic matter content .............................................................................. 2–31

(f) Substrate degradability .............................................................................. 2–31

(g) Compost quality .......................................................................................... 2–31

(h) Determination of compost stability .......................................................... 2–33

637.0210 Composting methods 2–36

(a) Passive composting piles ........................................................................... 2–36

(b) Windrow ....................................................................................................... 2–37

(c) Passively aerated windrows ...................................................................... 2–37

(d) Aerated static pile ....................................................................................... 2–38

(e) In-vessel systems ........................................................................................ 2–39

(f) Comparison of composting methods ....................................................... 2–40

(g) Controlled microbial composting ............................................................. 2–42

637.0211 Dead animal composting 2–44

(a) General ......................................................................................................... 2–44

(b) Dead poultry and small animal composting ............................................ 2–46

(c) Dead swine and large animal composting ............................................... 2–47

637.0212 Operational costs 2–50

(a) Availability and price of raw material ...................................................... 2–50

(b) Quantity and price of land available for the composting operation ..... 2–50

(c) Estimated costs of operation/production ................................................ 2–50

(d) Pre-startup cost ........................................................................................... 2–51

(e) Material handling ........................................................................................ 2–51

(f) Monitoring ................................................................................................... 2–51

(g) Operations after completion of composting ........................................... 2–51

637.0213 Compost end use 2–52

(a) Land application .......................................................................................... 2–52

(b) Marketing considerations .......................................................................... 2–53

637.0213 References 2–54

Glossary 2–59

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CompostingChapter 2

Appendixes

Appendix 2A Common Raw Materials for Farm Composting ..................... 2A–1

Appendix 2B Testing Material on the Farm ................................................... 2B–1

Tables Table 2–1 Dewar self-heating method for determining 2–34

compost maturity

Table 2–2 Stability of compost based on carbon dioxide 2–34

respired during incubation

Table 2–3 Volatile organic acids as an indicator of compost 2–35

instability

Table 2–4 Phytotoxicity as an indicator of compost stability 2–35

Table 2–5 Mortality rates and design carcass weights for 2–47

determining the size of a dead swine composting facility

Table 2–6 Mix for composting dead swine with sawdust 2–48

Table 2–7 Mix for composting dead swine with broiler litter 2–48

using sawdust/straw as a carbon source and bulking

agent

Figures Figure 2–1 Compost temperature ranges 2–2

Figure 2–2 Bacteria 2–4

Figure 2–3 Fungi 2–5

Figure 2–4 Actinomycetes 2–5

Figure 2–5 Higher organisms 2–5

Figure 2–6 Passive compost pile 2–36

Figure 2–7 Windrow method 2–37

Figure 2–8 Passive aerated windrow 2–38


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Figure 2–9 Aerated static pile 2–38

Figure 2–10 Bin 2–39

Figure 2–11 Rectangular agitated bed system 2–39

Figure 2–12 In-vessel silo 2–39

Figure 2–13 Rotating tube composter 2–40

Figure 2–14 Dead bird composter 2–45

Figure 2–15 Dead bird bin composting schematic 2–45

Examples Example 2–1 Determining the proportion of materials needed to 2–14

develop a mix based on the C:N ratio

Example 2–2 Determining the size of a dead swine composting 2–49

facility

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CompostingChapter 2Chapter 2 Composting

637.0200 Introduction

(a) Definition of composting

Composting is the controlled aerobic biological de-composition of organic matter into a stable, humus-like product called compost. It is essentially the sameprocess as natural decomposition except that it isenhanced and accelerated by mixing organic wastewith other ingredients to optimize microbial growth.

The potential benefits of composting manure andother organic wastes are improved manure handling;reduced odor, fly, and other vector problems; andreduced weed seeds and pathogens. Land appliedcompost improves soil fertility, tilth, and water hold-ing capacity. It is also free of offensive odors and canbe stored for extended periods. These qualities make itsuitable for use on the farm or for sale.

Composting is easily adapted to agricultural opera-tions because farms generally produce suitableamounts and types of waste for composting, haveadequate land, will benefit from the application ofcompost to the soil, and have the necessary equipmentalready available.

(b) Composting in the UnitedStates

Composting is more prevalent in the United States inresponse to concerns about agricultural pollution andthe encroachment of the urban population in ruralareas. The availability of cost-sharing to fundcomposting facilities, particularly in some criticalwatersheds, has also driven the growth of composting.

The Natural Resources Conservation Service helpsfarmers to design composting facilities as a part of awaste management system. Composting is used as atreatment component to convert manure and otherorganic material into a more environmentally stableproduct.

Composting is an alternative to more commonly usedmethods of managing manure and other organicwastes, such as poultry or livestock mortalities and

crop residue. It provides a means of developing anorganic source of fertilizer or soil conditioner. Be-cause composting is suited to a wide range of materi-als, it is possible for a composting operation to workin cooperation with other farms, municipalities, andindustry to compost their organic wastes, such asmanure from horse race tracks, food processingwastes, or yard trimmings.

(c) General procedure

The principal elements in planning a compost facilityinclude performing site investigations and developingthe recipe design, facility design, waste utilizationplan, and an operation and maintenance plan. Becausecomposting is a relatively fexible process, it is neces-sary to decide among alternative methods, locations,and materials. The decision depends on the manage-ment and economic aspects of the farm as well as onthe physical limitations of the site. The planner needsto present the landowner with the different alterna-tives so that the owner can make the final decision.


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2–2 (210-VI-NEH, February 2000)

637.0201 Principles ofcomposting

(a) General background

Composting is the process by which various aerobicmicro-organisms decompose raw organic material toobtain energy and material they need for growth andreproduction. The stable by-products of this decompo-sition, the biomass of both dead and living micro-organisms, and the undegradable parts of the rawmaterial make up the end product that is called com-post.

The organisms responsible for composting requirecertain nutritional and environmental conditions tosurvive and function. They require adequate amountsof macro- and micro-nutrients, oxygen, and water.These organisms experience optimal growth rates onlywithin certain temperature and pH ranges.

(b) Composting process

The composting process is carried out by a diversepopulation of predominantly aerobic micro-organismsthat decompose organic material in order to grow andreproduce. The activity of these micro-organisms isencouraged through management of the carbon-to-nitrogen (C:N) ratio, oxygen supply, moisture content,temperature, and pH of the compost pile. Properlymanaged composting increases the rate of naturaldecomposition and generates sufficient heat to destroyweed seeds, pathogens, and fly larvae.

The composting process can be divided into two mainperiods: (1) active composting and (2) curing. Active

composting is the period of vigorous microbial activityduring which readily degradable material is decom-posed as well as some of the more decay-resistantmaterial, such as cellulose. Curing follows activecomposting and is characterized by a lower level ofmicrobial activity and the further decomposition of theproducts of the active composting stage. When curinghas reached its final stage, the compost is said to bestabilized.

The compost pile passes through a wide range oftemperatures over the course of the active compostingperiod. As the temperature varies, conditions becomeunsuitable for some micro-organisms while at thesame time become ideal for others. The activecomposting period has three temperature ranges.These ranges are defined by the types of micro-organ-isms that dominate the pile during those temperatures(fig. 2–1) and are called psychrophilic, mesophilic, andthermophilic. Psychrophilic temperatures are gener-ally defined as those below 50 degrees Fahrenheit,mesophilic between 50 and 105 degrees Fahrenheit,and thermophilic above 105 degrees Fahrenheit.

Defining these temperature ranges does not meanmicro-organisms found in the pile during the meso-philic stage are not found during the psychrophilic orthermophilic stage. Rather, these ranges are defined tomake a rough delineation between temperatures atwhich certain classes of micro-organisms have peakgrowth rates and efficiencies. For example, mesophilicorganisms may inhabit the pile in the thermophilic orpsychrophilic temperature ranges, but will not domi-nate the microbial population because they are notfunctioning at optimal levels.

The initial stage of composting is marked by eitherpsychrophilic or mesophilic temperatures dependingon the ambient temperature and the temperatures ofthe compost mix material. A short lag period is typicalat the start of the composting process before thetemperature begins to rise rapidly. This lag period is

Figure 2–1 Compost temperature ranges

Temperatureplateau

Thermophilic(conversion)

Mesophilic(degradation)

Heating Substratedepletion

Psychrophilic(maturation)

105

50Tem

peratu

re °

F

Time



CompostingChapter 2

the time necessary for the development of the micro-bial population. As the microbial population begins todegrade the most readily degradable material and thepopulation increases, the heat generated by the micro-bial activity is trapped by the self-insulating compostmaterial. As the heat within the pile accumulates, thetemperature of the compost pile begins to rise. Thetemperature continues to increase steadily through thepsychrophilic and mesophilic temperature ranges asthe microbial population increases and diversifies.Depending on the operation, the compost pile typicallytakes from 2 to 3 days to increase beyond mesophilictemperatures and reach the thermophilic stage ofcomposting.

As the pile temperatures increase into the thermo-philic range, the pile becomes inhabited by a diversepopulation of micro-organisms operating at peakgrowth and efficiency. This intense microbial activitysustains the vigorous heating that is necessary for thedestruction of pathogens, fly larvae, and weed seeds.The diversity of the microbial population also allowsthe decomposition of a wide range of material fromsimple, easily degradable material to more complex,decay resistant matter, such as cellulose.

The temperatures continue to rise and peak at about130 to 160 degrees Fahrenheit. Once this peak isreached, microbial activity begins to decrease inresponse to a depletion in readily degradable materialand oxygen or to the excessively high temperature thatis detrimental to their function. Micro-organismsdegrade material by moving soluble componentsthrough their body walls as is done for simple com-pounds or by using extracellular enzymes to break thematerial down before it is taken into the cell body. Ifthe temperature becomes too high, the enzymes re-sponsible for the breakdown denature and becomenonfunctional so that the micro-organisms cannot getthe nutrition they need to survive. Elevated tempera-ture may not be lethal for all micro-organisms, but mayaffect their efficiency and further contribute to thedecrease in microbial activity. Still other micro-organ-isms form spores in response to excessive heating.Spores are the inactive form that some micro-organ-isms take to protect themselves from conditionsadverse to survival, such as heat and lack of moisture.These spores germinate once more favorable condi-tions return.

As microbial activity decreases, more heat is lost fromthe pile than is generated, and the pile begins to cool.As the temperature cools from thermophilic levels,different micro-organisms reinhabit the pile by migrat-ing from cooler spots while spores germinate as condi-tions become more suitable for survival. These micro-organisms serve to continue the decomposition pro-cess. The compost pile remains in the thermophilicrange from 10 to 60 days, depending on the operation.Once the temperature decreases to below 105 degreesFahrenheit, the curing period may begin or the pilemay be aerated to reactivate active composting.

At no set point is active composting determined to becomplete. It is usually considered complete when thepile conditions are such that microbial activity can notincrease enough to reheat the pile. This generallyhappens when the temperature has decreased tobelow 105 degrees Fahrenheit. Although microbialactivity is not as intense and most of the organicmaterial has already been degraded, curing is animportant part of the composting process.

Curing is marked by a lower level of microbial activityand is responsible for stabilizing the products resultingfrom active composting period. Stabilization includesfurther decomposition of organic acids and decay-resistant compounds, the formation of humic com-pounds, and the formation of nitrate-nitrogen. Anotherbenefit of curing is that certain fungi begin to inhabitthe pile and contribute to the disease suppressantqualities of the compost. Because microbial activityhas decreased and is operating at a lower level, littleheat is generated and the pile temperature continuesto decrease or remains at a low level. Proper manage-ment of moisture and oxygen is still required duringthe curing period to maintain microbial activity. Man-agement during the curing period is also required toensure that the pile is not recontaminated with weedseeds. This may require covering or relocating thecuring piles to reduce the potential for recontamina-tion.

The reactions that take place during curing are rela-tively slow and, as such, require adequate time. Thelength of the curing period varies with the type ofoperation, the length of the active composting period,and the intended end use of the compost. Short, activecomposting periods require extended curing periodsto allow for sufficient decomposition and stabilization.


CompostingChapter 2


Compost that will be used for sensitive end-uses, suchas application to sensitive crops or in potting media,also requires an extended curing period. Curing isgenerally considered complete when the pile afterrepeated mixings returns to ambient temperature. It isimportant to distinguish between cooling that is aresult of sufficient curing and cooling that is a result ofinadequate oxygen supply or moisture content. Curinggenerally lasts from 1 to 6 months.

(c) Microbiology

A variety of microbial populations develops in re-sponse to the different levels of temperature, mois-ture, oxygen, and pH within a compost pile. This mic-robial diversity enables the composting process tocontinue despite the constantly changing environmen-tal and nutritional conditions within the pile. Themicro-organisms responsible for composting degradea broad range of compounds from amino acids andsimple sugars to complex proteins and carbohydrates.This results in a thorough degradation of the compostmaterial. Temperature levels and available food supplygenerally have the greatest influence in determiningwhat class and species of organisms make up themicrobial population at a particular time.

Decomposition proceeds rapidly in the initial stages ofcomposting because of the abundant supply of readilydegradable material. The material is characterized by alow molecular weight and simple chemical structure.It is water soluble and can pass easily through the cellwall of the organisms, which allows it to be metabo-lized by a broad range of nonspecialized organisms.

As the readily degradable material is consumed andthe supply diminishes, more complex, less degradablematerial begins to be decomposed. This material ischaracterized by a high molecular weight, polymeric(long chain) chemical structure that cannot passdirectly into the cells. The material must be brokendown into smaller components through the action ofextracellular enzymes. Not all of the micro-organismspresent in the compost pile can produce these en-zymes, particularly simple organisms, such as bacteria.Such decomposition requires more specialized organ-isms, such as fungi. After the polymeric material ishydrolyzed into smaller components by these special-ized organisms, the resulting fragments can then beused by the nonspecialized organisms.

Micro-organisms that inhabit a compost pile are inthree classes: bacteria, fungi, and actinomycetes, ahigher form of bacteria. They can be anaerobes, aer-obes, or facultative anaerobes. Strict anaerobes do notuse oxygen and will die if exposed to oxygen. Aerobesuse oxygen, and facultative anaerobes use oxygen if itis available, but can function without it.

The micro-organisms within a compost pile can bepsychrophilic, mesophilic, or thermophilic dependingon the temperature range within which they experi-ence optimal growth rates. The psychrophilic tempera-ture range is defined as being below 50 degrees Fahr-enheit, mesophilic between 50 and 105 degrees Fahr-enheit, and thermophilic between 105 and 160 degreesFahrenheit.

(1) Bacteria

Bacteria (fig. 2–2) are small, simple organisms presentprimarily during the early stages of the compostingperiod. They are responsible for much of the initialdecomposition and include a wide range of organismsthat can survive in many different environmentalconditions. Although they are small relative to fungiand actinomycetes, they are present in significantlygreater numbers.

Bacteria are fast decomposers. They stabilize mostreadily available nutrients, such as simple sugars, aswell as digest the products of fungal decomposition.Some bacteria can degrade cellulose.

Figure 2–2 Bacteria



CompostingChapter 2

Bacteria function optimally within a pH range of 6 to7.5 and are less tolerant of low moisture conditionsthan other types of micro-organisms. Some bacteriaform endospores that enable them to withstand unfa-vorable environmental conditions, such as high tem-perature or low moisture. When the environmentbecomes more favorable for survival, the endosporesgerminate and the bacteria become active again. Thisfeature of certain bacteria helps to continue thecomposting process during the cooling phase thatfollows peak thermophilic temperatures.

(2) Fungi

Fungi (fig 2–3) are larger organisms than bacteria.They form networks of individual cells in strandscalled filaments. Fungi tend to be present in the laterstages of composting because of the nature of thematerial they decompose. Most fungi decay woodysubstances and other decay-resistant material, such aswaxes, proteins, hemicelluloses, lignin, and pectin.Fungi are less sensitive to environments with lowmoisture and pH than bacteria, but because most fungiare obligate aerobes (require oxygen to grow), theyhave a lower tolerance for low-oxygen environmentsthan bacteria. Fungi also cannot survive above atemperature of 140 degrees Fahrenheit.

Considering that fungi cannot survive in temperaturesgreater than 140 degrees Fahrenheit and that they areresponsible for much of the decay of resistant materialindicates that excessive temperatures are detrimentalto the composting process in terms of complete degra-dation. Therefore, while high temperature levels aredesirable for pathogen destruction, they must becontrolled to reduce the destruction of beneficialorganisms and its subsequent effect on the completionof decomposition.

(3) Actinomycetes

The actinomycetes (fig. 2–4) are the third major classof micro-organisms that inhabit a compost pile. Acti-nomycetes are technically bacteria because of theirstructure and size, but are similar to fungi in that theyform filaments and are able to use a variety of sub-strates. Actinomycetes can degrade organic acids,sugars, starches, hemicelluloses, celluloses, proteins,polypeptides, amino acids, and even lignins. They alsoproduce extracellular proteases and can lyse (disinte-grate or dissolve) other bacteria. Actinomycetes aremore prevalent in the later stages of composting whenmost of the easily degradable compounds have been

Figure 2–3 Fungi

Figure 2–4 Actinomycetes

Figure 2–5 Higher organisms

Protoza Rotifer


CompostingChapter 2


degraded, the moisture levels have decreased, and thepH has become less acidic.

(4) Higher organisms

Higher organisms (fig 2–5) begin to invade the com-post pile once the pile temperatures cool to suitablelevels. These organisms include protozoa, rotifers, andnematodes. They consume the bacterial and fungalbiomass and aid in the degradation of lignins andpectins. These higher organisms contribute to thedisease suppressive qualities of the compost.

(d) Chemical transformations

During the composting process, micro-organismsdegrade the raw material of the compost mix to syn-thesize new cellular material and to obtain the energyfor these catabolic processes. Several chemical trans-formations take place as complex compounds arebroken down into simpler ones and then synthesizedinto new complex compounds.

Before the micro-organisms can synthesize new cellu-lar material, they require sufficient energy for theseprocesses. The two possible modes of energy yieldingmetabolism for heterotrophic micro-organisms arerespiration and fermentation.

Respiration can be either aerobic or anaerobic. Inaerobic respiration, the aerobic micro-organisms usemolecular oxygen, O2, to liberate the bulk of the en-ergy from the carbon source, producing carbon diox-ide and water in the process (see equation 2–1).

C O H O CO H O Energy, , 4 2 2 2 2[ ] + → + + [2–1]

This conversion is not achieved through a singlereaction, but through a series of reactions. Thesereactions serve not only to liberate significant quanti-ties of energy, but also to form a large number oforganic intermediates that serve as starting points forother synthetic reactions.

Aerobic respiration is preferred over anaerobic respi-ration and fermentation for composting because it ismore efficient, generates more energy, operates athigher temperatures, and does not produce the samequantity of odorous compounds. Aerobes can also usea greater variety of organic compounds as a source ofenergy that results in more complete degradation andstabilization of the compost material.

In anaerobic respiration, the micro-organisms useelectron acceptors other than O2, such as nitrates(NO3

–), sulfates (SO42–), and carbonates (CO3

2–) toobtain energy. Their use of these alternate electronacceptors in the energy-yielding metabolism producesodorous or undesirable compounds, such as hydrogensulfide (H2S) and methane (CH3). Anaerobic respira-tion also leads to the formation of organic acid inter-mediates that tend to accumulate and are detrimentalto aerobic micro-organisms. Aerobic respiration alsoforms organic acid intermediates, but these intermedi-ates are readily consumed by subsequent reactions sothat they do not pose as significant a potential forodors as in anaerobic respiration.

Fermentation is the simplest means of energy genera-tion. It does not require oxygen and is quite inefficient.Most of the carbon decomposed through fermentationis converted to end-products, not cell substituents,while liberating only a small amount of energy.

Unassimilated protein as nitrogenous organic residueis broken down to obtain the nitrogen necessary forthe synthesis of cellular material in heterotrophicmicro-organisms. Nitrogenous organic residues, orproteins, undergo enzymatic oxidation (digestion) toform complex amino compounds through a processcalled aminization. Carbon dioxide (CO2), energy, andother by-products are also produced.

Proteins complex amino compounds+ →O2other products+ + +CO Energy2

The complex amino compounds formed can then besynthesized into the micro-organisms or undergoadditional decomposition into simpler products. Thegeneral reduction in complexity of the amino com-pounds proceeds from proteoses to peptones to aminoacids and acid amides (Hansen, et al. 1990).

The products of the digestion of the proteins andcomplex amino acids can only be used in the synthesisof new cellular material if sufficient carbon is avail-able. If not enough carbon or energy to incorporatethese amino compounds into the cells is available,unstable nitrogen forms and accumulates through theprocess of ammonification (see equation 2–2). Be-cause the ammonia group is characteristic of aminoacids, ammonia (NH3) or ammonium ions (NH4

+) willaccumulate.

R − + → − + +NH HOH R OH NH Energy3 3 [2–2]



CompostingChapter 2

The ammonium compound that is formed intercon-verts between two forms depending on the pH andtemperature of the pile. This interconversion betweenNH3 and NH4

+ is described by reaction shown inequation 2–3.

2 3 2 3 4 2 3 2 4 32NH H CO NH CO NH CO+ → ( ) ↔ + + −

[2–3]

Acidic conditions (pH<7) promote the formation ofNH4

+, while basic conditions promote the formation ofNH3. Elevated temperature also favors the formationof NH3 and, because of the low vapor pressure of NH3,it generally results in gaseous NH3 emissions from thepile.

Another key chemical transformation of the compost-ing process is nitrification, the process by whichammonia or ammonium ions are oxidized to nitrates.Nitrification is a two-step process. In the first step,NH4

+–N is oxidized to form nitrites (NO2–) through the

action of autotrophic bacteria that use the energyproduced by this conversion. The nitrites are thenrapidly converted to nitrates (NO3

–) by a differentgroup of micro-organisms called nitrifying bacteria.The reactions are shown in equations 2–4 and 2–5.

NH O NO H O H energy4 112 2 2 2 2+ + → − + + + + [2–4]

NO O NO energy212 2 3− + → − + [2–5]

Nitrification occurs during the curing period. Sincenitrites (NO2

–) are toxic to plants and nitrates (NO3–)

are the form of nitrogen most usable in plant metabo-lism, enough time must be allowed for the curingperiod so nitrates are the final nitrogen product in thecompost. In addition, because nitrification requiresoxygen, proper aeration of the compost pile must bemaintained during curing.

Another important nitrogen transformation is denitrifi-cation. Denitrification occurs in oxygen-depletedenvironments. It can be carried out by either aerobicor anaerobic bacteria. If denitrification is carried outby aerobic bacteria, nitrate is being used in place ofoxygen as a hydrogen acceptor resulting in the follow-ing progression of nitrogen:

NO NO N O N3 2 2 2− → − → ( ) → ( )nitrous oxide gas [2–6]

If denitrification is carried out by anaerobic bacteria,the general reaction is:

HNO H NH N O3 2 2 2− + → + [2–7]

Because nitrous oxide is an odorous compound andresults in the loss of beneficial nitrate-nitrogen, deni-trification is not desired and can be avoided by main-taining aerobic pile conditions. This, of course, isaccomplished with proper aeration.

(1) Nitrogen losses

A significant amount of nitrogen is lost during thecomposting process. The amount of nitrogen lost,however, varies widely and is somewhat dependent onthe material, method, and management methodsemployed. Martins and Dewes (1992) found averagenitrogen losses of 69.2 and 59.6 percent for poultrymanure and 53.6 and 57.1 percent for pig and cowmanure. Hansen, et al. (1990) found nitrogen lossesvarying from 3.7 to 32.0 percent when compostingpoultry manure. Nitrogen losses are a concern becauseof potential contamination of ground water, odorproblems, and final nitrogen content of the compost. Ifnitrogen conservation is a goal, it is possible to man-age the composting operation so the potential fornitrogen losses is reduced.

The three possible pathways for nitrogen losses duringthe composting process are gaseous emissions, leach-ing, and denitrification.

The primary path of nutrient losses during compostingis through gaseous emissions. Martin and Dewes(1992) found that 46.8 to 77.4 percent of the nitrogenlost was gaseous emissions. The majority of theseemissions are ammonia (NH3) with a small percentagenitrous oxide (N2O). The most important factors in therelease of NH3 from a compost pile are the pH, ammo-nium (NH4

+)/NH3 equilibrium, mineralization rates oforganic nitrogen compounds, C:N ratio, temperature,and pile aeration. A pH greater than 8 promotes theconversion of NH4

+ to NH3. The release of NH3 asgaseous ammonia is then promoted by the elevatedtemperature. Ammonia emissions also increase ifnitrogen has accumulated generally in response to alow C:N ratio or nitrogen-rich raw material, such aspoultry manure. Turning the compost pile to aerate thepile also influences the release of gaseous emissions.These losses increase as the frequency of turningsincreases. Gaseous NH3 emissions increase after pileturnings because the action of turning the pile releases


CompostingChapter 2


any gases that have built up in the pile. In addition,because turning rebuilds porosity and increases pileaeration, it recharges microbial activity so that moreNH3 is produced that is in turn potentially lost.

Leaching nutrient losses are primarily as bound nitro-gen, ammonium ions (NH4

+), and small amounts ofnitrates (NO3

–). Nitrates are a concern because of theirpotential to contaminate ground water although theyhave not been found in significant amounts in compostleachate (Rymshaw, et al. 1992). Martin and Dewes(1992) found that 9.6 to 19.6 percent of the total nitro-gen was drained off as leachate of which 76.5 to 97.8percent was ammonium nitrogen (NH4

+–N) and therest, 0.1 to 2.2 percent was nitrate nitrogen (NO3

–).

The greatest amount of leaching occurs during the first2 weeks of the composting period. Leaching after thattime generally occurs after a rainfall if the compostfacility is uncovered. The amount of leachate, thenitrogen content of the leachate, and the proportion ofnitrogen fractions within the leachate are dependenton a number of interrelated factors. These include thetype of material being composted, the point in theactive composting process, and the frequency of pileturnings. The amount of nitrogen lost as leachate isgreater for the material rich in nitrogen, such as poul-try manure. The majority of the leaching occurs duringthe first weeks of composting. As the compostingprocess continues, the amount of nitrogen lost throughleaching not only decreases, but the fraction of theleachate that is NH4

+–N also decreases as it is gradu-ally converted to NO3

––N.

The formation of nitrate-nitrogen is an indication of awell regulated, aerobic composting process. Theamount of leachate coming from the pile also tends toincrease after the pile is turned, perhaps in response tothe increased porosity and aeration that help increasethe microbial activity. Leaching is often independentof the addition of water, particularly during the latterstages of the active composting period. Leachate maybe collected and used to add moisture to the compostpile. Returning the nitrogen and carbon contained inthe leachate to the pile along with the decrease inmoisture that usually occurs during composting con-centrates the nutrients of a compost pile.

Denitrification is the process by which nitrates areconverted to nitrous oxide (N2O) or nitrogen gas (N2).This process is carried out under faculative anaerobic

conditions by aerobic and anaerobic bacteria. Nitrousoxide is also responsible for part of the odors gener-ated at a composting site.

(2) C:N ratio

Micro-organisms require certain nutrients in largeamounts. Examples of some of the macronutrientsrequired are carbon (C), nitrogen (N), phosphorus (P),and potassium (K). The relative amounts of carbonand nitrogen present have the greatest effect on thecomposting process and so are used as the primaryindicators of nutrient content. Carbon and nitrogen arealso the main nutrient focus because if these nutrientsare present in the proper ratio, the other nutrients alsotend to be present in the acceptable amounts.

Carbon is used both as a source of energy and forgrowth of microbes. In aerobic decomposition, part ofthe carbon is released as CO2 while the rest is com-bined with nitrogen for microbial growth. As a result,the carbon content of a compost pile is continuouslydecreasing.

Nitrogen is used for the synthesis of cellular material,amino acids, and proteins and is continuously recycledthrough the cellular material of the micro-organisms.Any nitrogen that is incorporated into the cells be-comes available again when the micro-organism dies.Because a large part of the carbon is continuouslyreleased while the majority of the nitrogen is recycled,the C:N ratio decreases over the composting period. If,however, the system experiences large nitrogen losses,the C:N ratio can increase.

An initial C:N ratio of 20:1 to 40:1 is recommended forrapid composting. However, C:N ratios as low as 14:1also compost well and are practical for compostinganimal mortalities. Higher C:N ratios work moreefficiently, but require large additions of a carbonsource, such as straw, that reduce the quantity ofmortalities that can be composted. If carbon is presentin excessive amounts relative to nitrogen so that theC:N ratio is above the optimal range, the compostingprocess slows. In this case nitrogen availability is thelimiting factor. With only limited nitrogen resources touse, micro-organisms take longer to use the excesscarbon. Several life cycles of organisms are required toreduce the C:N ratio to a more suitable level.

If the C:N ratio is too low because the raw material isrich in nitrogen, the limiting nutrient will be carbon. If



CompostingChapter 2

enough carbon is not available to provide the energyand material for the microbes to incorporate theproducts of protein decomposition, unstable nitrogenas NH3 or NH4

+ will form. This excess nitrogen may bereleased as gaseous ammonia, accumulate within thepile in toxic amounts, or leach out of the pile andpotentially contaminate ground or surface water.

The measured C:N ratio of a compost mix does notalways accurately reflect the amount of nutrientsavailable to the micro-organisms. Microbial systemsrespond only to available nutrients and those nutrientsthey are able to readily use. As such, it is not onlyimportant for the mix of raw materials to have aproper C:N ratio, but also that the nutrients in the mixbe in readily available forms. Material containingsimple sugars, such as fruit waste, decomposes rapidlywhile woody material bound by decay-resistant ligninsis more difficult to decompose. Most nitrogen sourcesare not resistant to decay except for keratin (any ofvarious sulfur-containing fibrous proteins that formthe chemical basis of epidermal tissue, such as horns,hair, wool, and feathers).

(3) Oxygen

Oxygen is necessary for the survival of aerobic micro-organisms. If sufficient oxygen is not provided tosustain aerobic micro-organisms, anaerobic micro-organisms begin to dominate the compost pile, slowthe composting process, and produce odors. A mini-mum oxygen concentration of 5 percent is required tomaintain aerobic conditions.

Oxygen can be supplied to the pile using either forcedor passive aeration. Regardless of the method ofaeration, the amount of air that is being supplied to thecompost pile does not necessarily reflect the amountof oxygen that is actually reaching the micro-organ-isms. The reason for this is that the micro-organismsrequire an aqueous environment in which to functionand, as such, are located within a thin liquid film onthe surface of the compost particles. In addition, thediffusion of oxygen through water is significantlyslower than that through air. Therefore, although airmay be entering the pile at a sufficient rate to providethe required oxygen, the oxygen may not be reachingthe micro-organisms at the correct rate. This factormust be taken into account when aerating the pile andwhen managing the moisture content of the pile.

(4) Water

Water is another essential component for the survivalof composting micro-organisms. The micro-organismsrequire an aqueous environment in which to move andtransport nutrients. Water is also necessary to act asthe medium for the chemical reactions of life.

Micro-organisms thrive in an aqueous environment,but if the material within the compost pile is saturated,oxygen is not able to penetrate the pile in sufficientamounts to maintain aerobic respiration. The idealmoisture content for composting must therefore be acompromise between achieving adequate moisture forthe micro-organisms to function and adequate oxygenflow to maintain aerobic conditions. The moisturecontent for composting is generally recommended tobe in the range of 40 to 65 percent. Below 15 percentmoisture, microbial activity ceases altogether.

The moisture content of the compost pile fluctuatesduring the composting process as water is lost to evapo-ration and added by precipitation. Moisture needs to bemonitored during the process to maintain sufficientmoisture and porosity. The moisture content generallydecreases over the composting period and, dependingon the climate, additional water may be required.

The type of material used in the compost mix alsoinfluences moisture content considerations. For ex-ample, a highly porous material can have a highermoisture content than a densely packed one.

Another function of moisture is to provide a mecha-nism for cooling. The heat generated during thecomposting process heats the pile's air and compostmaterial and evaporates the water. The majority of theheat provides the latent heat of vaporization. Thisdiversion of part of the heat generated provides somecooling to the pile. If the compost pile becomes toodry while it is heating during the thermophilic stage,the pile may begin to overheat because of decreasedevaporative cooling.

(5) pH control and adjustment

The pH levels of the raw material of the compost mixdo not significantly impact the composting processbecause different micro-organisms thrive at differentpH levels. The ideal range for microbial activity isbetween 6.5 and 8.0. Composting continues at ex-tremes, such as 5 and 9, but the process slows. By theend of the composting process, the pH generally


CompostingChapter 2


stabilizes between 7.5 and 8.0, regardless of thebeginning pH.

The pH levels vary in response to the raw materialused in the original compost mix and the productionof various products and intermediates over thecomposting period. The first several days of the activecomposting period are characterized by a drop in pHto levels between 4 and 5. This depression of pH canbe caused by the formation of organic acids in anaero-bic zones or the accumulation of organic acid interme-diates resulting from an abundance of carbonaceoussubstrate. Acidic conditions are generally detrimentalto aerobic micro-organisms, particularly bacteria, andslow the composting process. Composting will notstop, however, because a population of organisms,mostly fungi, eventually develops that can use theacidic compounds as a substrate. As these organismsconsume the acidic compounds, the pH is elevatedagain.

In most cases the pH does not need to be adjustedbecause of the natural buffering capacity of the com-post pile. The pH does become a concern when mate-rial high in nitrogen is to be composted. A basic pH(> 8.5) promotes the conversion of nitrogenous com-pounds to ammonia. This ammonia formation servesto further increase the alkalinity and not only slow therate of composting, but also promote the loss of nitro-gen through ammonia volatilization. In such cases thepH may need to be adjusted downward below 8. Theaddition of superphosphate has been shown to con-serve nitrogen when used with dairy manure inamounts equal to 2 to 5 percent of the dry weight ofmanure (Rynk 1992).

The pH must be adjusted upward only if the formationof acidic conditions in the initial stages of compostingcauses an extended lag period. By adjusting the pHupward, the lag time for an appropriate microbialpopulation to develop in acidic conditions is elimi-nated, and rapid composting begins sooner. A basic pHalso helps to control odors by preventing the forma-tion of organic acid intermediates that are responsiblefor most of the odorous compounds generated at thecomposting site. The additive generally used for thispurpose is lime (Ca(OH)2). A drawback to using anadditive to adjust the pH is that nitrogen losses andodors through ammonia volatilization increase be-cause the pH is alkaline for a longer period.

(6) Physical characteristics

The physical characteristics of the compost mix ingre-dients must also be considered when developing acompost mix. Different physical characteristics affectaeration, the amount of decomposition, and the abilityof a pile to maintain aerobic conditions. The threemain physical characteristics of the compost mix ofmain concern are porosity, texture, and structure.

Porosity is a measure of the air space within thecompost mix and influences the resistance to airflowthrough the pile. If the pores become filled with waterbecause of a high moisture content, then the resis-tance to airflow increases. Less oxygen reaches themicro-organisms, and anaerobic activity begins todominate. Porosity is improved by a more uniform mixof material that provides continuity of air spaces,proper moisture to allow adequate free air space, andlarger particles to increase the pore size and reducethe resistance to airflow.

Larger particles are desirable to promote the flow of air,but they also diminish the surface area of the particles.Because the majority of the microbial activity occurs onthe surface of the compost particles within a thin liquidlayer, the greater the amount of surface area exposed,the greater the amount of decomposition.

Texture is the relative proportion of various particlesizes of a material and is descriptive of the amount ofsurface area that is available to the micro-organisms.The finer the texture, the greater the surface areaexposed to microbial activity. Minimizing the particlesize by such methods as selection and grinding alsoincreases the overall surface area of the material in thepile that is exposed to microbial decomposition.

Structure refers to the ability of a particle to resistcompaction and settling. It is a key factor in establish-ing and maintaining porosity during the compostingprocess. Structure is important because even a mixthat has all of the necessary components may not beable to sustain rapid composting. If the pile begins tosettle and close off air spaces as the material decom-poses, the compost process slows. Highly absorbentmaterial tends to maintain better structure than lessabsorbent material.

The ideal particle size of the compost material musttherefore be a compromise between maximizing poros-ity, maximizing surface area, and increasing structure.



CompostingChapter 2

637.0202 Design ofcompost mixtures

Natural decomposition occurs in any pile of wastematerial even if the C:N ratio, moisture content, andaeration are outside the recommended limits forcomposting. Generally, however, decompositionproceeds at a rate too slow to be readily noticeableand also generates putrid odors. Composting is adirected effort to maximize the rate of natural decom-position, reduce the production of odors, and to de-stroy pathogens, weed seeds, and fly larvae. Thecompost mix must be designed to optimize conditionswithin the pile in terms of nutrition, oxygen and mois-ture content, and pH and temperature levels so that ahigh rate of microbial activity is achieved.

(a) Components of compost mix

The three components to a compost mix are the pri-mary substrate, amendment, and bulking agent. Theprimary substrate is the main waste material thatrequires treatment. The appropriate material to add tothe compost mix can be determined based on thecharacteristics of the primary substrate. An amend-

ment is any material that can be mixed with the pri-mary substrate to balance the C:N ratio, modify thepH, improve stability, and achieve the proper moisturecontent. More than one amendment may be added to acompost mix. A bulking agent is a decay-resistantmaterial whose main purpose is to provide structureand porosity to the pile. Some bulking agents undergolittle to no decomposition and can be screened fromthe finished compost and re-used. An amendment canalso be a bulking agent.

When forming a compost mix, the physical and nutri-tional characteristics of the different raw material aremanipulated to achieve ideal conditions for microbialactivity. The two main characteristics taken intoaccount are the C:N ratio and moisture content. If oneor both of these are not sufficient to maintain rapidcomposting, then an appropriate amendment must bechosen to balance them. More than one amendmentmay be required either because of the constraints onthe availability of materials or the characteristics ofthe amendment.

If the primary substrate cannot maintain good struc-ture, lacks porosity, and the C:N ratio and moisturecontent are not appropriate, an amendment is re-quired. If the primary substrate only requires adjust-ment of its structure and porosity, then only a bulkingagent, such as shredded rubber tires or wood chips, isnecessary.

Calculations can be made to proportion a compost mixthat has the correct C:N ratio and moisture content.However, in some situations proper balance of the C:Nratio and the moisture content using the availablematerial is not possible. In this case the mix should bedesigned so that either the C:N ratio or the moisturecontent is within the recommended ranges and theother as close as possible to the ideal range. Determin-ing which criteria, C:N ratio or moisture content, tomeet depends on the type of material and the com-posting method.

Generally, the compost mix is proportioned to meetthe recommended range for moisture content, if boththe C:N ratio and moisture content cannot be bal-anced. The reason is that a compost mix with im-proper moisture content has a more detrimental effecton the composting operation. Low moisture contenthalts the composting process, while high moisturecontent creates odors. High moisture creates odorsbecause the voids in the compost mix are filled withmoisture resulting in anaerobic conditions. The C:Nratio is then balanced so that it is above the recom-mended range because this will only slow the process.A C:N ratio below the recommended range, on theother hand, generates odors and leads to nutrientlosses through ammonia volatilization.

Calculations alone should not be depended on todevelop the proper compost mix. Intangibles of thematerial, such as water absorbency, degradability, andstructure, may alter the mix proportions (recipe)required to maximize the composting process. Forexample, a mix may have the correct C:N ratio andmoisture content, but will fail to compost because thecarbon is not readily available or the porosity is insuf-ficient for good aeration. Calculations provide a start-ing point for establishing compost recipes, but experi-ence provides the knowledge of which materialscompost the best and in what proportions.


CompostingChapter 2


(b) Typical raw material

The characteristics of different wastes are describedin appendix 2A. Specific characteristics, such as C:Nratio and N content, of various wastes are also tabu-lated in the appendix. Additional information is inchapters 4 and 10 of the Agricultural Waste Manage-ment Field Handbook.

The predominant raw material (PRM) used in mostonfarm composting operations is manure. Othermaterial considered raw material is any that poses amanagement problem in terms of handling, cost, orenvironmental safety. This can include crop residue,such as alfalfa seed screenings and grass straw.

Manure, as excreted, is a nitrogen-rich material withpoor structure. The amount of amendment that needsto be added depends on the consistency of the ma-nure. Manure that is collected from the barn alongwith the bedding does not require a great deal ofadditional material, if any. Separated solids often havesufficient dry solids content to be composted directlyfrom a separator. Slurries or semisolid consistencymanure can be composted if the proper method andamendments are used. Liquid manure generally is notrecommended for composting because of the dry, highcarbon material that must be added in amounts thatare generally not economically feasible. However,liquid manure has been successfully composted usingthe silo method (see 637.0210(e)(3)) and the passivelyaerated windrow method (see 637.0210(b)). The silomethod is suitable for liquid manure because it com-pletely contains material and can provide sufficientaeration. The passive aerated windrow method can beused with liquid manure when peat is used as anamendment. Peat has an excellent water-holdingcapacity and is able to give structure to the manure topromote composting. Peat also has the ability toabsorb odors and is resistant to changes in the pHlevels of the pile.

Typically, the amendments used for the compostingoperation are those that are readily available at little tono cost. The least cost and often the best source ofmaterial is onfarm material. Some that may be avail-able are crop residue, peanut shells, rice hulls, andspoiled hay or silage. The cost of raw material mayalso be reduced by using wastes obtained from munici-palities, such as leaves, grass clippings, newspapers,and cardboard.

Another potential source of raw material for compost-ing may be neighboring farms where a waste materialis posing a management problem. These wastes couldinclude manure, crop residue, straw, or hay. Thismaterial can often be obtained for the hauling or at aminimal cost.

A good source of raw material is horse manure. How-ever, manure from race tracks or show barns is prob-lematic because it often contains noncompostablematerial. When obtaining material from off-farmsources, consideration should be given to the distancethat the material must be transported as well as themethod of transportation.

Nonfarm sources of material from other than munici-palities include wood industry, landscapers, grocerystores, restaurants, and food, fish, and meat process-ing industries. The main concern when taking thesewastes is the noncompostable material, such as plas-tic, it may contain, required permits and regulations,and handling and storage requirements. If these fac-tors are not taken into consideration before compost-ing, it may prove to be a costly mistake. For example,it should be specified beforehand whose responsibilityit is to remove any noncompostable debris from thewaste. It may be costly for the farmer to separate thedebris. As such, many farmers insist on "clean" wastebefore they will accept it onto the farm for compost-ing. Another example is grass clippings. They requirespecial consideration because they can only be storedfor short periods before they become odorous. Grassclippings also tend to lose their structure quite easilyupon decomposition.

Fish processing waste includes material, such as fishgurry (residue from fish cleaning), breading crumbs,and shrimp, crab, lobster, and mussel shells. Thematerial also can be stored only for short periodsbecause it decomposes quickly and becomes odorous.This waste is, however, a good source of nitrogen.

A final group of raw material is fertilizer or urea. Theyare not wastes that need to be managed, but areneeded to adjust the C:N ratio of the mix. This materialprovides a concentrated source of nitrogen to raise theC:N ratio without affecting the moisture. Addingfertilizer and urea may also be most cost effective ifthe only options for other material to provide theneeded nitrogen would be more expensive. The poten-tial problem with using this concentrated nitrogen



CompostingChapter 2

material is that the nitrogen may be more readilyavailable than the carbon resulting in excess nitrogenaccumulation and loss.

(c) Determination of the compostrecipe

Quite often, particularly in farm situations, a compostrecipe is not specifically developed using calculations.A compost mix is instead developed by adding acarbon and nitrogen source together to achieve a pileof good structure that composts well. The benefit ofdeveloping a mix using calculations is that it providesa more accurate base from which to start even if themix calculated on paper is not the combination even-tually used.

The two possible ways to design a compost mix are to:• Determine the proportion of material necessary

to develop a mix based on the C:N ratio of thematerial.

• Determine the C:N ratio of the mix based on thequantity of the material available and then bal-ance the C:N ratio and moisture content accord-ingly.

The first approach, to determine the proportion ofmaterials necessary to develop a mix based on the C:Nratio of the material (Brinton, et al. 1988), is easy tocalculate if only two materials are being used. Theequations to start with are:

X a Y b CC C+ = [2–8]

X a Y b NN N+ = [2–9]

where:XC = carbon content of material Xa = mix proportion of material XYC = carbon content of material YC = carbon content of mixXN = nitrogen content of material XYN = nitrogen content of material Yb = mix proportion of material YN = nitrogen content of mix

Example 2–1 illustrates the use of equations 2–8 and2–9 to determine the proportion of materials needed todevelop a mix based on the C:N ratio.

C, N, XC, YC, XN, and YN are known while a and b arethe unknowns. The standard procedure for solvingtwo equations for two unknowns using either alge-braic manipulation or a determinant can be used tosolve for b or a.

The calculations become more complex as the numberof ingredients increases. Computer spreadsheets andprograms are a great time saving way to do the math-ematics.

Calculation of a complex mix may be simplified con-ducting a mix-ratio analysis for at least the two mostimportant C:N ingredients (the ingredients used in thegreatest amounts). If the main ingredients have beenmixed in the proper proportions to achieve the targetC:N ratio, any material added that has a C:N ratio closeto that targeted will not significantly change the C:Nratio of the mix.

If different material is being analyzed for use in acompost mix, the number of calculations can bereduced by being aware that as long as the C:N ratio ofthe ingredients to be removed and replaced is thesame, the new ingredient can be substituted pound forpound. Therefore, different ingredients can be ex-changed and added without significantly changing theC:N ratio so that one can experiment with other char-acteristics of the pile, such as porosity, particle size,structure, and moisture (Brinton, et al. 1988).

The other approach to designing a compost mix is todetermine the C:N ratio of the mix based on the quan-tities of the material available and then balance theC:N ratio and moisture content accordingly. If themoisture content and C:N ratio cannot be balancedwith two ingredients, it may be necessary to addothers. This will increase the number of calculationsnecessary. While meeting the target C:N ratio andmoisture content is important, other factors, such asdegradability, porosity, structure, precipitation andclimate, and C:N and moisture losses, need to beconsidered.

The C:N ratio of a compost pile generally decreases asthe carbonaceous material is decomposed and lost tothe environment as CO2 while nitrogen is being con-served. If nitrogen is lost through volatilization, leach-ing, or denitrification, the loss rate is slower and insmaller quantities than the carbon. The moisturecontent of a pile also experiences a net decrease over


CompostingChapter 2


Example 2–1 Determining the proportion of materials needed to develop a mix based on the C:N ratio

Determine: Mix proportions needed to attain a C:N ratio of 35:1 using a manure having 3.7% N and a C:Nratio of 15:1 and a straw having 0.7% N and a C:N ratio of 100:1

Given: XN = nitrogen content of material X = 3.7%, or 0.037XC = carbon content of material X = 0.037 x 15 = 0.555YN = nitrogen content of material Y = 0.7%, or 0.007YC = carbon content of material Y = 0.007 x 100 = 0.70C = 35N = 1

Solution: Solving Equations 2-8 and 2-9 simutaneously

X a Y b CX a Y b N

a ba b

C CN N

+ =+ =

+ =+ =

[ – ][ – ]

. .

. .

2 82 9

0 555 0 70 350 037 0 007 1

Multiply both sides of equation 2–9 by –100

0 555 0 70 353 7 0 70 100

3 145 6520 668

. .. .

..

a ba b

aa

+ =− + − = −

− = −=

Solve for b using equation 2-8

0 555 20 668 0 70 3511 471 0 70 35

0 70 35 11 47123 529

0 7033 61

. . .. .

. ...

.

( ) + =+ =

= −

= =

bbb

b

As a check, solve for b using equation 2–9

0 037 20 6680 0 007 1

0 765 0 007 1

0 007 1 0 765

0 2350 007

33 61

. . .

. .

. .

.

..

( ) + =

+ == −

= =

b

b

b

b

Therefore the mix proportions would be:

%.

. .. %

% . .

aa

a bb

=+

=+

=

= − = =

20 66820 668 33 61

38 08

100 3 8 07 61 92



CompostingChapter 2

the composting period as moisture is constantly beinglost through evaporation. Moisture loss from a com-post pile is generally greater than any additionsthrough precipitation. However, extremely wet andcold periods may result in a net gain of moisture.These conditions may require that the piles be placedunder roofing. Because the moisture content and C:Nratio of the compost mix will most likely decline, theyshould be targeted such that they are at the upper endof the acceptable range initially. Therefore, they willremain within the recommended ranges duringcomposting.

Moisture content can be calculated as follows:

Mi = ×Wet weight - Dry weightWet weight

100 [2–10]

where:Mi = percent moisture content (wet basis)

The general equation for determining the moisturecontent of a compost mix of material, such as col-lected manure, amendment, and bulking agent, is asfollows: (The equation may contain variables that arenot needed in every calculation).

M

W M W M W MH O

Wm

w w b b a a

m=

×( ) + ×( ) + ×( )+

×100 2100

[2–11]

where:Mm = percent moisture of the compost mixture

(wet basis)Ww = wet weight of primary raw material (lb)Mw = percent moisture of primary raw material

(wet basis)Wb = wet weight of bulking agent (lb)Mb = percent moisture of bulking agent (wet basis)Wa = wet weight of amendment (lb)Ma = moisture content of amendment (wet basis)H2O = weight of water added (lb) = G x 8.36 (G =

gallons of water)Wm = weight of the compost mix (lb) including wet

weight of primary raw material, bulkingagent, amendments, and added water

To determine the amount of amendment to add to thecompost mix to lower or raise its moisture content:

WW M M

M Maa

mb mb d

d aa

=× −( )

−( ) [2–12]

where:Waa = wet weight of amendment to be addedWmb = wet weight of mix before adding in amend-

mentMmb = percent moisture of mix before adding

amendmentMd = desired percent moisture content of mix

(wet bases)Maa = moisture content of amendment added

Equation 2–12 can be used for the addition of water bysetting Maa = 100%.

To estimate the C:N ratio from the fixed or volatilesolids content:

%%

.C

FS= −1001 8

[2–13]

WVS

c =1 8.

[2–14]

C NCN

WW

c

n:

%%

= = [2–15]

where:%C = percent carbon (dry basis)%FS = percent fixed solids, ash (dry basis)Wc = dry weight of carbonVS = weight of volatile solidsC:N = carbon to nitrogen ratio%N = percent total nitrogen (dry basis)Wn = dry weight of nitrogen

The weight of carbon and nitrogen in each ingredientcan be estimated using the following equations:

WN W

ndry=

×%

100[2–16]

WWC Nn

c=:

[2–17]

WC W

cdry=

×%

100[2–18]

W C N Wc n= ×: [2–19]


CompostingChapter 2


where:Wdry = dry weight of material in question

The dry weight of the material in question can becalculated using:

W WM

dry wetwet= × −100

100[2–20]

where:Wwet = wet weight of material in questionMwet = percent moisture content of material (wet

basis)

The C:N ratio and nitrogen content for the same typeof compost material varies from source to source andlocale to locale. As such, table values should only beused for developing rough estimates. Using localvalues or laboratory testing the compost materialsproposed for use is highly recommended to establishmore accurately the actual C:N ratio and nitrogencontent.

Equation 2–21 can be adjusted to accommodate a mixthat has no bulking agent or that has several amend-ments. Simply omit or add the necessary variables.Every material in the compost mix must be accountedfor in the equation.

RW W WW W Wm

cw cb ca

nw nb na= + +

+ + [2–21]

where:Rm = C:N ratio of compost mixWcw = weight of carbon in predominant raw

material (PRM) (lb)Wcb = weight of carbon in bulking agent (lb)Wca = weight of carbon in amendment (lb)Wnw = weight of nitrogen in PRM (lb)Wnb = weight of nitrogen in bulking agent (lb)Wna = weight of nitrogen in amendment (lb)

To calculate the weight of amendment to add toachieve a desired C:N ratio:

WW R R

N M R Raa

nm d mb

aa aa aa d

=× −( ) ×

× −( ) × −( )10 000

100

,[2–22]

WN W M R R

N M R Raa

m mb mb d mb

aa aa aa d

=× −( ) × −( )

× −( ) × −( )100

100 [2–23]

where:Wnm = weight of nitrogen in compost mix (lb)Wmb = weight of compost mix before adding

amendmentRd = desired C:N ratioRmb = C:N ratio of the compost mix before adding

amendmentNaa = percent nitrogen in amendment to be added

(dry basis)Raa = C:N ratio of compost amendment to be addedMaa = percent moisture of amendment to be addedNm = percent nitrogen in compost mix (dry basis)Mmb = percent moisture of compost mix before

adding amendment (wet basis)

For a compost mix that has a C:N ratio of more than40, a carbonless amendment, such as a fertilizer, canbe added to lower the C:N ratio to within the accept-able range. In this special case, equation 2–24 can beused to estimate the dry weight of nitrogen to add tothe mix:

WW W W

RW W Wnd

cw cb ca

dnw nb na= + + − + +( ) [2–24]

where:Wnd = Dry weight of nitrogen to add to mix

Although the addition of dry nitrogen reduces theinitial C:N ratio, the benefits are short-lived. Thefertilizer nitrogen is available at a faster rate than thecarbon in the organic material, leading to an initialsurplus of nitrogen. This surplus is either leached fromthe pile or lost as gaseous and odor-producing ammo-nia. Adding an organic amendment that is high innitrogen to the compose mix to reduce the C:N ratio isadvised.

After the amount of an amendment to add has beendetermined to correct the C:N ratio, the moisturecontent needs to be checked to determine if it iswithin the proper range. It may be necessary to gothrough several iterations of these calculations toachieve the desired mix.



CompostingChapter 2

637.0203 Monitoring andparameter adjustment

The compost pile must be monitored and the appropri-ate adjustments made throughout the compostingperiod. This is necessary to sustain a high rate ofaerobic microbial activity for complete decompositionwith a minimum of odors as well as maximum destruc-tion of pathogens, larvae, and weed seeds. Generally,the monitoring process is monitored observing tem-perature, odors, moisture, and oxygen and carbondioxide.

(a) Temperature

A convenient and meaningful compost parameter tomonitor is temperature. Temperature is an indicator ofmicrobial activity. By recording temperatures daily, anormal pattern of temperature development can beestablished. Deviation from the normal pattern oftemperature increase indicates a slowing of or unex-pected change in microbial activity. The temperatureshould begin to rise steadily as the microbial popula-tion begins to develop. If it does not begin to risewithin the first several days, adjustments must bemade in the compost mix.

A lack of heating indicates that aerobic decompositionis not established. This is caused by any number offactors, such as lack of aeration, inadequate carbon ornitrogen source, low moisture, or low pH. Poor aera-tion is caused by inadequate porosity that, in turn, canresult from the characteristics of the material orexcessive moisture. Material that is dense does nothave good porosity. A bulking agent must be added toimprove porosity. A mix of material that is too wetalso lacks good porosity because moisture fills the airspaces making oxygen penetration of the pores moredifficult. The addition of a dry amendment with goodabsorbency helps to decrease the moisture content ofthe pile and improves porosity.

The pile may also fail to heat because of excessiveheat losses. This is caused either because the pile istoo small or exposure to cold weather is promotingheat loss. A pile that is experiencing excessive heatloss because of its size requires a larger volume-to-

surface area ratio to retain heat. The surface areaexposed to the atmosphere is reduced while at thesame time the volume that is insulated by the pileincreases.

Another possible reason for the failure of a compostpile to heat is that the initial mix is sterile or does nothave a large microbial population. If the initial micro-bial population is small, it will take longer to developand grow. This is generally not a problem with wastematerial, such as manure or sludge, but can be aproblem with "clean" material, such as newspaper orpotato waste. The addition of some active compostingmaterial, such as manure or finished compost, can beused as sources of inoculant.

When fresh chicken litter is available, "hot litter" canbe used as an effective inoculant. Hot litter refers tochicken litter that is managed to maintain a highpopulation of micro-organisms. Litter fresh from apoultry house typically is high in micro-organisms.However, the number of micro-organism declinesrapidly if adequate water and air are not provided. Aconvenient way to keep fresh litter "hot" is to maintaina supply of chicken litter in a pile that is kept moistand is turned on a daily basis. As part of the hot litteris removed to operate the compost piles, an equalvolume of older litter is added to the hot pile, water isadded, and the pile turned for mixing and aerating.This method allows all the required water to be addedto the hot litter pile, and maintains a micro-organism-rich litter supply with an initial temperature of 120degrees Fahrenheit.

A pile that begins to cool after achieving thermophilictemperatures is nearing the end of the compostingprocess or has become unable to maintain aerobicconditions. A lack of moisture or aeration is generallythe cause in the latter circumstances. A loss of aera-tion can occur during composting as a result of a lossof structure and porosity as the material decomposesand the pile begins to collapse. Aerobic conditions canbe reestablished by turning or mixing the pile to re-build porosity.

Uneven temperatures within a pile indicate a nonuni-form mix of material and subsequently leads to unevendecomposition. Cold spots in the mix indicate sites ofanaerobic decomposition that have the potential toproduce odors and phytotoxic compounds. This isgenerally a problem with static piles that are not


CompostingChapter 2


turned during the composting period. Composting instatic piles may also be uneven because of compac-tion. As the material in the pile begins to decompose,the pile settles and begins to close off air spaces at itsbase. Therefore, the base of the pile undergoes decom-position to a lesser degree than the upper part.

Thermophilic temperatures indicate that intensemicrobial activity is taking place. Exceedingly highpile temperatures (>170 ºF), however, are not so muchan indicator of vigorous microbial activity as a signthat the pile is unable to control its temperature. A pilebegins to overheat if it traps too much of the heatbeing produced. Generally, the pile is too large or toodry to allow for enough cooling through evaporation.A greater amount of surface area needs to be exposedto allow for sufficient release of heat to the atmo-sphere, and adequate moisture needs to be main-tained.

A typical composting pile does not heat uniformlythroughout, but has a temperature gradient from thehot inner core to the cooler surface temperature. Toproperly assess the pile temperatures, measure thehottest temperature that is in the pile. Generally, thehottest temperature initially occurs at 12 to 18 inchesinside the compost pile and then penetrates deeperinto the pile with time. If the pile is not heating at the12- to 18-inch depth, it is a good indication that theentire pile is not heating. Temperature variations alongthe length of pile are also not uncommon because ofan uneven mix, varying degrees of microbial activity,and other varying conditions throughout the pile.Readings at 50-foot longitudinal intervals are recom-mended.

(b) Odor management

Next to temperature, odor is the most effective andsimple indicator of whether the pile conditions areaerobic and, also, to a certain degree, if nutrient lossesare occurring through ammonia volatilization. Odormanagement is an important aspect of the compostingoperation, particularly if the operation is in closeproximity to neighbors.

Odors may be detected before composting starts.These odors are generally caused by the raw materialitself. This is particularly true for material, such as fishprocessing waste and manure. However, these odors

generally disappear. After the material is incorporatedinto the compost pile, the odors are masked by theother material in the pile or eliminated because themicrobes in the compost mixture use the odorouscompounds as substrates.

Strong, putrid odors that sometimes smell of sulfur,indicate anaerobic activity, particularly when theseodors are accompanied by low temperatures. Anaero-bic conditions generally develop in response to highmoisture, low porosity environments. If excess mois-ture is not the cause, then the pile may be too large,leading to compaction and inadequate aeration, or theporosity of the material is insufficient. If ammoniaodors are produced by the compost pile, then it mayneed to be managed for nitrogen conservation, particu-larly if nutrient losses are a concern. Such manage-ment techniques include reducing the turning fre-quency and adding carbon-rich material to the mix.

Odor detection is subjective and therefore difficult toquantify or measure. Regardless, the best method ofodor detection is the human nose. Once odors haveformed and are detected, they are difficult to remove.The most effective approach is to manage pile condi-tions to minimize odor generation. If odors havedeveloped, the best solution is to modify conditionswithin the pile so that odor production is not contin-ued. Odor-masking chemicals are available; however,their use is restricted mostly to treating the air ofindoor composting operations.

(c) Moisture

The maintenance of proper moisture can be a problemfor a composting operation. The moisture conditionsin the pile vary constantly throughout the compostingperiod mainly because of the large amounts of evapo-ration and the addition of water through precipitation.Improper moisture can slow or stop the compostingprocess, lead to anaerobic conditions, and produceodors. A dry pile is not only detrimental to microbialactivity, but forms dust that carries odors and possiblefungal pathogens, such as Aspergillus fumigatus.

Maintaining the moisture level between 40 and 60percent alleviates these potential problems.

Problems associated with maintaining proper compostmoisture levels are related to the climate. In hot, aridclimates moisture is difficult to maintain because of



CompostingChapter 2

excessive losses caused by evaporation. If the pilebecomes too dry, the composting process can behalted prematurely. This becomes a problem particu-larly if the compost is to be sold in bagged form be-cause once the compost is rewetted, the compostingprocess begins again. Once composting begins, itquickly becomes anaerobic because of the lack of anoxygen supply. This eventually produces odors andphytotoxic compounds. In wet or humid climates,excess moisture is a problem. In these operations thepiles may require additional turnings to release mois-ture, the addition of a larger quantity of dry amend-ments, or roofing.

The simplest methods for correcting a low moistureproblem in a pile are to turn it after a rainfall or tospray water onto the pile during turning. A hose can beinserted under the insulating layer of a static pile sothat water penetrates. Water should be added gradu-ally to minimize moisture losses through runoff and toprevent the addition of too much water. The waterused to wet the pile can be runoff from the compostingarea that is collected in a pond or lagoon, irrigationwater, or liquid manure.

A simple, low technology method of checking themoisture content is called the squeeze test. If thecompost is damp to the touch, but not so wet thatwater can be squeezed out of a handful of the com-post, the compost has sufficient moisture to sustaincomposting. A more precise measure can be obtainedby weighing a sample before and after drying it in anoven or microwave or under hot air.

(d) Oxygen and carbon dioxide

Oxygen levels within the pile can also be used as anindicator of how the composting process is develop-ing. As aerobic activity increases, the oxygen con-sumption should also increase causing the oxygenlevels to decrease. Measuring oxygen levels to monitorthe composting process is not as accurate as measur-ing temperatures. Oxygen monitoring is most useful toshow that stability has been reached. Oxygen levelsremain low during the active composting period.However, as the pile reaches maturity and microbialactivity begins to slow, oxygen levels rise.

Because CO2 (carbon dioxide) is a product of aerobicrespiration, it can also be used as an indicator ofmicrobial activity. The CO2 levels should increase asmicrobial activity develops and decrease as the com-posting process approaches maturity.

(e) Monitoring equipment

The types of monitoring equipment used depend onthe degree of management the operator wishes toprovide. All operations require a thermometer toestablish normal temperature profiles, turning sched-ules, or microbial activity. Sophistication of the equip-ment varies. The simplest, least expensive is a dialthermometer with a 3-foot-long pointed probe. Themain drawback to this thermometer is that it takestime for the readings to stabilize. This becomes timeconsuming when temperature monitoring requires thata number of readings be taken. For these situations itmight be worth the additional expense to purchase afast-response thermometer. Other features that varybetween thermometers are whether the readout isanalog or digital and the length of the probe that canrange from 3 feet to 6 feet. Thermometers generallyrange in price from the least expensive dial (analog)thermometers and mid-priced digital, fast-responsethermometers, to the most expensive computerizedthermocouple thermometers.

A pH meter ranges in price depending on features,such as accuracy, temperature compensation, auto-matic calibration, and range.

CO2 and O2 testers are available with sensors, anaspirator, and a sniffer probe. Samples should betaken from the part of the pile where microbial activityis expected to be the most vigorous and O2 levelsexpected to be the lowest (inner core of the pile). Thepotential drawback to monitoring oxygen levels is thatlow O2 readings may not result from vigorous micro-bial activity, but a lack of oxygen penetration into thepile. Carbon dioxide levels should be just the oppositeof oxygen levels because carbon dioxide is a productof aerobic respiration.


CompostingChapter 2


637.0204 Odor generation

Odor generation is one of the primary concerns of anycomposting facility. It is important to know the differ-ent ways in which odors can be formed so that eco-logical conditions can be manipulated for their preven-tion and treatment.

Odors produced at the beginning of the compostingperiod are generally caused by the nature of the mate-rial used. Material, such as manure or fish processingwastes, often have a strong odor in the initial stages ofcomposting that diminishes as composting proceeds.Odors generated during the composting period resultfrom the production and release of odorous com-pounds through either biological (microbial respira-tion) or nonbiological means (chemical reactions).Odors can be in gaseous form or associated withparticulates, such as dust.

The main compounds responsible for odor generationare sulfur compounds, nitrogen compounds, andvolatile fatty acids.

Sulfur compounds are produced by various processes.They are produced biologically through the decompo-sition of sulfur containing compounds (cystine, me-thionine) or through the assimilation of sulfur com-pounds. Aerobic respiration produces significantly lessvolatile organics than anaerobic respiration. Volatilesulfur compounds are also produced nonbiologicallythrough the reaction of various compounds that accu-mulate within the pile.

Volatile fatty acids are also responsible for odor gen-eration. They are intermediates in carbohydrate me-tabolism and accumulate in anaerobic systems.

Although the generation of odors generally is associ-ated with anaerobic composting, aerobic decomposi-tion also generates odors, particularly through thevolatilization of ammonia. Decomposition of proteinsleads to the formation of ammonia or ammonium bythe process of ammonification. NH4

+ and NH3 thenreadily interconvert based on the pH of the environ-ment. NH4

+ is the preferred form in acidic conditionswhile NH3 exists in basic conditions. The vapor pres-sure of NH3 is low and readily volatilizes at low tem-peratures.

Ecological factors influence the amount and type ofodorous compounds produced and whether they arereleased. The initial chemical composition of thecompost mix, oxygen concentration, oxygen diffusionrates, particle size, moisture content, and temperatureinfluences odor production. High temperatures facili-tate the release of odors because of increased vaporpressure, increased rate of nonbiological reactionsthat produce odor generating compounds, and de-creased aerobic decomposition.

The production of odorous compounds within a com-post pile does not necessarily mean that odors will bereleased. These compounds can move to other parts ofthe composting pile where they are decomposed tononodorous compounds. For example, hydrogensulfide that is produced through anaerobic decomposi-tion can be converted to sulfur rather quickly in aero-bic zones. If this does not occur, then the compoundsare released into the atmosphere and odor results.

Biofilters are a proven effective treatment method forremoving odor during composting. They use micro-organisms to decompose odorous organic compounds.Because the majority of odor-generating compoundsreleased during composting are metabolic intermedi-ates that can be further metabolized into innocuousproducts, they are readily utilized by the micro-organisms.

Peat effectively adsorbs ammonia, which reducesammonia losses. It works best when used to filter theexhaust air coming from the pile instead of mixing itdirectly into the compost.

Soil filters also control odors, particularly thosecaused by gaseous products, such as ammonia andvolatile organic acids. Soil is an effective medium forremoving odors through chemical absorption, oxida-tion, filtration, and aerobic biodegradation of organicgases. Soil filters require a moderately fine texturedsoil, sufficient moisture, and the ability to maintain pHwithin a range of 7.0 to 8. 5.



CompostingChapter 2

637.0205 Additives,inoculums, starters

A lag time of several days occurs at the start of thecomposting period before the compost pile tempera-tures reach thermophilic levels. This lag time is re-quired for the microbial population to grow and de-velop. Compost additives are available that eitherreduce or eliminate this lag period and improve thecompost process.

Starters, also referred to as inoculums, consist ofmicrobes and enzymes. They are added to the initialcompost mix and generally make up about 10 percentof the mix. Additives are substances added to theinitial mix to adjust the C:N ratio or pH or to controlodors.

In theory, the addition of inoculums to the compostshould work for several reasons. Theoretically, theintroduction of a mass of enzymes and micro-organ-isms to the initial compost mix eliminates the need fora microbial population to grow and develop. As such,vigorous microbial activity and decomposition shouldbegin almost immediately. The introduction of certainmicro-organisms and enzymes to the pile may alsoimprove the decomposition throughout the compost-ing process. This encourages a more efficient andthorough degradation of the compost material, which,in turn, improves the quality of the final product.However, the differences between theory and realityare significant mainly because of the complex pro-cesses and microbial populations that exist within anygiven compost pile.

None of the beneficial effects of adding a microbialstarter are possible unless the inoculant is representa-tive of the microbial population that optimizes thecomposting process. Inoculums are also only useful ifthey supply micro-organisms not already present inthe waste, add to a population that is lacking, or aremore effective than those micro-organisms already inthe compost mix (Golueke 1991). The reality is thatthese requirements are extremely difficult to deter-mine, particularly given the extremely diverse andconstantly changing environmental and microbialaspects of a compost pile. Because the internal envi-ronment is constantly changing, the inoculum also

may not necessarily be as effective as expected. An-other reality is that micro-organisms that are intro-duced into the pile are not adapted to prevailing condi-tions within the waste as are the indigenous microbialpopulations. The microbial populations that developduring the composting process do so in response tothe breakdown of various substrates and environmen-tal conditions. Pile conditions may not be immediatelysuited to the microbial population that is being inocu-lated, resulting in a less than optimal performance.

Enzymes are another important component of inocu-lums and starters because they ultimately break downthe organic matter in the compost mix. Appropriateenzymes are even more difficult to pinpoint because oftheir specificity and their sensitivity to environmentalconditions, particularly temperature fluctuations.Enzymes denature at elevated temperatures. The useof enzymes in inoculums is also costly. This cost is noteasily justified by any significant differences seen inthe composting process when they are used.

Additives can also be material added to the initialcompost mix to adjust the C:N ratio or pH of the initialmix, or to attempt to control odors. Additives used toadjust the C:N ratio include fertilizers, urea, or otherconcentrated sources of nitrogen. These additiveslower the C:N ratio without altering the moisturecontent of the mix and often provide the requiredamount of nitrogen at a lower cost than some othersource. The drawback to using a concentrated sourceof nitrogen to lower the C:N ratio is that the nitrogen isavailable at a faster rate than the organic carbon. Thismay result in an accumulation of nitrogen that is lostas gaseous ammonia or leached from the pile.

Many claims are made by the suppliers of variousstarters. To evaluate the validity of these claims re-quires substantial knowledge of microbiology andenzymology. Therefore, the best determinant is to testthe starter in field conditions by inoculating one pileand using another pile as a control. The results of thecomposition of each pile are then compared to see ifthe starter performed as claimed.

The debate about whether inoculums or enzymesreally do improve the composting process is ongoing.An additional argument is that if they make a differ-ence, is this difference significant enough to maketheir use economically practical. This is especially of


CompostingChapter 2


concern because material can be successfully com-posted without adding inoculums and because inocu-lums and enzymes are fairly expensive to purchase.Whether the improvements that a particular inoculummay have on a process are from the microbial popula-tion that is introduced or from the additional nitrogenthat is put into some inoculums must be determined.For this, an additive, such as fertilizer that suppliesadditional nitrogen, can be used. Continued researchin this area is necessary to determine the effects ofinoculums and enzymes on the composting process.Topsoil or finished compost can be added to the pileto supply microbes.

637.0206 Pathogens

One of the aspects of composting that makes it anattractive alternative to the direct application of un-treated manure is the high degree of pathogen destruc-tion that is possible with a well-managed compostingoperation. The pathogen content of the compost isimportant because improperly treated compost can bea source of pathogens to the environment and, assuch, a threat to humans and animals. This depends onthe type of pathogen involved. The type and quantityof pathogens in the initial compost mix are dependenton the waste that is being composted. Animal patho-gens are in manure and on plant residue that has comeinto contact with any manure. Plant pathogens are inplant residue.

Pathogenic micro-organisms that may be in compostinclude bacteria, viruses, fungi, and parasites. Al-though parasites and viruses cannot reproduce apartfrom their host, they can often survive for extendedperiods. If they are not killed during the compostingprocess, they can survive until the compost is landapplied. At that time they may infect a new host.

Bacteria and fungi, by contrast, do not require a hostto reproduce. Even if their numbers are reduced in thecomposting process, their population may recover andincrease if conditions permit and given enough time.Therefore, it is not enough to reduce their numbers.Pathogenic bacteria and fungi must be killed in themature compost. Conditions unfavorable to patho-genic growth include a lack of assimilable organicmatter and a pile with moisture content of less than 30percent. Because such conditions are difficult toachieve in mature compost, as many pathogens aspossible should be destroyed during the compostingprocess.

Pathogens can be destroyed by heat, competition,destruction of nutrients, antibiosis, and time (Hoitink,et al. 1991). Antibiosis is the process by which a micro-organism releases a substance that, in low concentra-tions, either interferes with the growth of anothermicrobe or kills it.

Most pathogens do not grow at the optimum tempera-tures for composting. As such, exposure to high (ther-mophilic) temperature kills them. The few exceptions



CompostingChapter 2

to this are among fungal plant pathogens. Some ofthese pathogens can withstand temperatures over 180degrees Fahrenheit. Most pathogens originating fromanimals cannot survive above the 130 to 160 degreesFahrenheit temperature range.

Pathogens can also be destroyed as a result of compe-tition with the indigenous microbial population fornutrients and space. Pathogens are at a disadvantagebecause they are not as well adapted to the environ-ment as the indigenous population and their numbersare insignificant relative to the indigenous population.Pathogens must compete with the indigenous micro-organisms for sites of attachment on the waste par-ticles. However, because of the shear number ofindigenous microbes with which they must compete,the pathogens will be displaced.

The destruction of readily available nutrients alsocontributes to the destruction of pathogens. Nutrientrequirements of pathogenic micro-organisms arespecific. If their key nutrients are used by the compet-ing indigenous microbial population, then the patho-gens are deprived of nutrients, and they will die.

None of the mechanisms of pathogen destructiondescribed here result in high pathogen kill unlesssufficient time is allowed for them to take full effect.Most of these mechanisms are difficult to monitor andquantify; therefore, temperature and time are the mainindicators used to verify optimal pathogen destruction.

Good pathogen destruction is possible with the vari-ous composting methods if the windrows or piles aremanaged correctly. The two essential elements inachieving good pathogen destruction are:

• All of the material must be exposed to lethalconditions either simultaneously or successively.

• The exposure must last for a sufficient amount oftime to maximize its effectiveness.

In the windrow system, pathogens theoretically arekilled through the process of turning. During turningthe innermost layers that have the highest temperaturelevels and greatest degree of pathogen destruction areexchanged. The outermost layers that have not beenexposed to these lethal conditions are then allowed toreheat so that all material within the pile is exposed tothe lethal temperature conditions. In reality, however,the outermost and innermost layers are not simplyexchanged, but are instead thoroughly mixed so thatthe innermost layer is recontaminated with pathogensfrom the outermost layer. To counteract the effects ofrecontamination and ensure complete pathogen de-struction, either the frequency of turning or the dura-tion of the active composting must be increased. In-vessel systems also experience the same problemswith recontamination because they rely on mechanicalagitation to mix the compost.

Aerated static piles should, in theory, allow for betterpathogen destruction than turned windrows and thosemethods in which material is turned because staticpiles do not have the same potential for recontamina-tion. The top layer of insulating material should alsohelp to maintain even temperatures throughout thepiles so that pathogen destruction is increased. Inreality, however, the ideal mix of material generallycannot be achieved. The less than ideal mix results inshort-circuiting air through the pile leaving coolpatches of undegraded compost. These cool patchescontain pathogenic organisms that can survive thecomposting process and contaminate the finishedcompost.


CompostingChapter 2


637.0207—Health risks of acomposting operation

(a) Bioaerosols

A health concern in the operation of compostingfacilities is the presence of bioaerosols. Bioaerosolsare organisms or biological agents that are transportedthrough the air and, under certain specific conditionsmight cause health problems when inhaled in suffi-cient quantities (Biocycle, Jan 1994, p. 51). Bioaerosolsinclude bacteria, fungi, actinomycetes, arthropods,endotoxins, microbial enzymes, glucans, and mycotox-ins. They can act as toxicants, pathogens, and aller-gens. The mere presence of bioaerosols at acomposting site does not mean that they necessarilypose a health risk. They must also be present in adosage sufficient to cause an infection. Most peopleare not affected by bioaerosols. In many casesbioaerosols only affect individuals who are predis-posed to infection. Lowered immunity because ofdisease and some medications can render an indi-vidual vulnerable to infection.

The bioaerosols of main concern at composting facili-ties and most commonly mentioned in the literatureare Aspergillus fumigatus and endotoxins. Aspergil-

lus fumigatusis a secondary pathogen that infectsindividuals who are predisposed to infection becauseof lowered resistance because of diseases or disordersthat affect the immune system or lungs. Diseases ofsusceptible individuals include AIDS, leukemia, lym-phoma, and asthma. Medication, such as antibiotics orsteroids, that interferes with the normal flora withinthe respiratory tract that prevent infection and inflam-mation can also lower resistance. If Aspergillus

fumigatus infects an individual, that person candevelop allergic bronchopulmonary aspergillosis. If itis not detected and properly treated, aspergillosiscannot only become a chronic and debilitating pulmo-nary disease, but can also affect other tissues. Forexample, if exposure has occurred because of a punc-tured eardrum, the ear can become infected.

Aspergillus fumigatus is in various kinds of decayingorganic matter and in a variety of locations, rangingfrom households and hospitals to forests and compost-ing sites. This opportunistic fungus is heat tolerant and

is not destroyed by the thermophilic temperatures ofcomposting. It is also airborne and can be inhaled byhumans.

Another health concern of composting facilities is thepresence of endotoxins. Endotoxins are metabolicproducts of gram-negative bacteria that are part of thecell wall and will remain in the bacteria after it hasdied. Endotoxins are not known to be toxic throughairborne transmission, but can cause such symptomsas nausea, headache, and diarrhea.

The dispersion of bioaerosols is related primarily tothe amount of dust that is released and the materialbeing processed. Bioaerosols are in the highest con-centrations during such dust producing activities asshredding and screening and during the mixing ofvegetative material, such as wood chips and brush.Maintenance of the moisture content above 40 per-cent, particularly during shredding, screening, turning,and mixing, helps to reduce dust formation. Thebioaerosols released during composting, particularlyAspergillus fumigatus spores, generally are confinedto the composting area and have only minimal impactbeyond 300 feet from the composting site.

Bioaerosols are a more prominent problem withmunicipal solid waste composting facilities thanagricultural waste composting facilities. This is be-cause of the differences in the nature of the materialbeing composted and the amount of dust that is gener-ated at each. Screening and shredding produce thegreatest amount of dust and generally are not part ofagricultural waste composting operations. Bioaerosolsare not a major concern to the health of workersunless an individual is taking immuno suppressantmedication, is an insulin-dependent diabetic, or hassevere allergies. A simple, yet effective safety precau-tion is to wear a respirator that can filter out particlesas small as 1 micron.

A properly selected and worn respirator can provideprotection from dust and mold spores. Commonnuisance dust masks do not provide significant protec-tion. They generally have only one elastic attachmentstrap and do not seal well around the face. The respi-rator used should be approved by the National Insti-tute for Occupational Safety and Health (NIOSH) orthe Mine Safety and Health Administration (MSHA). Ifthe respirator or filter has a number preceded by theprefix TC, it is approved.



CompostingChapter 2

The two categories of respirators are air-purifying andsupplied-air. Air-purifying respirators are equippedwith filters through which the wearer breathes. Theserespirators do not supply oxygen and should not beworn in areas considered immediately dangerous tolife or health (IDLH). These areas include oxygen-limiting silos and highly toxic atmospheres, such asthose in tanks that contain or have contained manureand in compost leachate collection systems. Air-purifying respirators provide protection when turningdusty piles of compost, bagging compost, or handlingdusty hay. These respirators include mechanical filterrespirators (reusable or disposable) that trap particlesduring inhalation and powered air-purifying respira-tors that use a motorized blower to force air throughthe filtering device.

Supplied-air respirators are the only kind of respiratorthat may be used in leachate collection systems, tanks,and sumps that are considered IDLH. These respira-tors supply the wearer with fresh, clean air from anoutside source. The two types of supplied-air respira-tors are air line respirators and a self-containedbreathing apparatus (SCBA). Air line respiratorsprovide clean air through a hose that is connected to astationary air pump or tank. A self-contained breathingapparatus (SCBA) has a portable air tank that is car-ried on the back like those worn by scuba divers andfirefighters. Training is required to use a SCBA effec-tively. This training should be provided by a safetyprofessional, industrial hygienist, or product represen-tative of the SCBA manufacturer.

637.0208 Aerationrequirements

The correct amount of aeration for aerated static pilescan be determined using basic guidelines as refinedthrough trial and error. The correct amount of aerationmust be determined to provide for the desired amountof moisture removal and temperature control whilemaintaining aerobic degradation. Aeration rates areestablished for sludge composting, but are not wellestablished for agricultural operations because mostagricultural operations do not use forced aeration.Therefore, rates are often based on those used forsludge composting. In this section general calculationsare given to help estimate the aeration rates required.These calculations are taken from those suggested byHaug (1986). Various factors affect these estimates.

The supply of air to the compost pile satisfies threerequirements (Haug 1986):

• Oxygen demands of aerobic decomposition• Removal of moisture to facilitate drying• Removal of heat produced during decomposition

to control process temperatures and to preventmicrobial inactivation

The first requirement of forced aeration is to meet thestoichiometric oxygen demand. The compost pile musthave sufficient oxygen to carry out the microbialdecomposition of the organic matter. This is a difficultparameter to determine because of the heterogeneousnature of agricultural composting mixes. In addition,the amount of oxygen supplied does not necessarilyreflect the amount of oxygen that is reaching themicro-organisms because of the differential diffusionof oxygen in water and in air. Because stoichiometricoxygen demand is significantly less than that formoisture and heat removal, if the aeration demandsfor moisture and heat removal are being met, then thestoichiometric oxygen demand is also being met.

The aeration rate required for moisture removal can beestimated by taking into account certain environmen-tal factors and the amount of moisture that needs to beremoved from the initial mix of materials to achievethe final moisture content. This must consider that theamount of moisture in saturated air increases withincreasing air temperature. As such, a large amount of


CompostingChapter 2


moisture is removed once thermophilic temperaturesare reached. Even piles in climates with high ambienthumidity experience significant amounts of drying.This is because the relative humidity of the inlet airhas only a minor effect on moisture removal if thedifference in temperature between the inlet and outletair is greater than 45 degrees Fahrenheit (25 ºC) (Haug1986).

The amount of air needed for moisture removal can beestimated using the temperature of the inlet air andexit air and either standard psychrometric charts andsteam tables, or by using the following equations. Tocalculate the saturation water vapor pressure of theinlet and outlet air:

log10 PVSaT

ba

=

+ [2–25]

where:PVS = saturation water vapor pressure, mm Hga = constant, –2238 for water vaporb = constant, 8.896 for water vaporTa = absolute temperature (K)

inlet T = ambient T

outlet T = pile T

From equation 2–25, the actual water vapor pressurecan be estimated:

PV RH PVS= ( ) [2–26]

where:PV = water vapor pressure, mm HgRH = relative humidity, ratio of actual water vapor

pressure to saturation vapor pressurePVS = saturation water vapor pressure, mm Hg

The air that exits the composting pile is near satura-tion and about the same temperature as the compost-ing material. Therefore, the relative humidity withinthe composting pile is taken to be 100 percent. Usingthe results of equation 2–26, the specific humidity ofthe inlet and outlet air can be calculated:

wPVP

PV

P PV

a

t

=

=−( )

0 622

0 622

.

. [2–27]

where:w = specific humidity, ratio of the mass of water

vapor to the mass of dry air in a given volumeof gas mixture (lb water/lb dry air)

PV = water vapor pressure, mm HgPa = atmospheric pressure of dry air, mm HgPt = total atmospheric pressure, mm Hg

The net removal of water vapor is the differencebetween the specific humidity of the inlet air minusthe specific humidity of the outlet air.

The quantity of air needed to remove the requiredamount of water (lb dry air/lb compost mix) can thenbe calculated by:

=−

lb water to removew wo i

[2–28]

where:wi = specific humidity of inlet air, lb water/lb dry airwo = specific humidity of outlet air, lb water/lb dry

air

Air is required for heat removal because excessivetemperature destroys the beneficial micro-organismsresponsible for composting. Heat is required for vapor-ization and to heat the moisture and the dry air to theexit temperature. The heat comes from the energyliberated by the decomposition process that, in turn,depends on the amount of oxygen supplied. The heatlost to the surroundings is considered negligible. If theheat generated is greater than that lost to vaporizationand heating of the air and water, then the temperatureof the pile rises. This is desirable up to a point becausebiochemical reaction rates increase exponentially withtemperature. When the temperatures become too high,however, they inactivate the microbial populations.

The required air supply generally is determined by theamount of air needed to control heat removal, particu-larly if moisture removal is not a primary concern ofthe composting operation. The amount of heat thatneeds to be removed is difficult to pinpoint because ofthe number of variables involved. A starting point canbe estimated that can be fine tuned through trial anderror.



CompostingChapter 2

Heat generation is difficult to estimate for severalreasons. One reason is that complete decomposition ofthe original mix does not occur nor will it all be aero-bic. A second reason is that the rate of oxidationvaries. A third reason is that the heterogeneous natureof the compost mix is difficult to define in terms ofcomposition.

To determine the amount of heat that will be released,the oxygen requirements for decomposition must beestimated. This requires knowledge of the primarychemical formula of the composting material.Organics:

A B C D EC H O N O CO H O NHx y z a + → + +( )2 2 2 3Balance and determine: lb O2/lb organic compound

Amendment:

A B C DC H O O CO H Ox y z + → +2 2 2Balance and determine: lb O2/lb amendment

Nitrification:

NH O NO H O H3 2 3 2+ → + + +−

The oxygen required for nitrification is generallyneglected because the oxygen demand for nitrificationis significantly less than that for organic oxidation.Also, most of the ammonia is not oxidized, but lostthrough either volatilization or synthesis into micro-bial cells.

The total oxygen demand (lb O2/ lb compost material)can then be estimated by multiplying the quantity oforganics and amendment to be composted by thevalue obtained above for pound air per pound mate-rial.

The value as given by Haug (1986) to calculate theamount of heat released per pound of compost is 5,866Btu/lb O2. Therefore:

x h h h lb O lbv m a+ +( ) = ×5 866 2, / compost material

[2–29]

where:x = lb air/lb composthv = heat of vaporizationhm = heat required to heat moisture to exit tempera-

tureha = heat required to heat air to exit temperature

h x w w hv o i v= × −( ) × [2–30]

where:hv = heat of vaporization at exit (pile) T, Btu/lbwi = specific humidity of inlet air, lb water/lb dry airwo = specific humidity of outlet air, lb water/lb dry

air

h x w w w h T Tm o i i s o i= × −( ) + × × −( ) [2–31]

where:hm = heat required to heat moisture to exit Tx = pounds dry air required to maintain compost

temperature at some Towi = specific humidity of inlet air, lb water/lb dry airwo = specific humidity of outlet air, lb water/lb dry

airhwv = specific heat of water vapor, Btu/lb–ºFTo = exit T, ºFTi = inlet T, ºF

h x h T Ta air o i= × × −( ) [2–32]

where:x = pounds dry air required to maintain compost

temperature at some Tohair = specific heat of air, Btu/lb–ºFTo = exit T, ºFTi = inlet T, ºF

To determine the air supply needed, the quantity of airrequired must be converted into a rate. The averagerate of aeration can be estimated as:

xd

( ) × ( ) × ( ) × =2 000

0 074824

,

.

standard cubic feet per hourdry ton per day

[2–33]

where:x = pound dry air required to maintain compost

temperature at some Tod = duration of composting period, days0.0748 = density of air at standard condition (20 ºC,

760 mm Hg)

Because the oxidation rate varies throughout thecomposting period, this number is an estimate of theaverage rate of aeration and does not reflect the peakrate of aeration. Peak aeration rates are necessary at


CompostingChapter 2


certain times. Estimates of peak aeration rates forsludge composting as previously established are in therange of 4,000 to 5,000 standard cubic feet hour perdry ton.

Haug's article in the October 1986 edition of Biocycle,Composting Process Design Criteria (pp. 53–57),gives a design example and further description ofaeration requirements.

637.0209—Analysis of rawmaterials and compost

Depending on the needs of the operation and the end-use of the compost, laboratory analysis of the rawmaterial and finished compost may be prudent. Labo-ratory analysis of the raw material is important foroperations that are in the beginning stages of settingup a compost operation and are attempting to estab-lish a compost mix. Because the characteristics of theraw material vary between and within batches, litera-ture values may not be appropriate. Laboratory analy-sis allows the operator to formulate a more ideal mix.Laboratory analysis of the raw material may also beprudent to determine if it contains contaminants thatmay not degrade during the composting period. Forexample, heavy metals are in some cardboard thatmay be used in composting. Also, pesticides may beattached to some crop residue that may be used incomposting.

Analysis of the finished compost may be required todetermine nutrient content if the compost is to be soldon the basis of its fertilizer content. Knowing thenutrient content of the finished compost that will beland applied helps determine proper application rates.Simple analyses can be performed on the farm usingonsite testing equipment. More sophisticated analysesrequiring specialized equipment and methods need tobe performed by independent or agricultural laborato-ries.

A sample of either the raw material or compost forlaboratory analysis must be representative of the pile.To ensure that the sample describes the general quali-ties of the entire lot, several samples should be takenfrom different areas of the pile and then combined. Asample from this combined mix can then be taken foranalysis. Samples taken from a compost pile shouldnot be taken from the edges, outer surfaces, or center.These are all regions of either very low or very highmicrobial activity and are not representative of theentire pile. A compost pile that has been stored out-side and exposed to precipitation may also have differ-ent moisture and soluble salt concentrations at theedges and center of the pile. This is caused by waterpuddles that form at the base of the piles and theleaching of salts that concentrate at the center of thepile.



CompostingChapter 2

A sample should be tested as soon as possible after itis taken to reduce the risk of any changes in the char-acteristics that may occur as a result of exposure toconditions different from the pile. It is best to store thematerial covered in the refrigerator if testing cannot beperformed immediately.

The sample size should be convenient to work withand be suitable for the containers and equipment beingused. It should also be large enough to provide arepresentative sample yet not so large that it makesthe tests too time-consuming or difficult. The samplesize can range from a fraction of a pound to severalpounds depending on what the laboratory or indi-vidual doing the testing requires. Once a particular sizehas been chosen, it is best to use the same sample forany replicates. If samples are being tested by a labora-tory, the laboratory should be consulted as to whatsize sample is required.

Frequent analysis of material through independentlaboratories can become expensive; however, simpletests can be performed on the farm using onsite test-ing equipment. They include tests for moisture con-tent, density, pH, soluble salts, and particle size distri-bution. These tests are outlined in appendix 2B.

(a) Determining moisture content

The moisture content of a sample can be determinedquite easily either on the farm using available or field-test equipment or in a laboratory. Moisture contentcan be expressed on a wet basis, dry basis, or as thefixed solids content. The moisture content as ex-pressed on a wet basis gives the percentage of theoriginal wet sample that is water. This is useful fordetermining whether a compost mix has the correctmoisture for composting or if the finished compost issufficiently dry.

Moisture content expressed on a dry basis denotes themoisture content as a percentage of the sample after ithas been dried. The content remaining after a samplehas been dried are known as the total solids. Becausea dry sample is defined as the total solids of a sample,the dry basis moisture can also be expressed as unitsof moisture per unit of total solids. Dry basis moistureis useful when calculating moisture changes. This isbecause the total solids base remains constant even asthe material dries and results in a more accurate

description of the moisture changes that are occurringin the sample. The microbiological activity of compost-ing can, however, alter the total solids content overthe course of the composting period as organic matteris consumed and decomposed. In this case the mois-ture content expressed on a fixed solids basis is help-ful. Fixed solids are composed of inorganic matter andare biologically inert. This part of the sample is notconsumed or degraded by the microbiological activityof composting. Moisture content on a fixed solids basecan therefore be useful in describing the moisturechanges that occur in compost over the compostingperiod.

The first step in measuring the moisture content of agiven sample is to determine the weight of the sample.The container that holds the sample should beweighed empty first and then weighed again with thewet sample. This sample is then dried in stages until itno longer loses water weight. To ensure that theweight being lost is because of water losses and notorganic matter losses through volatilization, dryingmust be carried out at low temperatures over ex-tended periods. For this test, the sample should not betoo large because the larger the sample, the longer ittakes for the sample to dry. Sample sizes generallyrange from 10 to 100 grams. Depending on the size andmoisture content of the sample, drying requires 24 to72 hours at 140 to 220 degrees Fahrenheit. If a 600-watt microwave oven is being used, 6-minute intervalsat maximum power is recommended (Rynk 1992). Ifburning occurs during drying, the results are not validbecause organic matter is also lost in addition to thewater.

After the sample has been dried, it is weighed. Theweight of the water removed is the difference betweenthe weight of the wet sample and the weight of the drysample. The sample is dried again until the weight ofwater removed is less than 1 percent of the originalweight such that:

wet weight - dry weightoriginal weight of sample

× <100 1 [2–34]

The moisture content can then be determined basedon a wet or dry basis:

moisture content % wet basis( ) = [2–35]

wet weight - dry weightwet weight - container

×100


CompostingChapter 2


moisture content % dry basis( ) = [2–36]

wet weight - dry weightdry weight - container

×100

Fixed solids are defined as the weight remaining afterignition of the total solids at 600 degrees Celsius untilcomplete combustion. An alternative is to use a lowertemperature of 375 to 425 degrees Celsius to preventthe loss of inorganic solids. The sample should beexposed to these temperatures for a minimum of 8hours and a maximum of 24 hours.

The moisture content expressed on a fixed solids basisis calculated by:

Fixed solids % dry basis( ) = [2–37]

wt. dry sample - wt. remaining upon ignitionwt. dry sample

×100

Moisture content =wt. wet sample - wt. of dry sample

fixed solids wt. of dry sample% ×[2–38]

The water holding or water absorbing capacity of thecompost material is also pertinent because the idealmoisture at which a material will compost is related tothe water holding capacity of the material. Optimalbiological activity occurs at 60 to 80 percent of thewater holding capacity (Brinton 1993).

(b) Bulk density

The bulk density of a compost mix is the mass per unitvolume of the material. Bulk density of the compostmaterial is measured as opposed to the density of asingle particle. It is the mass per unit volume of thematerial, while particle density is the mass per unitvolume of a single particle. For example, if water isadded to the material and results in no change in thevolume of the material, then the bulk density of thematerial increases. The particle density, meanwhile,remains constant. Bulk density, therefore, is a measurenot only of the material, but also the air spaces withinthe sample such that it gives an indication of theability of air to move through the sample. It is a func-tion of the moisture content and compaction of the

material. Bulk density above 1,080 pounds per cubicyard or 40 pounds per cubic foot does not haveenough air spaces for adequate airflow. A high bulkdensity indicates the need for a bulking agent to im-prove porosity. Bulk density can also indicate theprogress of the composting process because it shoulddecrease over the composting period.

In determining the bulk density, the material must beplaced into the weighing container with the sameamount of compaction as occurs in the pile. This issomewhat difficult to judge, and overpacking orunderpacking the container will cause an overestima-tion or underestimation of the bulk density. The mar-gin for error can be reduced by taking several samplesand averaging the results. Equation 2–39 is used fordetermining bulk density:

Bulk density = [2–39]y

weight of filled container - weight of empty containerontainer volumec

(c) pH and soluble salts

One method of determining the pH and soluble saltscontent that can be performed using field-test equip-ment is the saturated paste method. A pH meter andsolu-bridge meter, USP grade calcium chloride, apaper or plastic drinking cup, and distilled or deion-ized water are required for this test.

A sample is prepared for testing by first filling the cuphalfway with compost. Next, the appropriate solutionis added to the cup in increments while stirring with aspatula or knife. Sufficient solution has been addedwhen a smooth paste has formed that will not losewater when the cup is held on its side. The solutionrequired for the pH test is a 0.01 M solution of calciumchloride. This can be prepared by dissolving a slightlyrounded teaspoon of the calcium chloride dissolved ina gallon of the distilled or deionized water. For the testto be accurate for soluble salts, distilled or deionizedwater must be used for the solution. The paste that isformed must sit for at least 4 hours. Just before themeasurements are taken, the paste should be stirred. Ifthe solution has dried, more solution needs to bestirred into a paste. The appropriate measurementscan then be taken by placing the pH meter or solu-bridge meter into the sample with the proper solution.



CompostingChapter 2

(d) Particle size distribution

Proper particle size distribution needs to be addressedonly if the compost is to be sold. Customers want acompost that is free of clods of compost. Compost thatis to be used in potting media or for nurseries requiresthat it has particles no larger than a certain size. Inthese cases a sieve is used to screen out particles thatare larger than desired.

(e) Organic matter content

The three methods that a laboratory may use to deter-mine organic matter are: Walkley-Black, loss on igni-tion through use of a muffle furnace, and combustionanalysis. The Walkley-Black method for determiningorganic matter content is the standard method that hashistorically been used. It involves the use of potassiumchromate. A more recently developed method is com-bustion analysis that determines the organic mattercontent using infrared sensors. This method requires asample 0.001 gram or smaller and gives the best andhighest value. Most laboratories still use Walkley-Black because of the expense of the equipment neededfor combustion analysis. Muffle furnaces give the mostvariation in results. The carbon content can then bedetermined by ashing a sample and calculating back toWalkley-Black.

(f) Substrate degradability

Just because a compost mix has the correct C:N ratioon paper does not necessarily guarantee that the pilewill begin rapid composting. The availability of thenutrients (carbon and nitrogen) is also a considerationand depends on the degradability of the substrate. Thebasic method for determining substrate degradabilityis to measure the amount of oxygen consumed by thesubstrate under conditions that do not cause ratelimitations from lack of nutrients, lack of oxygen,inadequate moisture, and unbalanced pH. The patternof oxygen consumption over time is then used todetermine the rate constant for decomposition of thesubstrate. The instrument used for this test is called arespirometer. Various respirometers include standardBOD bottle, the Warburg instrument, and an electro-lytic constant volume respirometer (Haug 1991).

The simplest method for measuring substrate degrad-ability is to use a standard BOD bottle. This bottle ischarged with an oxygen saturated solution containingboth water and the sample. The oxygen concentrationremaining in solution over time determines respiratoryconsumption. A drawback to this test for degradabilityis that only a limited amount of oxygen can be dis-solved in water. This limits the quantity of the samplethat can be analyzed. This method is difficult to applyto solid samples.

A manometric, constant volume respirometer (theWarburg instrument) is often used. It works under theprinciple that at constant temperature and volume, thechange in pressure as measured by a manometer willindicate the change in the amount of gas. This methodis limited to small, homogeneous samples and is ex-pensive.

An electrolytic, constant volume respirometer can alsobe used to measure substrate degradability. Theserespirometers use water electrolysis to restore theoxygen that is consumed in the reaction vessel. As thepressure in the reaction vessel decreases because ofthe consumption of oxygen and the absorption ofcarbon dioxide, an electrolytic cell is set into opera-tion. This electrolytic cell dissociates water into H andO. Oxygen is directed back into the reactor whilehydrogen is released to the atmosphere. The quantityof oxygen produced is then determined through mea-surement of the applied current and its duration.Electrolytic respirometers can range from home-builtto computerized and are well suited to analysis ofcompost substrates. They are also, however, relativelyexpensive.

R.T. Haug and W.F. Ellsworth's article in the January1991 issue of Biocycle, Measuring Substrate

Degradability, is an indepth description of setup anduse of a constant pressure respirometer (Biocycle1991).

(g) Compost quality

Compost quality is determined by its physical, chemi-cal, and biological characteristics. Because some ofthese characteristics are somewhat subjective, there isno set method of determining compost quality. Thedegree of compost quality required is also dependenton the end use and the sensitivity of that end use.


CompostingChapter 2


The physical characteristics used to determine com-post quality are particle size, texture, appearance, andabsence of noncompostable debris. These characteris-tics are important indicators of compost quality par-ticularly for compost that is to be sold. Characteristicsthat become less important when the compost isapplied to cropland are particle size, texture, andappearance. Particle size is dependent on the end-use.A particle less than 0.5 inch in diameter is generallyadequate for potting, potting media amendment, andsoil amendment grades of compost. A smaller particlesize (<0.25 inch) is necessary for compost of topdressing grade. The texture should be soil-like and thecolor dark brown to black. The difference in colorbetween composts can often be the deciding factor.The one feature that is important regardless of theend-use is that the compost be free of debris. Custom-ers do not want to find glass, plastic, or other suchdebris in their compost.

The chemical characteristics of the compost are im-portant to determine its value as fertilizer or a soilamendment, its potential toxicity to plants, and itsease of incorporation. The chemical characteristics ofinterest are organic matter content, moisture content,pH, metals, nutrients, and soluble salts.

The organic matter content of the compost as deter-mined through laboratory analysis does not necessar-ily reveal the amount of organic matter that will becontributed to the soil. This depends on the form ofthe organic matter. Well-degraded humus-like materialis the preferred form of organic matter as opposed toundecomposed material, such as wood. The type andamount of organic matter in the compost are espe-cially important if it is being used as a soil amendmentto restore organic matter to the land.

The desirable moisture content of the finished com-post is within a range of 30 to 50 percent. Compostwith a moisture content of more than 60 percent tendsto form clumps that are difficult to break apart and,consequently, is difficult to spread evenly over theland. Wet compost is also difficult to handle. The maindisadvantage of a dry compost is that it producessignificant amounts of dust. Dry compost that is highin organic matter content is also difficult to incorpo-rate into the soil because it tends to stay on the sur-face of the soil.

The pH of the compost should generally be within arange of 6 to 8. An acidic or basic compost can bedetrimental depending on the type of crop grown andthe sensitivity of its end use. More specifically, a pH of5.5 to 6.5 is recommended for potting soil and germi-nation mixes, and a pH of 5.5 to 7.8 for soil amend-ments, top dress, and mulch (Nilsson 1994).

A laboratory analysis indicates the nutrients presentand their amount. Several things about nutrient analy-sis are important to know. Kjeldahl nitrogen is animportant laboratory test because its results can beused to determine the amount of organic nitrogen inthe compost. Total Kjeldahl nitrogen (TKN) is the sumof the organic and ammonia nitrogen because thenitrate nitrogen is driven off in the test. If ammonianitrogen is determined individually, the organic nitro-gen can be determined by subtracting the ammonianitrogen from TKN. A test for nitrate nitrogen isneeded to obtain the total nitrogen (TN) for a com-post. Total nitrogen is the sum of the nitrate nitrogenand TKN.

The primary form of nitrogen in the compost immedi-ately following the active composting period is ammo-nium (NH4

+). In large amounts, NH4+ can be detrimen-

tal to some horticultural plants. As the compost isallowed to age, this ammonium nitrogen is graduallyconverted into nitrate-nitrogen. Compost of differentlevels of maturity should, therefore, be used as appro-priate for the specie of plants and the stages of growthdepending on sensitivity to pH levels and ammoniumnitrogen.

Other important nutrients for plant growth are phos-phorus, potassium, calcium, and magnesium. Thesenutrients in mature compost, like nitrogen, also havevarying nutrient release characteristics and plantavailability. The percentages of nutrients contained inthe compost are important as are their ratios in rela-tionship to each other. This is because the ratio of thenutrients can affect nutrient uptake and plant growth.The best example of this is the C:N ratio. If the C:Nratio of the compost is too high when land applied, thesoil micro-organisms compete with the plants for theavailable soil nitrogen for energy to degrade the addi-tional carbon. The resulting nitrogen immobilizationmay negatively affect the growth of the plants.



CompostingChapter 2

The metal content of the compost is important particu-larly when the compost is used on crops for humanconsumption. Metal content is a greater concern forcomposts produced using sewage sludge and munici-pal waste, particularly those that had brown bags orcardboard as a raw material.

Soluble salts can be harmful to plants by reducingwater absorption and producing conditions that aretoxic to the plants. When soluble salts cause harm isdependent on the type of salt, the salt tolerance of theplant, and how much compost is applied. For example,salts containing sodium may be more detrimental to aparticular plant than potassium salts. A compost highin soluble salts concentration is more detrimental tocompost used for potting soil or germination mixes.This is because there is little dilution with largeamounts of soil as compared to when compost is usedas a soil amendment or land applied to cropland.Potting soils, germination mixes, and topdressesgenerally require a soluble salts content below 2 to 4mmhos/cm (Nilsson 1994). When compost is used as asoil amendment or mulch, it is diluted with largeamounts of soil or applied to plants that are moretolerant of high salts. Therefore, these compost usescan have a higher soluble salts content, such as 12mmhos/cm.

A compost having desired physical and chemicalcharacteristics may not be considered a quality com-post if the level of microbial activity is too high for thecompost to be considered stable. Its use may inhibitthe growth of plants because of the continued activityof the micro-organisms competing with the growingplants for nutrients. Compost also may have diseasesuppressive qualities. The degree to which a compostis suppressive to disease is dependent on the rawmaterial used, the environment in which the materialwas composted, and the conditions during curing. Thedegree of maturity of the compost also affects itsdisease suppressive qualities. Immature compostspromote pathogens and can result in increased dis-ease. Highly stabilized compost will not support themicro-organisms responsible for disease suppres-sion—the biocontrol agents (Hoitink, et al. 1994).Research in the use of compost for disease suppres-sion is only in its beginning stages as is the technology.Because consistent disease suppression requires strictquality control, biological control of diseases is mostlylimited to composted bark and sphagnum peat.

King County, Washington, specifications for organicamendment for use in public works projects providesexample criteria for compost quality. The criteria is:

1. 100 percent shall pass through a 1-inch sievewhen tested in accordance with AASHTO T87and T88.

2. The pH range shall be between 5.0 and 8.5 whentested in accordance with WSDOT Test Method417.

3. Foreign material (plastic, mineral soils, concrete,metal) shall not be more than 2 percent on a dryweight or volume basis, whichever provides forthe least amount of foreign material.

(h) Determination of compoststability

Compost stability has implications for its curing anduse. A stable and mature compost is one that hascompleted the active composting period and has curedsufficiently so there has been further decomposition oforganic acids and decay-resistant compounds, theformation of humic compounds, and the formation ofnitrate-nitrogen. The use of immature compost forpotting media or for land application can damage orkill the plants because of excessive C:N ratio, ammo-nium-nitrogen, volatile organic acids, or other phyto-toxic compounds. A reliable test of compost maturityis required to prevent any damage that may be broughtabout by the application of immature compost. A testfor compost maturity also helps to determine whethera pile is suitable for storage. Several methods are usedfor measuring the stability of compost, but none hasproven to be completely reliable. In addition, thesetests are often sophisticated and expensive.

A simple and inexpensive test for determining com-post maturity is the Dewar self-heating test (Brinton,et al. 1993). In this test a sample of the compost istaken and cooled to room temperature. It is then putinto a Dewar flask, a double-walled vessel with avacuum between the walls to reduce the transfer ofheat. The temperature rise that occurs while thesample is in this flask indicates the stability of thecompost. The relation between the rise in temperatureand the stability of the compost is inversely propor-tional such that the more the sample heats, the lowerits stability. If the compost sample does not heat tomore than 20 degrees Celsius (68 ºF) above ambient,then the compost may be stored without any of the


CompostingChapter 2


problems caused by high levels of continuing micro-bial activity (e.g., anaerobic decomposition, odors,production of phytotoxic compounds). Table 2–1 givesthe rating and description of stability using the Dewarmethod to determine compost maturity.

A method developed by Woods End uses the oxidationand reduction (redox) potential of the compost tomeasure its stability. In this test the redox potential of

Table 2–2 Stability of compost based on carbon dioxide respired during incubation*

Carbon loss mg CO2– Rating of respiration Comments on stability Soil N-release

% of C/day C per g C (estimated)

0.0 – 0.2 0 – 2 very low rate advanced humification & stability low < 25%

0.2 – 0.8 2 – 8 moderately low expected for ripe composts medium 50%

0.8 – 1.5 8 – 15 medium rate normal for average manures medium 50%

1.5 – 2.5 15 – 25 med – high rate normal for fresh wastes medium 50%

2.5 – 5.0 >25 high rate very unstable – odorous! high > 75%

* Source: WERL: On-Farm Composting: Guidelines for Use of Dairy and Poultry Manures in Composting Formulations. USDA Report, p. 45.

a moistened sample is measured and then placed insaturated incubation for 24 hours. The redox potentialof a stable material does not change significantlyduring the incubation period. The greater the fall inredox potential, the lower the stability level. Texture,mineral species present, moisture content, and oxygensupply influence redox potential measurements. Themechanisms of this test are not clearly understood,but a low redox potential indicates low stability sincethe loss of gaseous nitrogen and odorous compoundsoccurs only when the redox potential is low. Falsehigh readings indicating stability are also possible ifconditions are such that decomposition is inhibited.These conditions include extremes in pH (those out-side the range of 6 to 9) or a lack of viable organismsto carry out the decomposition because of excessiveheating, sterilization, or antibiosis.

Measuring the decomposition rate as a function of theCO2 loss rate is another measure of the stability of acompost. The relative amount of organic carbonrespired during incubation at 34 degrees Celsius for 24hours is measured along with the weight loss. A lowdegree of carbon dioxide respired indicates advancedhumification and stability. This test is also used toestimate the nitrogen that will be released upon appli-cation. The less stable the compost, the greater theamounts of nitrogen that can be expected to be re-leased into the soil. Table 2–2 shows the stability ofcompost based on carbon dioxide respired duringincubation and the estimated nitrogen that will bereleased upon application.

Table 2–1 Dewar self-heating method for determiningcompost maturity *

Heat rise ºC (ºF) Rating Description of stabilityover ambient

0 – 10 ºC V Completely stable compost,(32 – 50 ºF) can be stored

10 -20 ºC IV Maturing compost, can be(50 – 68 ºF) stored

20 – 30 ºC III Material still decomposing,(68 – 86 ºF) do not store

30 – 40 ºC II Immature, active compost,(86 – 104 ºF) must remain in windrows

40 – 50 ºC I Fresh, very new compost(104 – 122 ºF)

* Source: WERL: On-Farm Composting: Guidelines for Use ofDairy and Poultry Manures in Composting Formulations. USDAReport, p. 20.



CompostingChapter 2

The determination of the presence of volatile organicacids is still another indicator of compost stability. Astable, high quality compost generally does not havevolatile organic acids. Examples of such acids areacetic and butyric acid. Their presence is indicative ofanaerobic fermentation and instability and are respon-sible for odors and phytotoxicity. Their presence maybe indicative of an unripe, unstable compost, but theirabsence does not necessarily indicate a stable, maturecompost. This test involves a distillation of water-soluble fatty acids at atmospheric pressure. Table 2–3shows the volatile organic acids ratings.

The best test of compost stability is to observe itseffect on plants. Phytotoxicity (poisonous to plants)can result from high levels of heavy metals, toxiccompounds, and organic acids as well as problemswith oxygen demand of the compost. Table 2–4 givethe classifications of compost stability as indicated byphytotoxicity.

Field tests generally are not used to test phytotoxicitybecause too many variables are introduced. Usingindicator plants to evaluate germination and growth ofplants in beds of compost is more practical. Cress seedplants are used for this purpose because of their rapidgrowth rates. Other indicator plants include wheat andlettuce seedlings in peat mixtures. The seeds areplanted in the compost. A standard method is to use100 plug holes for planting the individual seeds. Thisallows a visual representation of growth inhibition aswell as the determination of numerical percentages.

Table 2–3 Volatile organic acids as an indicator ofcompost instability*

VOA rating Level of VOA dry basis (ppm)

Very low < 200

Medium – low 200 – 1000

Medium 1,000 – 4,000

High 4,000 – 10,000

Very high > 10,000


Table 2–4 Phytotoxicity as an indicator of compoststability*

Percent inhibition Classification of toxicityof plants

81 – 100 I – Extremely toxic

61 – 80 II – Highly toxic

41 – 60 III – Toxic

21 – 40 IV – Moderately toxic

0 – 20 V – Slightly, non-toxic


Germination and growth are evaluated over a 5- to 8-day period depending on the type of seed used. Thepercentage growth must then be compared to controlgermination rates where seeds are not grown in acompost. A medium control often used is moistenedpaper towels. The major disadvantage of these tests isthat they are not useful for evaluating differences instability in the early stages of the composting process.

Another method is to evaluate cress seed germinationand root elongation in an aqueous extract from thecompost during an incubation period of 24 hours at 27degrees Celsius. A further test of compost stability andphytotoxicity is to test how long the compost cansustain a plant. Radishes and green beans are used forthese tests. Radishes are more sensitive while greenbeans are more tolerant.

The C:N ratio is an indicator of compost stability onlyup to a point. A compost with a high C:N ratio pre-vents the uptake of nitrogen by the plants because ofthe competition between compost micro-organismsand plants for nitrogen. A low C:N ratio, however, isnot necessarily an indicator of stability, particularly ifthe original C:N ratio is low as well. A reduced C:Nratio generally indicates that at least some decomposi-tion and stabilization have occurred.

Another method for determining compost stability isto determine the oxygen consumption rate by measur-ing the decrease in partial pressure of the oxygen of asealed vessel containing the compost.


CompostingChapter 2


Other possible tests that have the potential to beeffective are those that analyze the disappearance ofstarch, 5-day biochemical oxygen demand in aqueoussuspension, nitrogen transformations, and chemicaloxygen demand (COD). These methods are all poten-tially effective in determining compost maturity, butrequire sophisticated analytical techniques.

637.0210 Compostingmethods

Several composting methods are applicable to farmoperation. The method chosen is dependent on thequality, capital investment, labor investment, timeinvestment, and land and raw material availability. Thefour broad methods of composting developed for usein large-scale composting are passive piles, windrows,aerated static piles, and in-vessel systems.

(a) Passive composting piles

The passive composting pile method involves formingthe mix of raw material into a pile (fig. 2–6). The pilemay be turned periodically primarily to rebuild theporosity. Aeration is accomplished through the passivemovement of air through the pile. This requires thatthe pile be small enough to allow for this passive airmovement. If it is too large, anaerobic zones form.

Special attention should be given to the mixing of rawmaterial. The mix must be capable of maintaining thenecessary porosity and structure for adequate aerationthroughout the entire composting period.

The passive composting method requires minimallabor and equipment. The method is often used tocompost leaves. Because aeration is passive, thismethod is slow and the potential for development ofanaerobic conditions is greater. This, of course, in-creases the potential for odor problems.

Figure 2–6 Passive compost pile

Air



CompostingChapter 2

(b) Windrow

The configuration of windrows is elongated (fig. 2–7).These piles are turned regularly. Raw material is eithermixed before pile formation or mixed as a part of pileformation. Windrow shapes and sizes vary dependingon the climate and equipment and on the materialused. Typically, windrows are 6 to 10 feet high, 15 to20 feet wide, and are up to several hundred feet long.A wet climate requires a windrow shape that allowsmoisture runoff. A concave top may be required indrier climates to collect water and maintain pile mois-ture. Smaller windrows experience greater heat loss,while larger piles run the risk of anaerobic zones andodors. Dense material, such as manure, should bepiled at a lower height than fluffy material, such asleaves. Bucket loaders and backhoes can producehigher windrows than turning machines.

Windrows are aerated by passive aeration as in thepassive composting method. The porosity necessaryfor adequate passive aeration is maintained by regu-larly turning the windrows. Turning windrows alsoserves to mix the material; releases heat, water vapor,and gases; and composts material more evenly. Be-cause significant amounts of heat are released uponturning the windrow, turning prevents excessivetemperature accumulation within the windrow. Turn-ings are more frequent during the initial stages ofcomposting when the most intense microbial activitytakes place and temperature evolution is the greatest.

The schedule of turnings during composting variesfrom operation to operation depending on temperaturelevels in the pile, consistency of the manure, labor andequipment availability, season, and how soon the

compost is needed. The turning frequency can rangefrom several times weekly to monthly. The numberand frequency of turnings needed to achieve the de-sired quality compost is best determined throughexperience.

The amount of time required to finish the compostingprocess using the windrow method ranges from 3 to 9weeks. The duration is dependent on the type ofmaterial being composted and the frequency of theturnings. The more frequent the turnings, the shorterthe duration will be. For a 2-month composting period,five to seven turnings are typical. Curing generallylasts at least 1 month.

Commonly available farm equipment can be used forthe initial mixing and pile formation and for turning.Most windrow operations use bucket loaders formixing, pile formation, and turning. Manure spreadersare used to construct windrows. Backhoes, grappleloaders, potato diggers, and snowblowers are alsoused. Dump trucks, dump wagons, and bucket loaderscan be used for pile formation and material transport.Specialized windrow turners are available. The wind-row method is the most widely used by farmers be-cause of its adaptability and flexibility to farm opera-tions and its ability to produce quality compost.

(c) Passively aerated windrows

Passively aerated windrows (fig. 2–8) are not turned.Aeration is accomplished solely through the passivemovement of air through perforated pipes embeddedin the base layer of the pile. Another feature thatdistinguishes this method from turned windrows is theuse of a base layer and a top layer in windrow con-struction. The base layer is typically composed of peatmoss, straw, or finished compost. The main character-istic desired of this layer is that it be porous so that theair that is coming through the pipes is evenly distrib-ute. It also helps to insulate the pile and absorb mois-ture.

The top layer is composed of peat moss or finishedcompost and serves several functions. The first func-tion is to retain odors through the affinity of peat mossand finished compost for the molecules that causeodors. The top layer also deters flies and retains mois-ture and ammonia.

Figure 2–7 Windrow method

Air


CompostingChapter 2


Initial construction of this type of windrow requiresmore labor than other windrow methods. Once thewindrow is formed, however, the labor requirement isprimarily that necessary to monitor the temperatureand porosity of the pile.

As in the passive composting system, the key elementis to formulate a mix with good porosity and structureto allow for adequate aeration. Peat moss has been theprimary amendment with this method because of itsgood porosity and structural qualities. Passive aerationalso requires that the piles not be as high as those arefor the windrow method. The typical height is 3 to 4feet with a width of about 10 feet. The bottom and toplayers should each be about 6 inches thick.

(d) Aerated static pile

A variation on the passively aerated windrow methodis the aerated static pile (fig. 2–9). The main differencebetween a passively aerated windrow and an aeratedstatic pile is that the aerated static pile uses blowersthat either suction air from the pile or blow air into thepile using positive pressure.

The suction method of aeration allows better odorcontrol than positive pressure aeration, particularly ifthe air is directed through an odor filter. An odor filteris essentially a pile of finished compost that has anaffinity for odor causing molecules. Some other odor-treatment system can also be used to treat the air

coming out of the pile. The disadvantage of usingsuction is that not as much air can be pulled throughthe pile as can be pushed through using positive pres-sure.

The blowers used for aeration serve not only to pro-vide oxygen, but also to provide cooling. Blowers canbe run continuously or at intervals. When operated atintervals, the blowers are activated either at set timeintervals or based on compost temperature. Tempera-ture-set blowers are turned off when the compostcools below a particular temperature. Blower aerationwith temperature control allows for greater processcontrol than windrow turning.

A forced aeration static pile has a base layer and toplayer much like the passively aerated windrow. Thepurpose of the base layer for the aerated static pile isto distribute air evenly either as it enters or leaves theaeration pipes. This requires porous material, such aswood chips or straw. The top layer is generally com-posed of finished compost or sawdust to absorb odors,deter flies, and retain moisture, ammonia, and heat.

As with all static piles, the initial mix and pile forma-tion must have proper porosity and structure foradequate air distribution and even composting. Adecay-resistant bulking agent is required to providethe necessary porosity. Wood chips are a good ex-ample of a bulking agent. They undergo minimaldegradation during the composting process and can bescreened from the finished compost and reused.

Figure 2–9 Aerated static pile

Blower

Air

Figure 2–8 Passive aerated windrow

Compost, peatmoss, or strawbase

Compost or peatmoss cover

Perforated pipe



CompostingChapter 2

The use of forced aeration also requires additionalcalculations. The size of the blower as well as thenumber, length, diameter, and types of pipes to use foradequate aeration must be determined. Pipes andblowers interfere with pile formation and cleanupoperations. Aerated static piles are not commonlyused for farm-scale composting operations.

(e) In-vessel systems

(1) Bin

Bin composting (fig. 2–10) uses either constructedwooden bins, unused storage bins, or some otherappropriate vessel either with or without a roof. Somebins have aeration systems similar to those of forcedaeration static piles. The same principles as forcedaeration piles apply to these. The material innonaerated bins must be turned regularly to maintainaerobic composting.

(2) Rectangular agitated bed

The rectangular agitated bed method (fig. 2–11) useslong, narrow beds in which to compost and an auto-mated turner for periodic turning. The turner is sup-ported on rails that are mounted on either side of thebed for its whole length. As the turner moves along thebed, the compost is turned and moved a set distanceuntil it is ejected at the end of the bed. In some sys-tems blowers are also used to force air into the beds.

The duration of the composting process is determinedby the length of the bed and the turning frequency. Anextended curing period is generally required.

(3) Silo

The silo method (fig. 2–12) is a rapid compostingmethod that requires a prolonged curing stage. Com-post material is loaded into the silo at the top andremoved from the bottom using an auger. Aeration isprovided through the base of the silo so that air isforced upward through the compost material. Outletair can be collected from the top and directed to anodor treatment system, such as a biofilter.

(4) Rotating tube

The rotating tube (fig. 2–13) is a method that can beused where small amounts of waste require compost-ing. The compost mix is loaded in the upper part of thetube. The mix will rest on the first baffle plate. Whenthe tube has filled from the first baffle plate to the top

Figure 2–11 Rectangular agitated bed system

Figure 2–12 In-vessel silo

Compostout

Rawmaterals

in

Air

Turning device

Raw materials in

Compost out

Figure 2–10 Bin

Compostmix


CompostingChapter 2


of the tube, it is rotated to aerate the compost mix andempty the tube above the first baffle plate. This allowsadditional compost mix to be loaded in the tube.Ideally, the tube is operated so the composting pro-cess is complete by the time the material exits thetube. Tube size will be limited to what can be rotatedwhen it is filled to capacity.

(f) Comparison of compostingmethods

The advantages and disadvantages each compostingmethod is briefly summarized in this section.

(1) Passive composting piles

Advantages:

• They require the least management.• Once the piles are formed, they need only be

turned occasionally to restore porosity.• They have low capital costs. The equipment

needed to mix the raw material and form thepiles can be adapted from farm machinery al-ready in use.

End roller

Baffle arrangement

Stainless steel baffles

End roller

PVC

Figure 2–13 Rotating tube composter



CompostingChapter 2

Disadvantages:

• The composting process is very slow becauseaeration is passive and turnings are infrequent.

• Up to 1 year is required for the compost to be-come fully mature.

• The potential is greater for development of odorsbecause of the increased chance of anaerobicconditions brought about by compaction andlack of adequate aeration.

• The piles must be smaller than for other methodsto promote aeration that results because of spaceinefficiency.

• Because the piles generally are built without anyprotective covering, they are subject to theeffects of weather conditions. Cold weather canslow the process, while heavy precipitation canruin pile porosity and cause runoff and leaching.Excessive drying also stops the compostingprocess.

(2) Windrow method

Advantages:

• It is highly adaptable to common farm opera-tions. Already available facilities and equipmentcan be used to implement the windrow system.

• Flexibility of the turning schedule allows adjust-ments in the operation according to labor, equip-ment, and material availability.

• Electricity is not required for this method, so itcan be used in remote areas. This allows sitingthat provides adequate buffering between thecompost operation and neighbors and decreasesthe chance for nuisance complaints.

• They are periodically turned, so porosity andstructure of the mix are not as critical, whichallows a greater choice of amendments.

• Decreased need for secondary operations tofurther stabilize the compost.

• Curing can be accomplished.• Has the capacity to handle large amounts of raw

material if adequate land is available for the piles.• Turning the piles contributes to greater drying

and material separation (smaller particle size,finer texture) than a static pile, which increasesthe quality of the of the finished product.

Disadvantages:

• Similar to those of passive composting piles.• The open field or area location makes the piles

subject to weather conditions, particularly pre-cipitation and cold weather.

• Excess moisture in the pile can lead to anaerobicconditions that generate odors.

• The need for turnings, particularly in the initialstages, makes this method labor intensive.

• Pile turning can take significant time although itdepends on the skill of the operator and the typeof equipment used.

• Requires more management than other methodsbecause turnings must be frequent enough tomaintain porosity and thermophilic tempera-tures.

• Odors can be a problem, particularly after turn-ing the piles.

• Significant amounts of land and equipment areneeded. Land is required both for the piles, andfor movement of equipment and material on thecompost site.

• Equipment maintenance costs can accumulatebecause of increased wear and tear on the ma-chinery.

(3) Passively aerated windrows

Advantages:

• Turning is not required.• The top layer of straw or finished compost pro-

vides odor and nutrient retention.• Less expensive than forced aeration piles be-

cause the purchase of blowers is not required.

Disadvantages:• They are subject to the effects of the weather.• They are not appropriate for material that tends

to compact during the composting process andrequires turning to rebuild porosity.

• The initial mix is critical to maintaining goodaeration that limits the material that can be used.

• Perforations in the pipes can become cloggedwith material so that aeration is inhibited.

• Installation, removal, and damage to the pipesduring pile formation and cleanup can be aproblem.

(4) Aerated static piles

Advantages:

• They are more space efficient.• They can be larger than windrows because

aeration is forced rather than passive.• Space is not needed for turning equipment.• The increased aeration shortens the time re-

quired for composting.


CompostingChapter 2


• The time or temperature controlled blowersallow for close process control, which results inless temperature variation and a more consistentquality compost.

• Elevated temperatures increase pathogen kill.• The insulating layer on the pile helps to achieve

higher temperatures as well as prevent excessivelosses of ammonia.

• This layer reduces the intensity of odors.• Require lower capital investment than in-vessel

operations that employ forced aeration.

Disadvantages:

• Short-circuiting of the air in the pile can occur,which causes uneven composting and an incon-sistent product. It is more likely to happen whenthe raw material is not properly mixed to obtaingood porosity and structure.

• The pipe openings may become blocked, pre-venting aeration. This is difficult to correctduring composting because the pipes are buriedat the base of the pile.

• Installation, removal, and damage to the pipesduring pile formation and cleanup can be aproblem.

• Some capital investment is required to purchasethe necessary equipment for blowers and pipes.

• Forced aeration tends to dry the compost pileand, if excessive, will prevent stabilization of thecompost.

(5) In-vessel systems

Advantages:

• They are generally located indoors or under aprotective cover, which reduces the vulnerabilityof the compost material to the effects of weatheras well as the potential for odor problems.

• Good odor control within the composting facilityis possible by diluting the inside air with air fromthe outside or by directing odors to a treatmentsystem.

• The reduced exposure to the weather allows forgreater control of the quality and consistency ofthe product.

• They are space efficient. Rectangular agitatedbed or channel composters are space efficientbecause they use an automated turner that ismounted on channels. Bins and silos are spaceefficient because their containment walls allowthe material to be stacked higher than static pilesor windrows.

• Except for bins, these systems require less laborthan windrows because they use an automatedturning process or a self-turning mechanism.

Disadvantages:

• The high capital and operation and maintenancecosts associated with the required automatedturners.

• Breakdown can delay composting if equipmentrepairs cannot be made quickly.

• Silo and rectangular agitated bed systems en-courage shorter composting periods; however,the resulting product may not be fully stabilizedor have adequate pathogen kill.

• Bins filled too high can result in compaction andinadequate aeration.

• These systems have less flexibility than othersystems, particularly concerning location andequipment.

(g) Controlled microbialcomposting

Several management approaches are used in farmscale composting operations. One approach is tomanage the composting facility based on intuition,experience, and trial-and-error to develop a systemthat works best. Another approach, and perhaps abetter one, is to manage the compost facility from ascientific point of view. This method is called con-trolled microbial composting. It requires extra time,effort, and input of material to provide an optimumenvironment for growth of desired composting mi-crobes. Also required are regular monitoring and theuse of specialized equipment and material. However,investing the time and money necessary for this sys-tem results in a good quality finished compost within a6-week period. Quality compost can be producedwithout using the controlled microbial compostingmethod; however, it must be recognized that it gener-ally takes longer.

Controlled microbial composting is carried out using awindrow system. A mix of material is first developedsuch that the desired microbial population is estab-lished. This includes the addition of a microbial inocu-lant to ensure the presence of the desired micro-organisms. Clay and a soil high in organic matter arealso added. The clay is responsible for chelating (floc-culating) the compost material to improve the texture



CompostingChapter 2

and particle size of the compost. The purpose of thesoil with high organic matter is to help promote thedevelopment of the microbial population.

The compost pad generally is not modified with suchmaterial as gravel or concrete. Pile contact with thesoil promotes and maintains the microbial populationwithin the pile. The pile is formed by laying downmaterial in layers and mixing it using a specializedwindrow turner. Self-propelled and tractor-pulledturners can be used with this method. Some turnersinclude a watering system that wets the pile as it isturned. The windrows formed are generally 3 feet highwith widths approximately twice the height or asconstrained by the type of windrow turner being used.

Once the pile is formed, it is covered with a specialmaterial, generally referred to as fleece, composed of100 percent polypropylene. This material is resistant topH levels ranging from 2 to 13 as well as microbiologi-cal attack. It helps to maintain optimum conditionswithin the pile by protecting the pile from adverseenvironmental conditions. The cover repels precipita-tion and prevents drying by reducing the effects of sunand wind. It also possesses some insulation qualitiesthat reduce the drop in pile temperature caused by lowambient temperatures. Despite these qualities, theporosity of the cover is such that the necessary ex-change of gases between the pile and the surroundingair is not inhibited.

Windrows are monitored regularly for temperature,carbon dioxide, oxygen, and pH. Ideally, the windrowsare monitored on a daily basis and sometimes evenmore frequently during the initial stages of the activecomposting period. The pile is generally turned whencarbon dioxide levels exceed 20 percent or whenoxygen levels drop below 5 percent. The pile is alsoturned when the temperature exceeds 160 degreesFahrenheit or if the pile begins to cool prematurely.The pile must be turned daily or more often during thefirst several days through the first week of the activecomposting period and less frequently as thecomposting process continues.

Extensive testing may be performed on the final com-post product. Tests include pH, nitrates, ammoniumnitrogen, total Kjeldahl nitrogen, phosphates, andsulfides. Other tests may be required for specializeduses of the compost. Kits are available to performbasic testing in the field. This precludes the need forlaboratory testing in many cases.

Because controlled microbial composting is a laborand equipment intensive method, it is not suitable formany farm operations. Even those farm operationsthat do use this method precisely, often use it in amodified form to better fit availability of labor andtime. For example, the pile may not be monitored orturned as often as is technically required. However,microbial inoculants are used and temperature andcarbon dioxide are monitored. This saves the expenseof purchasing oxygen and pH sensing equipment andlabor required to make these tests.

The extra time, labor, money, and equipment requiredfor this method does pay off because the final productis of very good quality, particularly in terms of textureand appearance. This method also produces thisquality product in a shorter period than a less inten-sively managed system. Good quality compost isparticularly important if it is being produced to sell. If,on the other hand, the compost is being produced withthe intent to applying it onfarm, quality and appear-ance may not be as great an issue. For compost to beused onfarm, it is often not necessary or practical touse such a labor-intensive system.


CompostingChapter 2


637.0211 Dead animalcomposting

(a) General

Composting dead animals is an excellent alternative tothe traditional methods of their disposal, particularlyburning and burying. It provides an environmentallysafe and relatively inexpensive method of convertingthe carcasses into a useful and often marketableproduct. The composting system for this purpose iseasy to implement and requires minimal labor, time,and capital investment.

Dead animal composting has mostly been directedtoward broiler mortality, but has also been used tosuccessfully manage swine, turkey, and even bovinemortality. To ensure complete decomposition of largeranimals requires a longer composting period. A way toshorten the time required is to make cuts in the larger

muscles and open the gut to increase the surface areaexposed to biological activity.

The most used method for dead animal composting isthe bin method. It can be either a single stage or two-stage method that uses primary and secondary bins.The single stage method is recommended for smalleroperations that do not have the necessary equipment,such as front-end loaders available. The two-stagemethod is recommended for those operations thathave a high mortality and the necessary equipment forthe composting operations. A third stage may benecessary for larger animals, such as turkeys andmature swine.

Material is loaded into the primary bin (fig. 2–14) inlayers (fig. 2–15). The temperature rises to highsbetween 135 and 150 degrees Fahrenheit within 2 to 4days and remains elevated for several days. Once thetemperature begins to cool from the peak temperature,usually within 7 to 10 days, the material is unloadedfrom the primary bins. The material from the primarybins is then either loaded into the secondary bins or,

Figure 2–14 Dead bird composter

Removablewall

Removable wall

Concrete pad

Manurestorage

Secondarybin

Primarybin

Secondarybin

Primarybin

Plan view

ConcretePad

Metal roof

Purlins

Elevation side view



CompostingChapter 2

for single-stage operations, placed in curing piles. Thematerial in the secondary bins reheats and cools after7 to 10 days. Curing is generally done in an enclosedstorage area or outside under a protective tarp. Thisallows new material to be loaded into the secondarybins. The finished compost can be sold or applied tothe land.

The recipe for dead animal composting should beformulated so that a C:N ratio between 13:1 and 15:1and a moisture content of 40 to 60 percent are ob-tained. Dead animal bodies have a low C:N ratio.Poultry manure, often used in the recipe for dead

animal composting, also has a low C:N ratio unless itcontains a significant amount of bedding material. Toachieve a recipe having the recommended C:N ratio, acarbon amendment must be added to the mix. Strawor sawdust is generally used.

The carbon amendment can be omitted from the mixto reduce costs. Without it, however, the mix has areduced C:N ratio with decreased aeration, and theammonia odor increases upon mixing. As such,composting without the carbon amendment results ina tradeoff of decreased expenses for increased odorsas well as the risk of incomplete composting. This

Figure 2–15 Dead bird bin composting schematic

Repeatlayer

Repeatlayer

First layeronly

Manure

Litter cover layer

Chickens

Straw

Manure

Chickens

Straw

Manure

Chickens

Straw

Manure

Concrete


CompostingChapter 2


incomplete stabilization also results in decreasedcompost quality and possibly inadequate pathogendestruction if the temperature is not maintained above130 degrees Fahrenheit for 15 to 20 days.

Dead animal composting facilities should be sitedwhere drainage and ingress and egress are good. Forbin composting, a permanent structure, such as binsconstructed of treated lumber or concrete, within apole-frame building with concrete floors is the mostdesirable. This type facility offers easier overall opera-tion and management, especially during inclementweather, and is more aesthetic (fig. 2–15). Bins canalso be constructed of bales of low-quality hay. Thistype of construction is less expensive and providesflexibility that a permanent structure would not have,such as the number of bins and their location. Balebins can also be used along with a permanent struc-ture facility to provide additional composting capacitywhen the need arises. These bins are constructed withlarge round bales (5 to 6 feet in diameter placed end-to-end to form walls for three-sided enclosures).Some states may require that composters be roofed.

Compost produced from dead poultry or other animalsis generally lower in nitrogen than broiler litter be-cause of losses through denitrification and volatiliza-tion. It is higher in P2O5 and K2O than broiler litterbecause of the reduction in volume that is typically 25to 30 percent and a mass reduction of about 15 per-cent.

(b) Dead poultry and small animalcomposting

The following guidelines for composting dead poultryare adapted from those developed by Auburn Univer-sity for the NRCS based on their study (McCaskey1993):

• Use only approved plans for construction ofcompost facilities.

• Provide 200 cubic feet of primary bin capacityper 20,000 birds on hand and an equal amount ofsecondary bin capacity. For example, a poultryproducer with a flock of 40,000 birds per broodwould need 400 cubic feet of primary bin and 400cubic feet of secondary bin capacity.

• Remove poultry mortalities daily from poultryhouses.

• Use one of the following recipes (amounts areexpressed as parts per weight basis):— With carbon amendment (peanut hulls or

chopped hay or straw)Litter ............................ 3 to 4 partsCarbon amendment ... 0.2 to 0.4 partsMortalities ................... 1 partWater ........................... 0.5 to 1 parts

— Without carbon amendmentLitter ............................ 4 to 6 partsMortalities ................... 1 partWater ........................... 0.75 to 1 parts

Compost ingredients should be added to achieveabout 30 to 40 percent moisture in the initial mixregardless of recipe used.

• Monitor compost to see that a temperaturegreater than 122 degrees F for at least 5 days asan average throughout the composting mass isachieved. This temperature and time criterioncan be achieved during either the primary orsecondary composting stages or as the cumula-tive time of greater than 122 degrees Fahrenheitin both stages.

• Leave primary compost in the bin until the tem-perature reaches its maximum and then shows asteady decline for 1 week. If the maximum tem-perature during primary composting is less than122 degrees Fahrenheit, the compost should bemixed and aerated to encourage heating. This isaccomplished by moving the compost to thesecondary bin. This step, mixing and aeration,should be repeated until the compost hasachieved at least 5 days of temperatures greaterthan 122 degrees Fahrenheit. Generally, heatingduring primary and secondary composting isadequate. When the compost has achieved atemperature greater 122 degrees Fahrenheit forat least 5 days, the composting process is ad-equate to eliminate the bacterial pathogensListeria monocytogenes, Escherichia coli

0157:H7, and Salmonella typhimurium.

• Store stabilized compost until it is convenient toland apply it or prepare it for sale to others. Usethe secondary bin for stabilized compost storageor remove the compost from the secondary binand place it in a facility where it is protectedfrom the weather. Compost to be land appliedshould be tested for N-P-K and applied at ratesappropriate for the type of crop grown.



CompostingChapter 2

(c) Dead swine and large animalcomposting

Determining the size of a dead swine compostingfacility is similar to sizing a poultry composting facil-ity. The method is given in a step-by-step fashionbelow and is illustrated in example 2–1. The best datafor herd mortality can be obtained from the swineproducer. If information from the producer is notavailable, the mortality rate and carcass design weightvalues in table 2–5 should be used.

(1) Method to determine the size of a dead

swine composting facility

Step 1. Determine the weight (lb) of dead animals peryear for each size of animal using the following equa-tions.

Baby pigs:S LPS PPL MR DW PDA× × × × =

where:S = number of sowsLPS = number of litters per sowPPL = pigs per litterMR = mortality rate (% expressed as decimal)DW = design weight (lb)PDA = weight of dead animals (lb/yr)

Sows nursery pigs, boars and finishing hogs:S MR DW PDA× × =

Step 2. Determine the average weight of dead ani-mals per day (AWDAD):

TPDAAWDAD

365 days=

Step 3. Determine the primary bin size for thecomposting facility. Primary bin size for compostingdead swine can be determined using one of two vol-ume factors (VF).

• When sawdust is used as the composting carbonsource with no added nitrogen source, VF = 20cubic feet per pound of dead animal per day(Fulhage 1992).

• When sawdust is used as composting carbonsource and poultry litter, swine manure, or othernitrogen source is added to adjust the carbon-to-nitrogen (C:N) ratio, VF = 10 cubic feet perpound of dead animal (Henry 1995).

Sample using sawdust as a carbon source with nonitrogen source added:

AWDAD x VF= TPV

where:AWDAD = average weight of dead animals per day

(lb/d)VF = volume factorTPV = total primary bin volume required

The typical height of a compost pile when in an open(hay bale) facility is 4.5 feet, thus the floor area of asingle primary bin is:

TPVCH

PBFA=

where:TPV = total primary bin volume requiredCH = height of compost pilePBFA = primary bin floor area (for a single bin)

The second and third stage bins require the samevolume as the first stage bin.

(2) Loading the first stage bin

A typical stage one composting bin is loaded using thefollowing sequences according to type of facility andthe materials used for composting.

Table 2–5 Mortality rates and design carcass weightsfor determining the size of a dead swinecomposting facility

Animal type Mortality rate (%) Design weight (lb)

Baby pigs 20 52 litters/sow/year10 piglets/litter

nursery pigs 2-3 50

sows 6 400

boars 550

finishing hogs 2 220


CompostingChapter 2


Composting with no nitrogen adjustment:

Sawdust is layered with the dead animals for compost-ing according to the recipe in table 2–6. The first layeris 1 foot of sawdust. To speed composting and preventexcess bloating, an incision should be made into theabdomen of any pig larger than 50 pounds. After eachpig is placed in the composter, it is covered with 6inches of sawdust. The sawdust is sloped so thatrunoff will be directed from the facility. When the binreaches a height of 4.5 feet, a 6-inch minimum layer ofsawdust is placed on top and sloped to shed water.

The C:N ratio of the mixture in table 2–6 is approxi-mately the carbon source with a minimum of 6 inchesof 300:1, which is the C:N ratio for sawdust (Henry,1990). This is much higher than that desired for deadanimal composting because no outside nitrogensource is used.

Composting with a nitrogen adjustment

The composting process will be more efficient if thenitrogen concentration in the mixture is adjusted.Table 2–7 shows the recipe for composting dead swinewhen poultry litter is used to adjust the carbon-to-nitrogen ratio.

The C:N ratios of two mixtures above are 15:1 forsawdust and 17:1 for straw. These ratios are lowerthan typical composting C:N ratios of 25:1. However,they correspond with values for dead animal compost-ing in the NRCS, Agricultural Waste Management FieldHandbook (NRCS, 1992).

Loading sequence when composting with litter

A typical stage one composting bin is loaded using thefollowing sequence and according to the prescribedmix in table 2–7:1. 1 foot of dry litter is placed on the floor of the binto soak up excess moisture. This is not part of therecipe in table 2–6.

2. A 6-inch layer of carbon source/bulking agent isplaced on top of the manure to aid aeration under thecarcasses.

3. A uniform layer of carcasses is added on top of thecarbon source with a minimum of 6 inches of litteradded next to the sidewalls to keep the carcassesaway from the sidewalls.

4. A minimum of 6 inches of litter is immediatelyadded to cover the top of the carcasses.

5. The second and each subsequent combination ofcarbon source, carcasses, and liner (batch) starts witha layer of carbon source, then a layer of carcasses, andthen a layer of litter added in proportion required inthe prescribed mix (table 2–5). A minimum of fourbins should be planned for proper sequencing of thecomposting process.

6. When the loading of the primary bin is completedan additional 6-inch cap of litter is added to the top ofthe compost mix. This 6 inches of litter is in additionto the litter that was added to the top of the last batch.

Table 2–6 Mix for composting dead swine with sawdust

Weight ratio Volume ratio

Sawdust 1.5 5.5

Carcasses 1.0 1.0

Water l/

1/ Water is added as needed to maintain a damp spongeconsistency.

Table 2–7 Mix for composting dead swine with broilerlitter using sawdust/straw as a carbon sourceand bulking agent

Weight ratio Volume ratio

Sawdust/straw 1.0/0 3 3.0/1.0

Litter 2.0 4.0

Water 0.7 0.5

Carcasses 1.0 1.0



CompostingChapter 2

Example 2–2 Determining the size of a dead swine composting facility

Given: A hog operation having 300 sows. Sows have 2 litters per year with 10 pigs in each litter.

Required: Determine the bin size for a dead swine composting facility using sawdust as carbon source.Additional nitrogen from another source will not be added to compost mix..

Solution:

1. Determine total weight of dead animals per year using data from table 2–5:Baby pig mortality

PDA = S x LPS x PPL x MR x DW= 300 sows x 2 litters/sow x 10 pigs/litter x 0.20 x 5 lb/pig= 6,000 lb

Sow mortalityPDA = S x MR x DW

= 300 sows x 0.06 x 400 lb/sow= 7,200 lb

TPDA = Baby PDA + Sow PDA= 6,000 lb + 7,200 lb= 13,200 lb

2. Determine the average weight of dead animals per day:AWDAD = TPDA/365

= 13,200 lb/365 days= 36.2 lb/day

3. Determine primary bin size for composting facilityTPV = AWDAD x VF

= 36.2 lb/day x 20 ft3/lb= 724 ft3

The second and third stage bins require the same volume as the primary bin. The third stagevolume could possibly be reduced by 15 percent. However, this would limit use of this bin forthe third stage only.

Using a compost pile height of 4.5 feet, the floor area of the bin would be:Floor Area = 724 ft3/ 4.5 ft =161 ft2


CompostingChapter 2


637.0212 Operationalcosts

The costs involved in the production of compost varyconsiderably depending on the material, method,equipment, and final use of the compost. These costsshould be analyzed before implementing a compostoperation to determine economic feasibility.

(a) Availability and price of rawmaterial

The main raw material for most onfarm composters ismanure or dead animals. Other onfarm material can beused as amendments, such as crop residue and spoiledstraw. Just about any waste produced onfarm thatwould have high disposal costs or that presents han-dling difficulties should be considered. Compostingthis material reduces costs and improves its handlingproperties. The advantage of obtaining most or all ofthe raw material from onfarm is that material costs aregenerally minimal.

Potential off-farm sources of raw material includeother farms, municipalities, racetracks or stables, andfood, fish, or wood processors. Preferable off-farmmaterial is available either free or with a tipping feeand is compatible with a composting operation. Mu-nicipalities often pay a tipping fee to the compostoperator for yard waste, such as grass and leaves andfor cardboard and paper. The magnitude of the tippingfees varies depending on the cost of other methods ofdisposal available to the municipalities.

The cost of wood chips and sawdust varies dependingon the supply and competition for other uses. Strawthat has limited use for other purposes can generallybe obtained at nominal prices per bale.

The cost of transportation for raw material must beconsidered in the evaluation. A material that is free forthe taking may not be cost effective if the expense ofhauling is excessive and must be paid by the compostfacility.

(b) Quantity and price of landavailable for the compostingoperation

A production cost is associated with the land occupiedby the compost operation. The value and amount ofland available influences the type of compostingmethod used. Depending on the method, 1 acre of landcan handle anywhere from 2,000 to 10,000 cubic yardsof compost per year. If land availability is not a con-straint, the method used determines the amount ofland needed.

To minimize production costs, the operation should bescaled for the most efficient utilization of equipmentand land. For example, if land is scarce or expensiveand considerable material is to be composted, a space-saving method, such as the in-vessel system, should beconsidered. Static pile methods, however, would suitfarms with adequate amounts of land and small vol-umes of compostables. Windrows are good for opera-tions with adequate land and the need to handle largequantities of compost.

(c) Estimated costs of operation/production

After establishing the basic costs, the material, tasks,and equipment that will be used in the compost opera-tion should be well in mind. Using this knowledge,production costs can be estimated to determine theeconomic feasibility of the operation. If the analysisreveals that it is not economically feasible, adjust-ments can be made before significant amounts of timeand money have been invested into the operation.

Production costs vary considerably from operation tooperation and from month to month. This depends notonly on the material, operation, and market, but alsoon other uncontrollable factors, such as costs of labor,fuel, land, and equipment purchase and maintenance.A difficult item to determine is the profit from sale ofcompost. Compost operations may fail if overoptimis-tic estimates are used in the evaluation for marketingand sales.



CompostingChapter 2

(d) Pre-startup cost

Costs associated with startup generally are one-timecosts. These costs include the value of the land to beused and the labor, time, equipment, and capital in-vestment involved in site preparation. Site preparationshould include the cost of the planning that must bedone to acquire necessary permits. Actual site prepara-tion costs include the necessary grading, surfacing,drainage, and landscaping. Site preparation also in-cludes any necessary surfacing of access roads to thecomposting site.

(e) Material handling

Material handling is the primary cost in the productionof compost. It includes both capital investment andlabor and equipment investment. The amount of capi-tal invested in material handling equipment dependson the method used and the availability of onfarmequipment to the composting operations. Equipmentneeded that is not available onfarm must be obtained.Some automated turning equipment can be quiteexpensive. The options that could be consideredinclude joint ownership by several farmers, leasing, orpurchasing used equipment.

The cost associated with turning the piles must beconsidered. This cost depends mainly on the volumeand bulk density of the material being turned and theequipment used for turning. High volumes and densematerial require more time. The cost of turning de-creases as the composting process advances becauseof the reduced volume of the material. The volumedecreases by 50 to 80 percent over the duration of thecomposting period.

The skill and experience of the operator along with thepower and size of the machinery also influence thecost. A skilled operator can turn a windrow moreeffectively and in a shorter period than an inexperi-enced operator. Specialized windrow turners are oftenfaster and provide a more thorough mixing and shred-ding of the material than a front-end loader or otheradapted farm equipment. The negative aspect to theseturners is that they require a large capital investment.If farm equipment is to be used, the increased wearand tear and subsequent maintenance costs alsocontribute to the operating costs.

(f) Monitoring

Relative costs of monitoring and testing equipmentwere described previously in the section on monitor-ing equipment (637.0203(e)).

(g) Operations after completion ofcomposting

If the compost is to be sold, it may require additionalprocessing, such as screening and bagging. Screeningis necessary particularly if the compost is to be sold inbagged form. Bagging can be accomplished by hand orby machine. Either method of bagging requires a laborinvestment and, depending on the machinery used forbagging, a minimal to substantial capital investment.Additional space and a roofed area for longer termcuring and higher quality may also be necessary.Permits, licensing, and additional testing and reportingmay also be required by regulation if the compost issold for off-farm purposes.

The primary alternative to selling the finished compostis to apply it to the land. This may also be consideredan operational cost of the compost operation.


CompostingChapter 2


637.0213 Compost end use

(a) Land application

The many different and often intangible effects thatcompost can have on soil and plant growth makes itdifficult to determine precisely what the applicationrates should be for land applied compost. The variedqualities and characteristics of the finished compostand the feedstocks used to produce the compost makeit difficult to suggest application rates. Ongoing re-search is extensive on the application of compost toagricultural and horticultural crops. This researchevaluates the effects of compost on soil nutrientcontent, soil conditioning properties, and diseasesuppression. Compost marketed on the basis of itsfertilizer content requires licensing by most Statedepartments of agriculture.

Compost serves its most important function as a soilconditioner through the addition of humus and organicmatter to the soil. Humus is the dark, carbon-rich, andrelatively stable residue that is a product of the de-composition of organic matter. The addition of humusand organic matter increases the water and nutrientholding capacity of the soil, decreases the soil bulkdensity, and improves the soil aeration and porestructure. These improvements result from the directeffects of the compost material itself and the indirecteffects brought about through the promotion of soilmicrobial activity and earthworms.

The changes in the soil brought about by the additionof compost stimulate root growth. An increased rootsystem makes a plant more drought resistant becauseit is able to obtain more water from the soil. Theincreased root system also allows the plant to increaseits nutrient uptake. Increased water and nutrientretention capacities of the soil because of increasedorganic matter provided by the compost also reducesleaching.

While the main value of compost is in its improvementof soil structure and water holding capacity, compostdoes contain many nutrients. These nutrients are notpresent in the same quantities per unit of volume asinorganic fertilizer, however, and will require higherapplication rates. The advantage of using compost as a

fertilizer is that it releases nutrients slowly, generallyunder the same warm, moist soil conditions requiredfor plant growth such that nutrient release is matchedwith plant uptake. This results in a more efficientutilization of nitrogen and a decreased potential fornitrogen leaching. The potential for leaching does stillexist when conditions are suitable for nutrient releasefrom the compost, but no plants are available to utilizethe nitrogen. This can occur, for example, in early fallafter crops have been harvested, but soil moisture andtemperature for plant growth and nutrient release arestill adequate.

A nutrient analysis of the compost makes known theamount of nutrients present in the compost. However,the analysis does not indicate the amount of nutrientsthat is immediately available to the plants or howmuch will be released in subsequent seasons. Variousstudies have found a wide range of values for theamount of nitrogen available during the first growingseason. This variation is attributed to the nitrogencontent of compost and its mineralization rates beinghighly variable. The amount of nitrogen availableduring the first growing season ranges anywhere from8 to 35 percent of the total nitrogen, depending on theraw material and method used to make the compost. Itis generally assumed that 10 to 25 percent of thenutrients are available during the first growing season.Compost produced from manure generally has ahigher nitrogen content than other composts. Of themanures, poultry manure compost has the greatestfertilizer value. While compost applied at reasonablerates may not provide sufficient nutrients to com-pletely replace commercial fertilizers, it can reducethe amount that is normally applied.

Compost has other nutritional benefits. These includeprovision of a stable supply of ammonium, an in-creased cation exchange capacity to hold nutrients inthe soil, and a buffering capacity to prevent acidicconditions that are damaging to plants.

Compost functions as a disease suppressant by in-creasing the microbial activity in the soil. The in-creased number and diversity of soil micro-organismsgive beneficial organisms a competitive edge overpathogens. Research for the use of compost as adisease suppressant has mostly been concentrated inthe area of composted bark and sphagnum peat be-cause of the need for strict quality control. This is not



CompostingChapter 2

a feature of agricultural composting operations. Inaddition, the use of compost for its disease suppres-sive quality has been emphasized in container mediaand nurseries. Chen and Hadar (1986) found thatcomposted separated manure was effective as a peatsubstitute in container media and was suppressive tosoil-borne pathogens, such as Pythium, Rizoctonia,

and Fusarium.

Compost application rates vary depending on themineralization rates and whether the compost is usedas a soil inoculate or as a primary nutrient source.Compost applied as a soil inoculate to improve soiltilth and organic matter content should be of highquality and applied at a rate of 2 to 3 tons per acre.When compost is used as a primary nutrient source,the application rate increases to more than 100 wettons per acre. At 100 tons per acre, the application rateis more than 2 inches thick. It is recommended thatapplication rates not exceed 50 dry tons per acre(4 yd3/1,000 ft2). Compost is generally spread on landat a thickness of 1 to 2 inches. If it is applied at a rategreater than this, it becomes too difficult to incorpo-rate into the soil.

A major concern when applying compost to land is thepresence of viable weed seeds in the compost. Ofcourse, weed seeds present in the compost can lead toweed problems. To destroy any weed seeds that maybe in the raw material, the compost pile must sustainthermophilic temperatures. It is also possible, how-ever, to recontaminate the finished compost pile withweed seeds. This is particularly a problem if the pile isstored outdoors. To prevent such recontamination,either place the pile under protective covering or inareas where exposure to weed seeds is minimized.

Compost generally has the best effects on plantgrowth when applied in conjunction with commercialfertilizer. When used alone, compost improves soil andhas been shown to increase crop yields and cropheight, particularly during the initial stages of growthand during times of drought. The use of an immaturecompost or one with weed seeds, pathogens, orsoluble salts can have a negative effect and promotedisease. In addition, potash and phosphorus must notbe overapplied when spreading.

(b) Marketing considerations

Marketing places additional managerial demands onthe composting operation that may outweigh thepotential revenues. However, when a waste must beutilized off-farm, marketing it as compost will be lessdifficult than marketing the raw material. The mainchallenges in marketing compost are to establish amarket and then consistently meet the quality de-mands of that market. The successful sale of compostdepends on the establishment of an adequate clientelebase and then consistently meeting their expectationsin both volume and quality.

Many retail stores sell compost produced by large,commercial operations that can produce high qualitycompost less expensively than smaller agriculturaloperations. Nurseries and landscapers are also begin-ning to branch out into composting yard waste andtrimmings. The compost market for their products ismostly other landscapers and nurseries. The marketfor agricultural compost is generally home gardeners.This market is generally local with the compost soldonsite, through local stores, or in bulk to certainbuyers. Selling the compost onsite or to selectedparties also saves on packaging, advertising, andpromotion.

The sale of compost produced on the farm provides anopportunity for another source of revenue. The advis-ability of selling compost, however, must be carefullyevaluated because the additional demands it places onthe farm operation may not result in profit when allthe costs are considered. In addition, regulations mayrequire the compost to meet certain requirements,especially if it is to be sold as a fertilizer. In somecases nitrogen may be added to the final compostproduct to increase its fertilizer value.


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637.0213 References

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Barton, Lionel T., and Raymond C. Benz. Compostingpoultry carcasses. In MP317, Cooperative Exten-sion Service, University of Kansas.

Bell, R.G. 1973. High-rate composting of municipalrefuse and poultry manure. Canadian Agricul-tural Engineering 15(1).

BioCycle: Journal of Waste Recycling. January 1994, p.51 (formerly Compost Science and CompostScience/Land Utilization). Published monthly byThe JG Press, Inc.

Brinton, Jr., William F. 1988. Compost demonstration.Time and Tide RC&D.

Brinton, Jr., William F. Agricultural and horticulturalapplications of compost.

Brinton, Jr., William F., and Mary W. Droffner. 1994.Microbial approaches to characterization ofcomposting process. Compost Science andUtilization 2(3):12-17.

Brinton, Jr., William F., and Milton D. Seekins. 1988.Composting fish by-products: a feasiblity study.Woods End Research Laboratory, Maine.

Brinton, Jr., William F., and Milton D. Seekins. 1994.Evaluation of farm plot conditions and effects offish scrap compost on yield and mineral compo-sition of field grown maize. Compost Science andUtilization 2(1):10-17.

Brinton, Jr., William F., Eric Evans, and Jonathan W.Q.Collinson. 1993. On-farm composting: guidelinesfor use of dairy and poultry manures incomposting formulations. Woods End ResearchLaboratory, Maine.

Brodie, Herbert L. 1993. Multiple product compostrecipes. American Society of Agricultural Engi-neers Paper, St. Joseph, MI.

Brodie, Herbert L., and Lewis E. Carr. 1991. Low inputcomposting of crab waste. American Society ofAgricultural Engineers Paper 916004, St. Joseph,MI.

Buchanan, M., and S. Gleissman. 1991. How compostfertilization affects soil N and crop yield.Biocycle, December:72-77.

Chen, Y., and Y. Arnimelech. 1986. Supply of nutrientsby organic additives. In The Role of OrganicMatter in Modern Agriculture, Martinus NihjoffPublishers, Dordrecht.

Chen, Y., and Y. Hadar. 1986. Composting and use ofagricultural wastes in container media. In Com-post: Production, Quality, and Use, M.deBertoldi, M.P. Ferranti, P. L'Hermite, and F.Zucconi, eds., Elsevier Applied Science, NY.

Compost Science and Utilization. Published by The JGPress, Inc.

deBertoldi, M., M.P. Ferranti, P. L'Hermite, and F.Zucconi, eds. 1986. Compost: production, quality,and use. Elsevier Applied Science, NY.

Donald, James O., and John P. Blake. 1992. Deadpoultry composter construction. In Poultry By-Product Management, Auburn University,Alabama Cooperative Extension Service.

Donald, James O., John P. Blake, Kevan Tucker, andDavid Harkins. 1994. Mini-composters in poultryproduction. Alabama Cooperative ExtensionService, Auburn University.

Emerton, B.L., C.R. Mote, H.H. Dowlen, J.S. Allison,and W.L. Sanders. 1988. Comparison of forcedand naturally aerated composting of dairy ma-nure solids. American Society of AgriculturalEngineers Paper, St. Joseph, MI.

Fabian, Eileen A., Tom L. Richard, David Kay, DavidAllee, and Joe Regenstein. 1992. Agriculturalcomposting: a feasibility study for New Yorkfarms. Cornell University, Ithaca, NY.

Fulhage, C. 1992. Composting Dead Swine. WQ 225.University of Missouri Extension.



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Gillett, James W. 1992. Issues in risk assessment ofcompost from municipal solid waste: occupa-tional health and safety, public health, andenvironmental concerns. Biomass and Bioenergy3 (3-4):145-162.

Goldstein, Jerome, ed. 1991. The BioCycle guide to theart and science of composting. The JG Press,Inc., Emmaus.

Golueke, Clarence G. 1991. Inoculums and enzymes.In The BioCycle Guide to the Art and Science ofComposting. Jerome Goldstein, ed., The JGPress, Inc., Emmaus.

Golueke, Clarence G. 1991. Principles of composting.In The BioCycle Guide to the Art and Science ofComposting, Jerome Goldstein, ed., The JGPress, Inc., Emmaus, pp. 13-40.

Gonzalez, J.J., M. Medina, and I.C. Benitez. 1989. Slurrycomposting options. BioCycle, July.

Goodfellow, M., M. Mordarski, and S.T. Williams, eds.1984. The biology of the actinomycetes. Aca-demic Press, Inc., London.

Grobe, K., M. Buchanan. 1993. Agricultural markets foryard waste compost. Biocycle, September.

Hammouda, G.H.H., and W.A. Adams. 1986. The de-composition, humification, and fate of nitrogenduring the composting of some plant residues. InCompost: Production, Quality, and Use, eds. M.deBertoldi, M.P. Ferranti, P. L'Hermite, and F.Zucconi, Elsevier Applied Science, NY.

Hansen, Robert C., Christoph Marugg, Harold M.Keener, Warren A. Dick, and H.A. Hoitink. 1991.Nitrogen transformations during poultry manurecomposting. American Society of AgriculturalEngineers Paper 914014, St. Joseph, MI.

Hansen, Robert C., Harold M. Keener, and H.A.Hoitink. 1988. Poultry manure composting:system design. American Society of AgriculturalEngineers Paper No. 88-4049. St. Joseph, MI.

Hansen, Robert C., Harold M. Keener, Warren A. Dick,Christoph Marugg, and H.A. Hoitink. 1990. Poul-try manure composting ammonia capture andaeration control. American Society of Agricul-tural Engineers Paper 904062, St. Joseph, MI.

Haug, Roger T. 1986. Composting process designcriteria. BioCycle 27:53-57.

Haug, Roger T. 1990. An essay on the elements of odormanagement. Biocycle October:60-67.

Haug, Roger T., and W.F. Ellsworth. 1991. Measuringcompost substrate degradability. BioCycle 32:56-62.

Henry, S. T. 1990. Composting broiler litter—Effects oftwo management systems. Clemson Univ., MSThesis.

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CompostingChapter 2

(Adapted from the On-Farm Composting Handbook, NRAES-54, Northeast Regional Agricultural Engineering

Service, 152 Riley-Robb Hall, Cooperative Extension, Ithaca, NY 14853-5701)

Actinomycete A group of micro-organisms, intermediate between bacteria and true fungi,that generally produce a characteristic branched mycelium. These organ-isms are responsible for the earthy smell of compost.

Aerated static pile Forced aeration method of composting in which a freestanding compostingpile is aerated by a blower moving air through perforated pipes locatedbeneath the pile.

Aeration The process by which the oxygen-deficient air in compost is replaced by airfrom the atmosphere. Aeration can be enhanced by turning.

Aerobic An adjective describing an organism or process that requires oxygen (forexample, an aerobic organism).

Agitated-bed An in-vessel composting method in which the material is contained in a binor reactor and is periodically agitated by a turning machine or by augers.Some means of forced aeration is generally provided.

Agricultural waste Waste normally associated with the production and processing of food andfiber on farms, feedlots, ranches, ranges, and forests. May include animalmanure, crop residue, and dead animals. Also agricultural chemicals,fertilizers, and pesticides that may find their way into surface and subsur-face water.

Ambient air temperature The temperature of the air near the compost pile.

Amendment See Composting amendment and Soil amendment.

Ammonia (NH3) A gaseous compound of nitrogen and hydrogen. Ammonia, which has a

pungent odor, is commonly formed from organic nitrogen compoundsduring composting.

Ammonium (NH4+) An ion of nitrogen and hydrogen. Ammonium is readily converted to and

from ammonia depending on conditions in the compost pile.

Anaerobic An adjective describing an organism or process that does not require air orfree oxygen.

Anion An atom or molecule with a negative charge (for example, nitrate, NO3).

Aspergillus fumigatus Species of fungus with spores that cause allergic reactions in some indi-viduals. It can also cause complications for people with certain existinghealth problems.

Availability, nutrient See Nutrient availability.

Glossary


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Bacteria A group of micro-organisms having single-celled or noncellular bodies.Bacteria generally appear as spheroid, rod-like, or curved entities, butoccasionally appear as sheets, chains, or branched filaments.

Bedding Dry absorbent material used to provide a dry lying surface for livestock.Bedding material, such as sawdust and straw, absorb moisture from live-stock waste, the soil, and the environment.

Bin composting A composting technique in which mixtures of material are composted insimple structures (bins) rather than freestanding piles. Bins are considereda form of in-vessel composting, but they generally are not totally enclosed.Many composting bins include a means of forced aeration.

Biochemical oxygen demand The quantity of oxygen used in the biochemical oxidation of organic matter (BOD) in a specified time, at a specified temperature, and under specified condi-

tions; normally 5 days at 20 degrees Celsius unless otherwise stated. Astandard test used in assessing the biodegradable organic matter in munici-pal wastewater. See also Chemical oxygen demand.

BOD See Biochemical oxygen demand.

Bucket loader A vehicle that employs a hydraulically operated bucket to lift material.Includes farm tractors with bucket attachments, skid loaders, and largefront-end loaders.

Bulk density Weight or mass per unit of volume of a material made up of many indi-vidual particles. For example, the weight of a pile of wood chips divided bythe volume of the pile is the bulk density. This is different from the particledensity, which in this case equals the weight of a single wood chip dividedby its volume). See also Density.

Bulking agent An ingredient in a mixture of composting raw material included to improvethe structure and porosity of the mix. A bulking agent is generally rigid anddry and often has large particles (for example, straw). The terms bulking

agent and amendment are commonly used interchangeably. See alsoComposting amendment.

C Chemical symbol for carbon.

Carbon dioxide (CO2) An inorganic gaseous compound of carbon and oxygen. Carbon dioxide is

produced by the oxidation of organic carbon compounds duringcomposting.

Carbon-to-nitrogen ratio The ratio of the weight of organic carbon (C) to that of total nitrogen (N) in(C:N ratio) an organic material.

Cation A atom or molecule that has a positive charge (for example, ammonium,NH4

+).

Cellulose A long chain of tightly bound sugar molecules that constitutes the chiefpart of the cell walls of plants.



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Chemical oxygen demand (COD) A measure of the oxygen-consuming capacity of inorganic and organicmatter present in water or wastewater. It is expressed as the amount ofoxygen consumed from a chemical oxidant in a specified test. It does notdifferentiate between stable and unstable organic matter and thus does notnecessarily correlate with biochemical oxygen demand. See also Biochemi-cal oxygen demand.

CO2

Chemical symbol for carbon dioxide.

COD See Chemical oxygen demand.

Compost A group of organic residue or a mixture of organic residue and soil that hasbeen piled, moistened, and allowed to undergo aerobic biological decom-position.

Composting Biological degradation of organic matter under aerobic conditions to arelatively stable humus-like material called compost.

Composting amendment An ingredient in a mixture of composting raw material included to improvethe overall characteristics of the mix. Amendments often add carbon,dryness, or porosity to the mix.

Compost stability See Stability of compost.

Contamination Any introduction into the environment (water, air, or soil) of micro-organ-isms, chemicals, wastes, or wastewater in a concentration that makes theenvironment unfit for its intended use.

Cubic yard A unit of measure equivalent to 27 cubic feet or 22 bushels. A box that is 1yard wide, 1 yard long, and 1 yard high and has a volume of 1 cubic yard. Acubic yard is often loosely referred to as a yard (for example, a one-yardbucket).

Curing Final stage of composting in which stabilization of the compost continues,but the rate of decomposition has slowed to a point where turning orforced aeration is no longer necessary. Curing generally occurs at lower,mesophilic temperatures.

Damping off disease The wilting and early death of young seedlings caused by a variety ofpathogens.

Decomposers The micro-organisms and invertebrates that cause the normal degradationof natural organic materials.

Degradability Term describing the ease and extent that a substance is decomposed by thecomposting process. Material that breaks down quickly and/or completelyduring the timeframe of composting is highly degradable. Material that resistsbiological decomposition is poorly degradable or even nondegradable.

Denitrification An anaerobic biological process that converts nitrogen compounds tonitrogen gas or nitrous oxide.


CompostingChapter 2


Density The weight or mass of a substance per unit of volume. See also Bulk den-sity.

Endotoxin Metabolic products of gram-negative bacteria that are part of the cell walland will remain in the bacteria after it has died.

Enzymes Any of numerous complex proteins produced by living cells to catalyzespecific biochemical reactions.

Evaporative cooling The cooling that occurs when heat from the air or compost pile material isused to evaporate water.

Forced aeration Means of supplying air to a composting pile or vessel that relies on blowersto move air through the composting material.

Fungus (plural fungi) A group of simple plants that lack a photosynthetic pigment. The individualcells have a nucleus surrounded by a membrane, and they may be linkedtogether in long filaments called hyphae. The individual hyphae can growtogether to form a visible body.

Gram-negative bacteria Bacteria that test negative to the Gram stain procedure. The Gram stainprocedure was originally developed by the Danish physician Hans ChristianGram to differentiate pneumococci for Klesbsiella pneumonia. The proce-dure involves the application of a solution of iodine to cells previouslystained with crystal violet or gentian violet. This procedure producespurple iodine-dye complexes in the cytoplasm of bacteria. The cells thatare previously stained with crystal violet and iodine are next treated with adecolorizing agent. The difference between Gram-positive and Gram-negative bacteria is in the permeability of the cell wall to these purpleiodine-dye complexes when treated with the decolorizing solvent. WhileGram-positive bacteria retain purple iodine-dye complexes after treatmentwith the decolorizing agent, Gram-negative bacteria do not.

Grinding Operation that reduces the particle size of material. Grinding implies thatparticles are broken apart largely by smashing and crushing rather thantearing or slicing. See also Shredding.

Humus The dark or black carbon-rich, relatively stable residue resulting from thedecomposition of organic matter.

Hydrogen sulfide (H2S) A gas with the characteristic odor of rotten eggs, produced by anaerobic

decomposition.

Immobilization, nitrogen Conversion of nutrient compounds from an inorganic form, available toplants, into the organic tissue of micro-organisms (or other plants). Thenutrients are unavailable until the micro-organisms die and the microbialtissues containing the nutrients decompose. Nitrogen immobilizationoccurs when material with a high C:N ratio is land applied. The micro-organisms that use the carbon also assimilate the available nitrogen, ren-dering it unavailable to plants.



CompostingChapter 2

Inoculum (plural inocula) Living organisms or material containing living organisms (bacteria or othermicro-organisms) that are added to initiate or accelerate a biological pro-cess (for example, biological seeding).

In-vessel composting A diverse group of composting methods in which composting material iscontained in a building, reactor, or vessel.

K Chemical symbol for potassium.

Land application Application of manure, sewage sludge, municipal wastewater, and indus-trial waste to land either for ultimate disposal or for reuse of the nutrientsand organic matter for their fertilizer value.

Leachate The liquid that results when water comes in contact with a solid and ex-tracts material, either dissolved or suspended, from the solid.

Lignin A substance that, together with cellulose, forms the woody cell walls ofplants and the cementing material between them. Lignin is resistant todecomposition.

Liquid manure (thin slurry) Manure that has had sufficient water added so that it can be pumped easily.Normally fibrous material, such as chopped straw or waste hay, is notpresent. See also Manure.

Litter, poultry Dry absorbent bedding material, such as straw, sawdust, and wood shav-ings, that is spread on the floor of poultry barns to absorb and conditionmanure. Sometimes the manure-litter combination from the barn is alsoreferred to as litter.

Manure The fecal and urinary excretion of livestock and poultry. Sometimes re-ferred to as livestock waste. This material may also contain bedding,spilled feed, water, or soil. It may also include waste not associated withlivestock excrete, such as milking center wastewater, contaminated milk,hair, feathers, or other debris. See also Liquid manure, Semisolid manure,Slurry manure, and Solid manure.

Manure storage A storage unit to keep manure contained for some period before its ulti-mate utilization or disposal. Manure storage is generally classified by typeand form of manure stored and/or construction of the storage; for example,above- or below-ground liquid manure tank, earthen storage basin, or solidmanure storage. See also Manure.

mho See mmho.

Microbe See Micro-organism.

Micro-organism An organism requiring magnification for observation.


CompostingChapter 2


mmho (plural mmhos) A millimho. One thousandth of a mho (pronounced mo with a long O). Amho is a unit of measurement for electrical conductivity that is the basisfor measuring soluble salt concentration. (mho is the backward spelling ofohm, the unit of measurement for electrical resistance.)

Moisture content The fraction or percentage of a substance made up of water. Moisturecontent equals the weight of the water part divided by the total weight(water plus dry matter part). Moisture content is sometimes reported on adry basis. Dry-basis moisture content equals the weight of the water di-vided by the weight of the dry matter.

Mulch A material spread over the soil surface to conserve moisture and porosityin the soil underneath and to suppress weed growth. Grass clippings,compost, wood chips, bark, sawdust, and straw are common mulch mate-rial.

N Chemical symbol for nitrogen

Nitrate-nitrogen A negatively charged ion made up of nitrogen and oxygen (NO3–). Nitrate is

a water soluble and mobile form of nitrogen. Because of its negativecharge, it is not strongly held by soil particles (also negative) and can beleached away.

Nitrification The biochemical oxidation of ammonia nitrogen to nitrate.

Nutrient-holding capacity The ability to absorb and retain nutrients so they are available to the rootsof plants.

Organic matter Chemical substances of animal or vegetable origin, consisting of hydrocar-bons and their derivatives.

P Chemical symbol for phosphorus.

Passive aeration Air movement through composting windrows and piles that occurs bynatural forces including convection, diffusion, wind, and the tendency ofwarm air to rise (thermal buoyancy).

Passive composting Method of composting in which there is little management and manipula-tion of the materials after they are mixed and piled. Turning occurs infre-quently (for example, monthly). Forced aeration is not provided.

Passively aerated windrow A composting method in which windrows are constructed over a series ofcomposting perforated plastic pipes that serve as air ducts for passive aeration. Wind-

rows are not turned.

Pathogen Any organism capable of producing disease or infection. Often found inwaste material, most pathogens are killed by the high temperatures of thecomposting process.



CompostingChapter 2

Peat Unconsolidated soil material consisting largely of organic matter accumu-lated under conditions of excessive moisture. The organic matter is notdecomposed or is only slightly decomposed.

pH A measure of the concentration of hydrogen ions in a solution. pH is ex-pressed as a negative exponent. Thus, something that has a pH of 8 has 10times fewer hydrogen ions than does something with a pH of 7. The lowerthe pH, the more hydrogen ions present and the more acidic the material is.The higher the pH, the fewer hydrogen ions present and the more basic itis. A pH of 7 is considered neutral.

Phytotoxic An adjective describing a substance that has a toxic effect on plants. Imma-ture or anaerobic compost may contain acids or alcohols that can harmseedlings or sensitive plants.

Pollution The presence in a body of water (or soil or air) of a substance (pollutant) insuch quantities that it impairs the body's usefulness or renders it offensiveto the senses of sight, taste, or smell. In general, a public-health hazard maybe created, but in some instances only economic or aesthetics is involved,as when foul odors pollute the air

Porosity A measure of the pore space of a material or pile of material. Porosity isequal to the volume of the pores divided by the total volume. Incomposting, the term porosity is sometimes used loosely, referring to thevolume of the pores occupied by air only (without including the pore spaceoccupied by water).

Poultry litter See Litter, poultry.

Primary raw material (PRM) See Primary substrate.

Primary substrate The main waste material that requires treatment. Also called primary rawmaterial.

Pythium A fungal plant pathogen that causes seed, seedling, and root rots on manyplants. These fungi are most active under conditions of high moisture.

Recipe The ingredients and proportions used in blending together several rawmaterials for composting.

Redox potential Oxidation and reduction (Redox) reactions are defined as reactions inwhich electrons are transferred. The species receiving electrons is reducedand the donating electron is oxidized. Redox reactions determine themobility of many inorganic compounds as well as biologically importantmaterial such as nitrogen and sulfur. In addition, redox conditions governthe particulars for the biological degradation of complex carbonaceousmaterial. Redox potential is an intensity parameter of overall redox reac-tion potential in the system similar to the concept of pH. Redox is not thecapacity of the system for specific oxidation or reduction reactions.”

Root rot A disease of plants characterized by discoloration and decay of the roots.


CompostingChapter 2


Saturated paste A laboratory technique in which solid particles are rendered into a paste sothat characteristics, such as pH and soluble salt concentration, can bemeasured.

Semi-solid manure Manure that has had some bedding added or has received sufficient airdrying to raise the solids content such that it will stack, but has a lowerprofile than solid manure and seepage may collect around the outside. Itmay be pumped with positive displacement jumps or handled with a front-end loader. See also Manure.

Sewage sludge Solid part of waste from sewage treatment plants. Contains human waste.

Shredding An operation that reduces the particle size of material. Shredding impliesthat the particles are broken apart by tearing and slicing. See also Grinding.

Slurry manure Slurry manure has a near liquid consistency. It can be handled with conven-tional, centrifugal manure pumps and equipment, but the solids contentmay be too high for irrigation equipment. See also Manure.

Soil conditioner A soil additive that stabilizes the soil, improves its resistance to erosion,increases its permeability to air and water, improves its texture and theresistance of its surface to crusting, makes it easier to cultivate, or other-wise improves its quality.

Soil structure The combination or arrangement of primary soil particles into secondaryparticles, units, or peas. Compost helps bind primary soil particles toimprove the structure of soil.

Solid manure Manure that has had sufficient bedding or soil added or has received suffi-cient air drying to raise the solids content to where it will stack with littleor no seepage. It is best handled with a front-end loader. See also Manure.

Stability of compost The rate of change or decomposition of compost. Generally, stability refersto the lack of change or resistance to change. A stable compost continuesto decompose slowly and has a low oxygen demand.

Stoichiometric oxygen The oxygen proportioned in the exact right amount needed for, in thecontext of composting, biological degradation of organic matter underaerobic conditions.

Structure of composting mix The ability to resist settling and compaction. Structure is improved by or raw material large, rigid particles.

Texture of composting mix Characteristic that describes the available surface area of particles. A fine or raw material texture implies many small particles with a large combined surface area. A

course texture implies large particles with less overall surface area.

Thermophilic Heat-loving micro-organisms that thrive in and generate temperaturesabove 10 degrees Fahrenheit (40 °C).

Tipping fees Fees charged for treating, handling, and/or disposing of waste material.



CompostingChapter 2

Turning A composting operation that mixes and agitates material in a windrow pileor vessel. Its main aeration effect is to increase the porosity of the windrowto enhance passive aeration. It can be accomplished with bucket loaders orspecially designed turning machines.

Vermin Noxious or objectionable animals, insects, or other pests, especially thosethat are small; for example, rats, mice, and flies.

Volatile compound A compound or substance that vaporizes (evaporates) at relatively lowtemperatures or is readily converted into a gaseous by-product. Examplesinclude alcohols and ammonia. Volatile compounds are easily lost from theenvironment of a composting pile.

Windrow A long, relatively narrow, and low pile. Windrows have a large, exposedsurface area that encourages passive aeration and drying.

Yard waste Leaves, grass clippings, yard trimmings, and other organic garden debris.


CompostingChapter 2


2A–1(210-VI-NEH, February 2000)


CompostingChapter 2

(Adapted from the On-Farm Composting Handbook,

NRAES-54, Northeast Regional Agricultural Engi-

neering Service Cooperative Extension.)

The list of materials appropriate for composting isalmost endless. Only materials commonly available tofarmers are described here and summarized as fol-lows:

Bark Paper mill sludgeCardboard Peat mossCattle manure Poultry manureCrop residue Sawdust & shavingsFertilizer & urea Seaweed & otherFinished compost aquatic plantsFish processing wastes Septage & sewer sludgeFood processing wastes Slaughterhouse &Fruit & vegetable wastes meatpacking wasteGrass clippings Spoiled hay & silageHorse manure StrawLeaves Swine manureLime Wood ashLivestock manure Wood chipsNewspaper

These materials are described here and are listed intable 2A–1 along with their characteristics (percentnitrogen, C:N ratio, moisture content, and bulk den-sity).

Other materials abundant on the farm or availablelocally may be good components of a composting mix.Trucking raw materials beyond 50 miles is usuallycost-prohibitive, so farmers should seek out localsources of clean organic materials. They should beevaluated in the same manner as the materials de-scribed below.

Cattle manure

Nitrogen-rich and very wet. Moisture content and C:Nratio depend on the amount of bedding used, manage-ment practices, type of operation, and climate. Gener-ally requires a large amount of dry, high-carbonamendment (often two to three volumes of amend-ment per volume of manure). Relatively low odor riskif composted within a few weeks. Decomposesquickly. Bedded pack manure is moderately dry with agood C:N ratio. Liquid manure or slurries must bescreened or dried unless only small amounts are usedin the composting mix. Some trash may be present.Overall, a very good composting material.

Poultry manure

Very high nitrogen content and moderately moist.Needs a high carbon amendment. Litter with sawdustor wood shavings is well suited to composting andmay be partly composted when removed from thebarn. Nitrogen loss and odor from ammonia is a poten-tial problem because of the high nitrogen content andhigh pH. Low pH amendments may be needed to lowerthe alkalinity. Decomposes quickly. The high nitrogencontent can result in a fertilizer-grade compost. Goodto very good composting material.

Horse manure

Usually contains large amounts of bedding; therefore,dry with a high C:N ratio. Composts well alone or asan amendment for wet cattle manure. Low odor poten-tial. Decomposes quickly, especially if bedding isstraw. Often available at little or no cost from localstables, racetracks, pleasure horse owners, fairs, andschools. Some stable wastes contain medicationcontainers, soda cans, and other trash. Excellentcomposting material.

Swine manure

Nitrogen-rich and very wet. Needs a dry, high-carbonamendment. Strong potential for odors. High moisturecontent and odor make composting more difficult thanother manure. With bedding, solids separation, and/orodor-control measures, it can be a fair to goodcomposting material.

Other livestock manure

Sheep, goat, rabbit, and other livestock manure areusually good for composting. It is collected mostlyfrom bedded manure packs and is, therefore, relativelydry with a high C:N ratio. Without bedding, the manureis nitrogen-rich and wet. Bedded material may be usedas an amendment to other livestock manure. Relativelylow odor potential. Decomposes quickly. Goodcomposting material.

Crop residue

Variable characteristics depending upon the material,but generally moderate to high moisture and moderateC:N ratio. The C:N ratio and moisture content dependon the age and the amount of fruit and seeds present.Generally, older vegetation is drier and contains lessnitrogen. Usually very good structure and gooddegradability. Some residue may be dry and high incarbon (cornstalks). Plant pathogens are a concern if

Appendix 2A Common Raw Materials for FarmComposting


CompostingChapter 2

2A–2 (210-VI-NEH, February 2000)

compost does not reach a high temperature in all partsof the pile. Excellent to good composting amend-ments, depending on the material.

Spoiled hay and silage

Moderately dry to wet, depending on conditions.Moderate to high C:N ratio. In most cases, availableonly occasionally. Added to compost mix as a disposalmethod and not as a reliable amendment. Good struc-ture and degradability. Possible problems include odorand leachate from silage and weed seeds in hay. Mod-erate composting material.

Straw

Dry and carbonaceous. Good degradability. Providesvery good structure and odor absorption. If used asbedding, it can precondition manure for composting.Availability and cost can be disadvantages. Excellentcomposting amendment.

Sawdust and shavings

Dry and carbonaceous. Moderate to poor degrad-ability; sawdust degrades faster than shavings. Goodmoisture and odor absorption. Can also have a dualuse as bedding. Usually available at a moderate to lowcost. Good to moderate composting amendment.

Leaves

Relatively dry. High in carbon. Good degradability ifshredded. Moderate moisture absorption. Low odorpotential. Composts alone or as an amendment. Oftencontains trash, rocks, plastic bags, and so on—espe-cially if collected from streets. Large quantities avail-able, but seasonal supply requires storage and/orspecial handling and scheduling. Leaves can be ob-tained free, or a tipping fee may be available. Good tomoderate composting material.

Wood chips

Dry and high in carbon. Large particle size providesexcellent structure, but poor degradability. Often usedas a bulking agent for forced aeration composting.Must be screened from final compost, but can bereused. Moderate to low cost. Has a competing use asa mulch product. Chips from preservative-treated andpainted wood should not be used. Very good bulkingagent, but poor amendment otherwise.

Bark

Qualities are similar to that of wood chips except for agiven tree species, bark contains slightly more nitro-gen and easily degradable compounds. May becomposted alone for use in potting media or formulch. Good bulking agent, but poor as a generalamendment. Good material for specialty compostproducts (mulch, potting media) though thecomposting time is relatively long.

Grass clippings

Moderately wet to dry. Slightly low C:N ratio. Decom-pose quickly. Moderate to high odor potential depend-ing upon management. Good source of nitrogen forleaf and yard waste mixtures. Usually available free, ora tipping fee may be available. Good compostingmaterial if mixed with coarse material. Alone, grassclippings tend to compact and become anaerobic.

Newspaper

Dry. High carbon content. Moderate degradability.Potential for dual use as bedding. Good moistureabsorption, but poor structure and porosity. Blackinks are generally nontoxic. Large quantities of col-ored inks and glossy paper are best avoided or shouldbe analyzed because of possible heavy metals andother contaminants. Available in large quantities atlittle or no cost, or a tipping fee may be available. Mayneed shredding and some sorting initially. Possibleproblems include storage, dust, and trash around thefarmstead. In general, a good to moderate amendmentdepending upon the structure of the mix.

Cardboard

Dry and high carbon content. Good degradability.Good moisture absorption and structure. Large quanti-ties available for little or no cost, or a tipping fee maybe available. Shredding, storage, and some sorting maybe needed. Staples in cardboard boxes may need to beremoved. Glues in corrugated cardboard may containhigh boron levels. Good to fair amendment.

Finished compost

Compost can be recycled as an amendment for wetwastes, either alone or in combination with otheramendments. Moderately dry. Moderate to low C:Nratio. Provides a good initial supply of micro-organ-isms. Frequent recycling may potentially lead to highsalt concentrations, but otherwise no significant



CompostingChapter 2

disadvantages. Loss of compost product after recy-cling is small. Good amendment, especially for lower-ing the mix moisture content without raising the C:Nratio.

Peat moss

Acidic fibrous material that has resulted from years ofanaerobic decomposition. Low in nitrogen. Highlyabsorbent of water, nutrients, and odors. May holdover 10 times its weight in water. Except in regionswhere natural deposits exist, peat moss is expensivepartly because of its competing uses as an amendmentfor potted plants and other horticultural crops. Peatmoss passes through the composting process virtuallyunchanged, producing potentially high valued com-post. Its odor and water-absorbing qualities make it anexcellent amendment, but cost limits its use.

Fruit and vegetable waste

Peels, tops, trimmings, culls, damaged or spoiled fruit.Moderate to wet with a moderate to low C:N ratiodepending upon the nature of the waste. Except forpits, good degradability. Poor to fair structure. Stand-ing piles of many fruits and some vegetable wastequickly collapse into a wet mess once decompositionbegins. The potential for tipping fees exists. Slight tomoderate risk of odor problems. Possible trash frompacking operations and markets. Good to faircomposting material.

Food processing waste

Variable characteristics depending upon the process.Filter press cakes generally are moderately dry andhave high to moderate carbon content. Other foodprocessing by-products are generally wet with moder-ate to low C:N ratios. Possible problems include highrisk of odors, vermin (rats, mice, flies), contaminantsfrom machinery and cleaning solutions used at theprocessing plant, and poorly degradable components,such as pressing aids. A major advantage is the oppor-tunity to receive a tipping fee. Good to poorcomposting material depending upon the nature of thewaste.

Slaughterhouse and meatpacking waste

Paunch manure, blood, miscellaneous parts. Wet andlow C:N ratio. Good degradability. High risk of odorsand vermin. More restrictive regulations may apply.Large amounts of amendment are required to lowermoisture content and control odors. Except forpaunch manure, composting should be considered

only if direct land application and other options arenot practical.

Fish processing waste

Racks, frames, heads, tails, shells, gurry. Variablecharacteristics depending on waste, but generallymoderately to very wet and high in nitrogen. Lobster,crab, shrimp, and mollusk shells provide good struc-ture. All but mollusk shells decompose quickly. Thehigh risk of odor along with the high moisture requirelarge amounts of dry amendment and/or special han-dling. More restrictive regulations may apply. Potentialfor tipping fee. Wet material—racks or gurry—aretroublesome, and composting should be consideredafter other options. Shells are moderate to goodcomposting material if managed properly.

Seaweed and other aquatic plants

Water hyacinth, pond cleanings, wastewater treatmentspecies. High to moderate moisture content, depend-ing on previous drying. C:N ratios vary from low(seaweeds) to moderate (water hyacinth). Gooddegradability. Generally poor structure, especially forseaweeds. Good sources of minor nutrients, but saltcontent of seaweed is a possible problem if used inlarge quantities. Possible trash and weed seeds in-cluded with beach cleanings. Low to moderate odorrisk. Good composting material with added structure.

Paper mill sludge

Wet or moderately wet if pressed. Moderate to highC:N ratio. Requires a dry amendment with nitrogen—adifficult combination. Good degradability, but poorstructure. Slight to moderate risk of odor if misman-aged. Organic contaminants are occasionally found inpaper sludge. Potential for tipping fee. Faircomposting material.

Wood ash

Very dry with little or no carbon and nitrogen. Con-tains a fair amount of other nutrients, particularlypotassium. The concentrations of heavy metals may bea concern with some ashes. In a composting mix,wood ash would absorb moisture and raise the pH ofthe mix. It has also been proposed as an odor-adsorbing agent. Handling is difficult as the ash is afine powder that blows around and creates dust.Particles tend to cement together after they becomewet. Tipping fees may be available. Fair to goodcomposting amendment for wet acidic mixes. Shouldnot be used if the pH is high.


CompostingChapter 2


Septage and sewage sludge

Raw and digested. Nitrogen-rich and very wet. Re-quires two to four volumes of dry amendment pervolume of sludge. Septage and raw sludge decomposequickly, digested sludge moderately. Strong odorpotential for septage and raw sludge, strong to moder-ate for digested. Possible contamination from humanpathogens and heavy metals. Special regulations applyfor pathogen reduction. Restrictions on land use applyfor heavy metals. Composting this material usuallyinvolves operational and land application permits,process monitoring, and product analysis. The oneadvantage is the opportunity to collect a fee forcomposting this material. In general, sewage sludgeand septage bring many restrictions and regulations.Though exceptions exist, it is best to avoid this mate-rial for farm composting operations.

Fertilizer and urea

Fertilizers, urea, or other concentrated nitrogensources are sometimes considered as additives tolower the C:N ratio of high carbon material, such asleaves. Although such material does reduce the initialC:N ratio, the benefits are short-lived. Nitrogen fromsuch sources tends to be available much more quicklythan the carbon in the organic material. Initially, theavailable carbon and nitrogen are in balance; but asthe easily available carbon is depleted, a surplus ofnitrogen soon develops. Eventually, the excess nitro-gen is lost as ammonia.

Lime

Like fertilizers, lime is also considered as an additive,either to adjust pH or to control odors. Generally, limeis an unnecessary ingredient and can be detrimental.pH adjustment is rarely necessary in composting. Iflime is used for odor control, it can raise the pHenough to cause an excessive loss of ammonia. Thesame effects should be expected for other concen-trated sources of alkalinity including cement kiln dustand wood ash.



CompostingChapter 2

Table 2A–1 Typical characteristics of selected raw materials

Material Type of value % N C:N ratio Moisture content Bulk density(dry weight) (weight to weight) % (wet weight) (lb/yd3)

Crop residue and fruit/vegetable-processing waste

Apple filter cake Typical 1.2 13 60 1,197Apple pomace Typical 1.1 48 88 1,559Apple-processing sludge Typical 2.8 7 59 1,411Cocoa shells Typical 2.3 22 8 798Coffee grounds Typical -- 20 -- --Corn cobs Range 0.4 – 0.8 56 – 123 9 – 18 --

Average 0.6 98 15 557Corn stalks Typical 0.6 – 0.8 60 – 73a 12 32Cottonseed meal Typical 7.7 7 --- ---Cranberry filter cake Typical 2.8 31 50 1,021(with rice hulls) Typical 1.2 42 71 1,298Cranberry plant (stems, leaves) Typical 0.9 61 61 ---Cull potatoes Typical -- 18 78 1,540Fruit wastes Range 0.9 – 2.6 20 – 49 62 – 88 --

Average 1.4 40 80 --Olive husks Typical 1.2 – 1.5 30 – 35 8 – 10 --Potato-processing sludge Typical -- 28 75 1,570Potato tops Typical 1.5 25 -- --

Rice hulls Range 0 – 0.4 113 – 1,120 7 – 12 185 – 219Average 0.3 121 14 202

Soybean meal Typical 7.2 – 7.6 4 – 6 -- --Tomato-processing waste Typical 4.5 11a 62 --Vegetable produce Typical 2.7 19 87 1,585Vegetable wastes Typical 2.5 – 4 11 – 13 -- --

Fish and meat processing

Blood wastes (slaughterhouse Typical 13 – 14 3 – 3.5 10 – 78 ---waste and dried blood)

Crab and lobster wastes Range 4.6 – 8.2 4.0 – 5.4 35 – 61 --Average 6.1 4.9 47 240

Fish-breading crumbs Typical 2.0 28 10 --Fish-processing sludge Typical 6.8 5.2 94 --Fish wastes Range 6.5 – 14.2 2.6 – 5.0 50 – 81 --

(gurry, racks, and so on) Average 10.6 3.6 76 --Mixed slaughterhouse waste Typical 7 – 10 2 – 4 -- --Mussel wastes Typical 3.6 2.2 63 --Poultry carcasses Typical 2.4b 5 65 --Paunch manure Typical 1.8 20 – 30 80 – 85 1,460Shrimp wastes Typical 9.5 3.4 78 --

See footnotes at end of table.


CompostingChapter 2


Table 2A–1 Typical characteristics of selected raw materials—Continued


Manure

Broiler litter Range 1.6 – 3.9 12 – 15a 22 – 46 756 – 1,026Average 2.7 14a 37 864

Cattle Range 1.5 – 4.2 11 – 30 67 – 87 1,323 – 1,674Average 2.4 19 81 1,458

Dairy tiestall Typical 2.7 18 79 -- Dairy freestall Typical 3.7 13 83 --Horse—general Range 1.4 – 2.3 22 – 50 59 – 79 1,215 – 1,620

Average 1.6 30 72 1,379Horse—race track Range 0.8 – 1.7 29 – 56 52 – 67 --

Average 1.2 41 63 --Laying hens Range 4 – 10 3 – 10 62 – 75 1,377 – 1,620

Average 8.0 6 69 1,479Sheep Range 1.3 – 3.9 13 – 20 60 – 75 --

Average 2.7 16 69 --Swine Range 1.9 – 4.3 9 – 19 65 – 91 --

Average 3.1 14 80 --Turkey litter Average 2.6 16a 26 783

Municipal waste

Garbage (food waste) Typical 1.9 – 2.9 14 – 16 69 --Night soil Typical 5.5 – 6.5 6 – 10 -- --Paper from domestic refuse Typical 0.2 – 0.25 127 – 178 18 – 20 --Pharmaceutical wastes Typical 2.6 19 -- --Refuse (mixed food, paper, etc.) Typical 0.6 – 1.3 34 – 80 -- --Sewage sludge Range 2 – 6.9 5 – 16 72 – 84 1,075 – 1,750 Activated sludge Typical 5.6 6 -- -- Digested sludge Typical 1.9 16 -- --

Straw, hay, silage

Corn silage Typical 1.2 – 1.4 38 – 43a 65 – 68 --Hay—general Range 0.7 – 3.6 15 – 32 8 – 10 --

Average 2.10 -- -- --Hay—legume Range 1.8 – 3.6 15 – 19 -- --

Average 2.5 16 -- --Hay—nonlegume Range 0.7 – 2.5 -- -- --

Average 1.3 32 -- --Straw—general Range 0.3 – 1.1 48 – 150 4 – 27 58 – 378

Average 0.7 80 12 227Straw—oat Range 0.6-1.1 48-98 -- --

Average 0.9 60 -- --Straw—wheat Range 0.3-0.5 100-150 -- --

Average 0.4 127 -- --

See footnotes at end of table.



CompostingChapter 2

Table 2A–1 Typical characteristics of selected raw materials—Continued


Wood and paper

Bark—hardwoods Range 0.10 – 0.41 116 – 436 -- --Average 0.241 223 -- --

Bark—softwoods Range 0.04 – 0.39 131 – 1,285 -- --Average 0.14 496 -- --

Corrugated cardboard Typical 0.10 563 8 259Lumbermill waste Typical 0.13 170 -- --Newsprint Typical 0.06 – 0.14 398 – 852 3 – 8 195 – 242Paper fiber sludge Typical -- 250 66 1,140Paper mill sludge Typical 0.56 54 81 --Paper pulp Typical 0.59 90 82 1,403Sawdust Range 0.06 – 0.8 200 – 750 19 – 65 350 – 450

Average 0.24 442 39 410Telephone books Typical 0.7 772 6 250Wood chips Typical -- -- -- 445 – 620Wood—hardwoods Range 0.06 – 0.11 451 – 819 -- --(chips, shavings, and so on) Average 0.09 560 -- --Wood—softwoods Range 0.04 – 0.23 212 – 1,313 -- --(chips, shavings, and so on) Average 0.09 641 -- --

Yard waste and other vegetation

Grass clippings Range 2.0 – 6.0 9 – 25 -- --Average 3.4 17 82 --

Loose Typical -- -- -- 300 – 400 Compacted Typical -- -- -- 500 – 800Leaves Range 0.5 – 1.3 40 – 80 -- --

Average 0.9 54 38 -- Loose and dry Typical -- -- -- 100 – 300 Compacted and moist Typical -- -- -- 400 – 500Seaweed Range 1.2 – 3.0 5 – 27 -- --

Average 1.9 17 53 --Shrub trimmings Typical 1.0 53 15 429Tree trimmings Typical 3.1 16 70 1,296Water hyacinth—fresh Typical -- 20 – 30 93 405

a Estimated from ash or volatile solids data.b Mostly organic nitrogen.


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CompostingChapter 2Appendix 2B Testing Materials on the Farm

(Excerpted from On-Farm Composting Handbook,

NRAES-54, Northeast Regional Agricultural Engi-

neering Service Cooperative Extension)

A few characteristics of raw material and compost canbe determined on the farm using simple proceduresthat require only available or inexpensive equipment.The characteristics include density, moisture, content,pH, and soluble salts. At a minimum, a good weighingscale is required. The scale should be able to readnumbers that are at least one-hundredth the size of thesample (for example, 1/8 ounce for a 1-pound sampleor 1 gram for a 100-gram sample). Scales that can readto 0.1 gram are preferable. Other equipment requireddepends on the specific test.

Laboratory safety

The tests described here are not hazardous, but a fewsimple safety precautions need to be observed. Glovesshould be available and worn when hot containers arehandled. Safety glasses or goggles should also beavailable. Work areas should be well vented. Observeappropriate equipment precautions. For example, donot use metal containers in a microwave oven and donot leave a microwave oven unattended while samplesare being heated.

Samples

The first step in testing material is obtaining a repre-sentative sample. The sample should reflect the overallqualities of the material being tested. It is best tocollect many samples from different locations in a pileand/or from several piles. Mix these samples and thendraw subsamples to be tested from the mixture. If asingle sample is taken, collect it from a location that istypical of the whole pile. Avoid taking samples fromthe center, edge, and outer surface. These areas aremore likely to have different qualities from the bulk ofthe material in the pile.

Samples can lose moisture and undergo other changesin the time that elapses between collecting and testingmaterial. Therefore, samples should be collectedshortly before testing. If they must be collected sometime in advance, they should be refrigerated in acovered container or at least kept away from heat,sunlight, and other conditions that might alter theircharacteristics.

The sample size should be convenient to work withand suited to the testing equipment and containers.Establish a standard sample size so that testing proce-dures are consistent. The calculations can sometimesbe simplified by using samples sizes that have roundnumbers, such as 100 grams, 1 pound, or 1 liter, thenweighing the dried sample. In general, the larger thatthe sample is, the more accurate the testing resultswill be. However, this must be balanced with practical-ity. For example, larger samples take a longer time todry for moisture content determinations.

Density

Density is calculated by dividing the weight of a sub-stance by the volume that it occupies. In compostingwork, a material's bulk density generally is required.Bulk density is the mass of a pile or container ofmaterial divide by the volume of the pile or container.The volume includes the air spaces between particles.For example, the density of a pile of wood chips (bulkdensity) is more important to know than the density ofan individual wood chip (particle density).

Density can be determined by filling a container ofknown volume and weight with the material to betested and then weighing the filled container. Thedensity equals the filled container weight minus theempty container weight divided by the containervolume.

Density = filled container wt - empty container wt

container volume

( )

When determining the bulk density, the material needsto fill the container with nearly the same degree ofcompaction that occurs in the storage or field stack. Itmust not be packed down; otherwise, the bulk densitywill be overestimated. Filling the container properlycan be tricky; therefore, it is best to obtain and weighseveral samples and then average the results.

Moisture content

Moisture content is the portion of a material's totalweight that is water. It is often expressed as a percent-age. The nonwater portion of a material is referred toas dry matter.


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Moisture content can be determined by drying asample of material to remove the water and thenweighing the dried sample. Follow these steps:

1. Weigh the container.2. Weigh the wet sample and the container.3. Dry the sample (see sections on drying below).4. Weigh the dried sample and container.5. Subtract the dried weight from the wet weight

and determine the moisture content, as ex-plained below.

The difference between the wet weight and driedweight is the weight of water removed from thesample. The moisture content equals the weight ofwater removed (that is, wet weight of the sampleminus its dry weight) divided by the wet weight minusthe weight of the container. Note that this is the wet-basis moisture content. The moisture content on a drybasis is the wet weight minus dry weight divided bythe dry weight minus the container weight. To obtainthe moisture content in percent, multiply this ratio by100:

Moisture content % = wet wt - dry wt

wet wt - container wt( )

×100

Where the wet weight is the total weight of the sampleincluding the container.

The goal in drying a sample is to remove the waterwhile minimizing the loss of volatile dry matter com-pounds, such as ammonia and organic acids. Samplesare dried at a relatively low temperature over a longperiod because high temperatures increase the drymatter loss, especially if a sample burns. There is atradeoff between accuracy and speed. Lower tempera-tures and larger samples generally improve accuracy,but increase drying time.

The general procedure involves weighing the wetsample and then drying it until the sample no longerloses weight. To determine this, the sample must bedried in stages and then weighed after each stage. Thesample is dry when its weight remains constant be-tween two consecutive drying stages. For compostingpurposes, the sample can be considered dry if itsweight changes by less than 1 percent of the originalwet weight (for example, 1 gram for a 100-gramsample). The required drying time varies with thetemperature, drying equipment, sample size, andsample moisture. After a number of experiments,

typical drying times can be established. The generalguidelines that follow provide a starting point, butexperimentation is still necessary to establish routineprocedures for specific equipment and sample charac-teristics.

Methods for determining moisture content on the farmdiffer in the way that the sample is dried. Three com-mon methods include air drying, conventional ovendrying, and microwave oven drying. Although theresults produced by these methods are less accuratethan laboratory procedures, they are satisfactory foralmost all composting situations.

Air-drying is perhaps the simplest method for deter-mining the moisture of a sample. First obtain theweight of the sample container and then weigh thecontainer full of material. The larger the sample, themore accurate the results (that is, a gallon sample ismore accurate than a pint sample). Next, spread thesample material no more than 0.5-inch-thick on paperin a warm room that has a fan to improve air circula-tion. Allow the sample to dry for 24 to 48 hours, stir-ring occasionally to obtain uniform drying of all par-ticles. Pour the material back into the sample con-tainer and weigh again. It may be necessary to repeatthese steps, weighing every several hours, until theweight loss is negligible. Air-drying removes most, butnot all, of the water in the sample material; therefore,the actual moisture content tends to be underesti-mated. However, for most composting situations, air-drying produces acceptable moisture content esti-mates.

Samples can be more thoroughly dried in a conven-tional heated-air oven at temperatures between 140and 220 degrees Fahrenheit. An oven temperature of212 degrees Fahrenheit is a good compromise betweenspeed and accuracy for most composting material.Rough estimates for drying a 4-ounce (100-gram)sample range from 24 hours (219 °F) to 72 hours(141 °F). Experimentation and periodic weighing arenecessary to determine the required time for a giventemperature and sample material. Drying can bequickened by spreading the sample in a thin layer.

Drying time is considerably reduced by using a micro-wave oven to dry samples. Again, experimentation isnecessary to determine the drying time for a givenmicrowave oven and sample. As a start, use a 4-ounce(100-gram) sample of moist material and heat it for 8

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CompostingChapter 2

minutes at full power in a microwave oven with atleast 600 watts of power. For a less powerful micro-wave oven, increase the heating period (or reduce thesample size). For relatively dry material, such asfinished compost, decrease the heating period to 6minutes. After this initial heating, remove the samplefrom the oven and weigh it. Then reheat the sample foranother 2 minutes, rotating it 90 degrees from itsoriginal position when replacing it in the oven. Afterreheating, weigh the sample again. Continue the cycleof heating and weighing at 1-minute intervals until theweight change is negligible. If you notice the samplebecomes burned or charred, start a new trial using lesspower and/or shorter heating times. After determiningthe required drying time for a particular microwaveoven, sample size, and material, a continuous dryingperiod can be used.

Microwave drying is a convenient and relatively accu-rate method of determining moisture content; how-ever, care must be taken to avoid overheating and spotburning of the sample. Spreading the sample in a thinlayer is helpful. Samples must be placed in microwavesafe containers. Metal should not be placed in a micro-wave oven. A paper plate is a good container becauseit is light weight and the sample can be spread. Formaximum accuracy, paper containers should be pre-heated to remove moisture.

pH and soluble salts

Saturated paste method—This method is the mostcommon and reproducible method used for measuringpH and soluble salts. This method can be mastered byalmost anyone because it is simple and requires easilyavailable supplies. The equipment needed includes apH meter and a solubridge meter. Simple batteryoperated pH and solubridge meters are available atreasonable costs, and they are easy to operate.

Because compost is rich in ammonium, the solutionsused for preparing samples for measuring pH andsoluble salts are different. Therefore, separate prepa-rations must be made for each measurement. Whenmeasuring pH, use only a 0.01 M solution of calciumchloride. This is equivalent to approximately a slightlyrounded teaspoon of U.S.P. grade calcium chloridedissolved into a gallon of distilled or deionized water.For measuring soluble salt, use either distilled ordeionized water alone, without calcium chloride.

To make a saturated paste, use a paper or plasticdrinking cup half filled with compost. Depending onwhich test you are conducting, add the appropriatesolution in small quantities and stir constantly with astirring spatula, kitchen knife, or plastic plant label. Asaturated paste is achieved when there is just enoughwater to make a smooth paste of the compost so thatwhen the cup is held in a horizontal position, all of thewater will be held by the compost and none will flowto the sides of the cup. This mixture should be allowedto stand with the container covered at room tempera-ture for at least 4 hours, preferably overnight, beforemeasurements are taken. Just before taking measure-ments, stir the saturated paste. If it appears to havedried, either the distilled or deionized water or thecalcium chloride solution must be added before mea-suring. If several samples are being tested, rememberto rinse your stirring tool before stirring the nextsample. The measurements are taken by plunging thebase of the instruments into the saturated paste andtaking readings as soon as the numbers stabilize.


CompostingChapter 2

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Compost a Gem

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