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Waste Management

ALBERT J. VAN OOSTROM

Albert van Oostrom and Associates, Hamilton, New Zealand

I. INTRODUCTION

II. WASTE CHARACTERISTICS

III. WASTE SOURCES AND WASTE MINIMIZATIONA. Overview and PrinciplesB. StockyardsC. Blood Collection and ProcessingD. Trimming, Cutting, and BoningE. Viscera ProcessingF. RenderingG. Hide and Skin Processing

IV. WASTEWATER TREATMENTA. Primary Physical TreatmentB. Physicochemical TreatmentC. Anaerobic TreatmentD. Aerobic Treatment and Biological Nitrogen RemovalE. DisinfectionF. Land Treatment

V. SOLID WASTE MANAGEMENT

VI. CONCLUDING REMARKS

ACKNOWLEDGMENTS

REFERENCES

I. INTRODUCTION

In meat processing, as in most other industries, the production of some waste is un-avoidable. The primary product of processing livestock is edible meat for human con-sumption. All other materials that leave the meat processing plant are by-products orwaste.

Copyright © 2001 by Marcel Dekker, Inc. All Rights Reserved.

For this chapter, by-products are defined as the non-meat components of the animal(and the products produced from them) that are saleable and generate revenue for the meatprocessor. By-products can include blood and renderable “waste” tissue, as well as thedried blood, tallow, and meat and bone meal produced from such raw materials. The man-ufactured by-products are also known as co-products, and the revenues from these productscan greatly affect the profitability of a meat processing operation. Waste is defined in thischapter as materials resulting from meat and by-product processing operations that have nocurrent economic value to the processor and that normally have a cost associated with theirdisposal. By this economic definition, what constitutes a waste or by-product can changewith market conditions, and with the development of new economic uses for waste materi-als. Drawing from an adage, one man’s trash is another’s treasure.

Although the meat processing industry has made major advances in waste reductionand by-product recovery, it still produces and discharges a significant amount of solid, liq-uid, and gaseous waste. This waste must be carefully managed and disposed of to avoid cre-ating a nuisance or pollution hazard, and to minimize disposal costs.

With increasingly strict standards and restrictions being imposed on how wastes canbe disposed of, and on the levels of pollutants considered acceptable in the receiving envi-ronment, many meat processors face the challenge of improving their waste managementpractices in a cost-effective way. To do this they need to develop waste management strate-gies based on the following hierarchy:

1. waste avoidance and reduction at source2. waste recovery, reuse, and recycling3. waste treatment and disposal

This chapter summarizes the principles and practices of waste management in themeat processing industry. The reader will be taken on a journey through this hierarchy ofwaste management. Along the way we will look at the sources and characteristics of thewaste, as well as consider the potential effects of waste discharges on the receivingenvironment.

II. WASTE CHARACTERISTICS

Meat processing operations use large volumes of hot and cold water to maintain hygienicprocessing conditions. Various quantities of blood, fat, gut contents, feces, and other organicmatter are washed down the drain, producing a wastewater with a not surprising resemblanceto uncooked, unclean meat soup. In addition to these organic wastes, the wastewater mayalso contain small amounts of soil and grit from preslaughter washing of the animals, as wellas detergents and other chemicals used during processing and cleaning.

The volume of wastewater produced in slaughterhouses can vary greatly (Table 1),depending on the types of animal processed, the cost of the water, a company’s attitude towater conservation, and the extent of by-product processing undertaken on site.

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Table 1 Typical SlaughterhouseWastewater Volumes (liters·animal�1)for Processing Different Animals

Cattle 1200–5000Pigs 250–1000Sheep and lambs 200–800


As it leaves the processing operations, the wastewater contains varying amounts ofcoarse separable material. The first treatment step almost always involves primary treat-ment by screening or sedimentation to remove such solids.

Primary treated wastewater typically contains high levels of 5-day biochemical oxy-gen demand (BOD5, a measure of organic matter that can be aerobically biodegraded infive days at 20°C), nitrogen, phosphorus, and fecal microorganisms, all of which are pol-lutants of concern.

The characteristics of the primary treated wastewater vary widely from plant to plant,and at different times, depending on the amount of water used, the kinds of livestockslaughtered, and the processing operations undertaken. Table 2 gives the ranges of wastew-ater characteristics typical for slaughterhouses. By comparison, the concentrations of ni-trogen and organic matter in slaughterhouse wastewater are typically 5 to 10 times higherthan those in domestic wastewater (sewage). Definitions and measurement techniques for

Waste Management 637

Table 2 Typical Concentration Ranges for Pollutants in the CombinedScreened or Settled Wastewater from Slaughterhouses (all units are g·m�3,except for fecal coliforms, which are counts per 100 ml)

Pollutant Concentration range

Chemical oxygen demand (COD) 2000–6000Soluble COD 1200–36005-day biochemical oxygen demand (BOD5) 1000–3000Total suspended solids (TSS) 200–2000Fat, oil, and grease 100–1000Total Kjeldahl nitrogen (TKN) 100–300Ammoniacal nitrogen (NH3-N) 10–80Total phosphorus (TP) 10–30Fecal coliforms 107–108

Table 3 Mean Percentage ofSettled Meat ProcessingWastewater That is Soluble (orColloidal), as Determined byFiltration Through a WhatmanGF/C Glass Fiber Filter

Pollutant %

COD 60TKN 79Fat 16Total solids 80TP 76Carbohydrates 61

Source: From Cooper, 1982.


the various wastewater pollutants discussed in this chapter can be found in Standard Meth-ods for the Examination of Water and Wastewater (APHA, 1998).

A high proportion of many components of screened or settled wastewater is in a sol-uble or colloidal form, as determined by filtration techniques (Tables 3 and 4). Therefore,these components are difficult to recover from the wastewater by physical means, but aremore easily broken down by biological treatment than if they were present primarily ascoarse particles.

It is also useful to know the mass of each pollutant produced in meat processing op-erations (by determining waste stream volumes as well as pollutant concentrations) and torelate this to a unit of production, such as dressed carcass weight, live weight, or numberand type of animals processed. Although this may seem an obvious calculation, there arevery few published data expressing pollutants on a unit production basis. Some data forU.S. meat packinghouses are given in Table 5. The large variation reflects differences inthe extent and nature of processing undertaken at different plants.

To permit comparison between plants, waste production data should be accompaniedby a description of the processing operations, as well as the nature of any treatment up-stream of the sampling site. Table 6 indicates the range of pollutant loadings expected inthe primary treated wastewater from cattle and sheep/lamb slaughterhouses with cuttingand boning operations, but with minimal by-products processing done on-site.

Similarly, when measuring waste from a specific by-product processing operation, itis useful to relate waste generation to a measure of by-product producted. Some such dataare presented in the next section.

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Table 4 Fractions of COD in 1 mm-screened Slaughterhouse Wastewater (total CODof 2870 g·m�3), as Determined by Filtration through 7.4 �m and 0.45 �m Filters

Fraction Nominal size range Percent total COD

Coarse suspended solids 1 mm–7.4 �m 45–55Colloidala 7.4 �m–0.45 �m 20–30Soluble �0.45 �m 20–30

a Some of this colloidal fraction is commonly considered part of soluble CODSource: From Sayed et al., 1988.

Table 5 Mean Pollutant Loadings in the Wastewater from U.S. Packinghouses (dataare combined from four surveys; the ranges in parentheses represent one standarddeviation about the mean; units are kg per tonne live weight)

BOD5 TSS Grease Nitrogen

13.3 (8.6–18.0) 10.3 (5.5–15.1) 5.2 (0.2–10.2) 1.3 (0.9–1.7)

Source: From Witherow et al., 1973.


III. WASTE SOURCES AND WASTE MINIMIZATION

A. Overview and Principles

1. Waste Minimization and Recovery

When organic materials enter the wastewater stream they add to its treatment and disposalcosts. Some of the particulate solids can be removed by primary treatment systems, but af-ter such treatment the wastewater still contains large quantities of soluble and fine particu-late matter, and is generally unsuitable for discharge to the environment. The cost to dis-charge primary treated wastewater to a municipal sewer (if allowed), or to further treat thewastewater on-site, is usually very high and is directly related to the wastewater volumeand pollutant content.

Meat processors therefore have a powerful economic incentive to minimize both wa-ter use and the amount of material that enters the wastewater stream. This can be achieved,for example, by using “dry-cleaning” techniques, such as sweeping. Often the material re-covered by dry-cleaning can be processed into products with a commercial value. Thus,waste minimization not only reduces wastewater treatment and disposal costs but also canreduce water supply cost and increase revenue.

Where wastewater pollutant loadings cannot practicably be reduced at source, thenext best option is usually to recover material from the wastewater stream by physicaland/or physicochemical treatment methods. Generally, this is best accomplished by segre-gating different types of solids-laden waste streams and recovering the solids as soon aspossible. These actions maximize the quantity and potential value of the materials recov-ered, as explained below.

2. Segregation of Waste Streams

When assessing opportunities for reducing and recovering waste in meat processing, it is use-ful to distinguish between potentially valuable wastes derived from animal tissues (e.g., blood,fat, meat and bone), and low- or no-value wastes, such as animal gut contents, feces, and urine.


Table 6 Typical Pollutant Loadings in the Total Screenedor Settled Wastewater from Slaughterhouses with MinimalBy-products Processing Done On-Sitea (data are based onunpublished measurements at New Zealand slaughterhouses;units are kg per tonne dressed carcass weight; dressed carcassweight is approximately 50% of live weight)

Cattle Sheep/lambs

COD 18–36 22–44Soluble COD 11–22 14–28BOD5 9–18 11–22TKN 1.2–2.4 1.5–3.0TP 0.12–0.24 0.13–0.26

a For plants with blood collection, dressing, cutting, boning, tripe re-covery (cattle), stripping of runners (sheep/lambs), and optionally gutcutting and washing. No on-site rendering, blood processing, orhide/skin processing except cooling with water.


Animal tissues can be processed into saleable by-products such as tallow, meat and bone meal,or dried blood. These materials contribute to waste only if they are washed down the drain.

Waste minimization and recovery applied to gut contents and fecal material is morecomplex. As with waste animal tissues, the collection of gut contents and feces in a rela-tively dry form (e.g., dry-dumping paunch contents and dry-collection of stockyardwastes), or their partial recovery from the wastewater by primary treatment, can consider-ably reduce the wastewater pollutant loading and wastewater treatment costs. However, drycollection can be difficult, and the collected solids pose a significant disposal problem formany slaughterhouses. Nevertheless, managing solid waste is usually more cost-effectivethan treating and disposing of it as a part of wastewater.

Initial segregation of wastewater streams containing animal tissues from those con-taining fecal matter and gut contents allows animal tissues to be recovered without con-tamination that can downgrade rendering products. Equally, feces and gut contents can berecovered with minimal contamination by animal tissue. This is important because animaltissue increases the potential for the recovered solids to generate odor and attract vermin,which can restrict the utilization and disposal of these solids. For example, gut contents andfeces that are relatively free of animal tissues can be stabilized by simple windrow com-posting, whereas more costly composting techniques or alternative treatment methods aregenerally required for similar wastes containing animal tissue.

3. Recover Without Delay and Close to Source

As soon as solids enter the wastewater stream they begin to break down and release solu-ble material. This release is increased by turbulence (e.g., by pumping) and by high tem-peratures. Therefore, if the solids cannot be recovered dry, they should be removed fromthe wastewater stream as quickly and as close to source as possible.

A schematic diagram of the main sources and flows of wastes, in relation to meat andby-product processing operations, is given in Fig. 1.

Some of the main sources of waste in meat processing, and techniques for their min-imization, are discussed in the following sections.

B. Stockyards

The waste from the holding and washing of livestock prior to slaughter is mostly fecal matterand urine, although some soil and grit may be present, depending on animal cleanliness. Thequantity and characteristics of the waste voided by the livestock depend on several factors:

Animal species, size, and recent dietHow long the animals were held off feed prior to arriving at the plantHow long the animals were held at the meat plant before slaughter

Quantities of waste voided by cows and sheep/lambs during holding in yards for 24 hoursare given in Table 7.

To reduce the amount of fecal waste, animals are sometimes fasted before transportto the meat plant. However, this practice is difficult for meat packers to control and can po-tentially result in significant dehydration of the animals and carcass weight loss. A furtheropportunity to reduce stockyard waste is to slaughter directly off the truck, and thus reducethe time the animals are held in the yards. However, the animals will have a fuller intesti-nal tract, and the wastewater management benefit of “tailgate” slaughter is realized only ifthe gut contents are recovered undiluted during later viscera handling.

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Figure 1 Generalized schematic of waste flows and treatment stages in relation to common meatand by-product processing operations. In accordance with good practice, waste streams containingpredominantly animal tissues are segregated and treated separately from those containing predomi-nantly fecal matter and gut contents. (Adapted from Jones, 1974.)

A key characteristic of stockyard waste is that typically 50% to 60% of the TKN issoluble organic nitrogen (i.e., soluble TKN less ammoniacal nitrogen). The urea compo-nent of urine is the dominant source of this organic nitrogen. This high proportion of urine-derived nitrogen limits the effectiveness of dry-cleaning and primary treatment techniquesin reducing the amount of nitrogen in the wastewater from stockyards.


The usual practice for managing stockyard wastes is to flush them into the wastewa-ter stream using large volumes of water, then to recover a proportion of the solids by sedi-mentation or screening. Typically only 17% to 49% of the organic load (measured as COD)and 4% to 12% of the nitrogen (measured as TKN) will be removed by such screening(Table 7). However, recoveries can be increased to 70% and 24%, respectively, by dry-cleaning the pens before washing (Table 7).

In Australasian slaughterhouses, stockyards for sheep and lambs are covered and typ-ically have a raised floor comprising a metal grating through which the feces and urine fall.Most of the urine drains to wastewater, but the feces are often allowed to accumulate un-der the grating for several days before being washed to the wastewater stream. The accu-mulated solids consolidate and dry somewhat, and can be recovered quite effectively ifscreened from the wastewater close to source before the solids disperse/dissolve signifi-cantly. Recovery could be improved by collecting the solids dry, but this is not common,as it requires machine access beneath the grating.

The dry recovery of fecal matter from cattleyards is more difficult than for sheep andlambs, as cattleyards usually have a solid floor and need frequent washing.

C. Blood Collection and Processing

1. Blood Characteristics

Blood contains high concentrations of total solids and nitrogen and has a high oxygen de-mand (Table 8), and thus only small losses of blood into wastewater can significantly in-crease pollutant loadings and treatment costs.

2. Blood Collection

It is normal practice to collect the blood from animals after slaughter. The blood may becollected by special hygienic techniques for edible purposes, but most commonly it is col-

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Table 7 Quantities of Material Voided by Pasture-Fed Sheep and Cattle during 24 h Holding,and the Percentage of the Pollutants Recoverable by Various Means (Units are g·animal�1 unlessotherwise specified)

Sheep and lambs

Cows Recoverable

Recoverable(%) by

Parameter Mean Range (%) by A Mean Range A B

COD 1265 765–1618 17–49 142 60–230 36 70Soluble COD 321 237–474 — — — — —TKN 109 68–158 3.6–9.2 14 9–19 11 24Soluble TKN 84 55–129 — — — — —NH3-N 28 12–43 — 5.3 3.0–8.0 — —TP 8.3 4.8–13 5.0–14 1.4 0.7–2.3 31 74

The different groups of animals had been withheld from pasture for 5–22 hours (cows) or 0–32 hours (sheep andlambs) prior to arrival at the meat plant.A � Treatment of wastewater with a 0.5 mm wedge-wire screen.B � Dry-cleaning pens followed by washing the pens and screening of wastewater with a 0.5 mm wedge-wirescreen.Source: MIRINZ, unpublished data.


lected for inedible processing by allowing it to drain into a collection pit or trough. By thetime a carcass has moved past the blood collection area, blood flow has usually slowed toa drip, but blood loss from the carcass can increase periodically due to hide pulling, brisketcutting, and head removal.

The quantity of blood that drips on the floor after the designated collection site is usu-ally much greater than it looks, as it is spread thinly over a large area and is regularlywashed away.

In an unpublished MIRINZ study, the volume of blood lost from carcasses at twobeef and two ovine processing plants was measured between the end of bulk blood collec-tion and evisceration. Blood loss in this period averaged 1.2 and 2.4 liters per beef animal,0.12 and 0.22 liters per lamb, and 0.14 and 0.31 liters per sheep. Detailed data for beef atone plant are shown in Table 9.

Blood rapidly coagulates once it has left the carcass. Much of the blood loss that oc-curs after the collection trough can be recovered by dry-cleaning the floor under the car-casses and heads using a squeegee, then pushing the amassed blood into the blood collec-tion system or scooping the mass into a holding bin for processing with other recoveredblood. A low nib wall on the floor around the areas to be dry-cleaned helps to contain theblood and minimize its dilution by wash water used in other operations.

Thorough dry cleaning is important for all areas where blood accumulates. At onebeef plant, on average 96 liters of coagulated blood lining the collection pit was washed intothe wastewater at each break in the working day. This loss of blood, which was equivalentto about 1 liter per animal processed, occurred because the pit design made dry-cleaningdifficult, and because staff did not realize how much blood was lost by washing nor its ef-fects in the waste stream. Good management and an understanding of downstream conse-quences are essential in waste minimization.


Table 8 Pollutants of Concern in UndilutedBovine or Ovine Blood

Typical concentration(g·m�3)

Total solids 200,000COD 300,000BOD5 200,000Total nitrogen 30,000Total phosphorus 200

Table 9 Volume of Blood Drip Lost at One Plant from Beef Carcasses and Detached HeadsBetween the End of Formal Blood Collection and Evisceration

Elapsed time after Mean bloodthroat cut volume collected

Site of blood loss (min:sec) (ml.animal�1)

Between collection through and hide puller 6:00–14:00 1540During hide removal 14:00–16:00 570From detached head 15:45–16:00 220Between hide puller and evisceration 16:00–18:00 100Total 2430


In addition to the sources of blood loss already mentioned, significant quantities ofblood unavoidably enter the wastewater streams from floor washing in carcass chillers, andfrom the initial washing and cooling of sheep skins and cattle hides. Blood is too sparse inthese situations to warrant dry recovery.

It is important that slaughterhouses maximize their blood recovery. An increase inblood recovery of, say, 2 liters per beef animal, which equates to about a 15% increase inblood yield, reduces wastewater COD and nitrogen loadings by 600 and 60 g per animal,respectively.

3. Blood Processing

For plants that process blood on site, additional losses to wastewater will occur to a degreethat depends on the processing method. Usually blood is dried to produce blood meal, asource of animal protein. The most popular method of producing blood meal involves co-agulating the blood proteins by steam injection, centrifuging the coagulum from the aque-ous fraction, and then drying the coagulum (Fernando, 1992).

The efficiency of the coagulation and separation steps, and thus the quantity of bloodsolids lost to drain in the aqueous phase, depends on:

The amount of water added to the blood during collection (less is better)How long the blood was aged prior to processing (ageing improves coagulation)The coagulation temperature (90° to 95°C is optimal)

Potentially the loss of blood solids in the aqueous fraction can be as low as 4%, butin practice a loss of less than 10% can be considered satisfactory (Pilkington, 1975). Addi-tional blood losses to wastewater will occur if a water scrubber is used to remove blood dustfrom the air stream from the blood dryer, and during the cleaning of blood holding tanks.At one facility an inefficient dust removal system on a blood dryer caused almost 10% ofthe product to be discharged to wastewater (MIRINZ, unpublished data).

Blood processing methods that use advanced techniques such as ultrafiltration to sep-arate the blood proteins from whole blood, or involve the drying of whole blood, produceless waste than the coagulation method, but are much less common due to their currenthigher cost.

As blood can be a major source of wastewater organic and nitrogen loading, plantsshould monitor their blood collection and processing efficiencies. For plants that processthe collected blood on-site, this simply requires relating the amount of dried blood pro-duced to the total carcass weight of source animals. For ruminants, a yield of 12 to 15 g ofdried blood per kilogram of dressed carcass weight is an achievable goal.

D. Trimming, Cutting, and Boning

Some animal tissue waste, typically meat and fat trimmings and fine debris from carcasssaws, is unavoidable in meat processing. Dry-cleaning methods should be used to collectthe solids close to source to maximize recovery for rendering. Gratings and perforated bas-kets in floor drains are normally used to prevent large pieces from entering the wastewater.If washed into the wastewater, a proportion of the smaller solids that pass through thesegratings cannot be recovered by screening and sedimentation, so producing a significantwastewater load requiring treatment.

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E. Viscera Processing

Slaughterhouses may carry out one or more viscera processing operations, all of which con-tribute gut contents and sometimes gut tissue to the wastewater stream.

1. Paunch Emptying

Cattle paunches (rumens) are processed for recovery of edible products (e.g., tripe), for petfood production, or for rendering. Clearly the paunch contents must be emptied if paunchesare destined for human or pet food. However, even when paunches are simply sent to ren-dering, they are normally emptied first.

The volume and characteristics of cattle paunch contents vary, depending on the sizeof the animal, its recent diet, and how long it has been off feed and water prior to slaugh-ter. Paunches of pasture-fed cattle typically contain 30 to 40 kg of material (Table 10), butthe paunch of a large animal may contain 80 kg or more.

Traditionally, the paunches are manually slashed and their contents flushed out withwater. This is called wet dumping (Witherow, 1974). Large volumes of water (100 to 200liters) are used to both clean the paunch sac (for tripe recovery) and carry the paunch con-tents out of the plant. When paunch contents are wet-dumped, only about 10% to 30% ofthe nitrogen and phosphorus and 40% of the total solids in the contents can be readily re-covered from the wastewater by screening or sedimentation. The resulting waste stream istherefore high in pollutants.

This loading on the wastewater system can be mostly avoided by dry dumping thepaunch contents or rendering the full paunches (Fig. 2). Dry dumping involves opening thepaunches to release their contents without the aid of water. After dry dumping, the paunchsac, still containing about 10% of its original contents, may be washed for tripe recovery orsent to rendering or pet food processing, as shown in Fig. 2. Compared with wet dumping,dry dumping can reduce the wastewater loading from paunch handling by 90% or more.

Dry-dumped paunch contents (8% to 10% total solids) are sometimes dewatered bybeing passed over a screen or through a screw press to make them more manageable. De-watering to 15% to 20% total solids removes free-draining liquid and this is all that is re-quired. The recovered liquid is obviously high in nitrogen, phosphorus and BOD5, but itsvolume is low, and the mass of soluble pollutants lost to wastewater is only about half thatof wet dumping systems. Nevertheless, the volume of liquid drained or squeezed from thedry-dumped solids should be minimized to maximize the waste management benefits ofdry dumping.

Conversion from a wet-dump to a dry-dump system is one of the best single methodsof reducing the wastewater pollutant loading at beef plants. For a beef slaughterhouse with


Table 10 Typical Composition of PaunchContents of Pasture-fed Cattle

Mass (g·paunch�1)

Total wet weight 40,000Total solids 4,000Total nitrogen 100Total phosphorus 35Sodium 100


minimal on-site by-products processing, conversion to dry dumping (with washing of thedry-dumped paunch sac) can reduce total plant wastewater solids, nitrogen and phosphorusloadings by 18% to 36%, 9% to 18% and 20% to 46%, respectively, depending on the ex-tent of solids dewatering (van Oostrom and Muirhead, 1996).

2. Viscera Cutting and Washing for Rendering

Condemned paunches and other inedible gut material are sent to rendering. Before ren-dering, this material is typically macerated in a mechanical gut cutter, followed by sepa-ration of tissue from gut contents, usually in a rotating wash screen. This washing in-creases the value of the gut tissues as a rendering feedstock, but the large volume ofwastewater produced contains a high pollutant loading from the gut contents and fromdislodged fat. Swan et al. (1986) found that the amount of TKN, fat, and COD in thewastewater from four gut cutting and washing systems ranged from 490 to 720 g, 2.0 to9.8 kg, and 17 to 33 kg per metric ton of dressed carcass weight, respectively. Fat losseswere greater for cutters that employed blunt, fast-moving blades than those with sharp,slow-moving blades.

A proportion of the gut solids can be recovered from this wastewater by fine screen-ing, but the collected solids are contaminated with fat, which may restrict how they can betreated or disposed.

The waste from gut washing can be avoided altogether by rendering the gut intact.However, this is generally not an economic option, as the rendering of gut contents de-grades tallow quality and value (by increasing the tallow free fatty acids and color), reducesthe meal protein content, and increases rendering energy costs. However, these disadvan-tages might be outweighed by reduced fat losses, a reduction in wastewater and solid waste

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Figure 2 Paunch handling methods.


loads, and simplification of materials handling equipment (Cooper, 1975). The economicfeasibility of this option has to be considered case by case.

3. Sausage Casings

When natural sausage casings are made from animal intestines, the first processing step in-volves squeezing fecal material out of the intestines. In a series of further steps the mucosaand other tissues are removed from the intestinal wall to produce a clean casing (the sub-mucosal layer). Large volumes of water are used to remove the intestinal contents and an-imal tissue wastes. To reduce water use, the cleaner wastewater streams are sometimes re-cycled to the initial processing steps.

During the processing of lamb intestines, the animal tissue waste consists of approx-imately 80% protein and accounts for 68% to 77% of the total COD, nitrogen and phos-phorus waste loading from the process (Fig. 3). It is good practice to keep the gut contentand waste tissue streams separate, to enable the fecal waste to be treated with other gut-con-tent wastewater, and the animal tissue waste to be rendered.

Another pollutant from casings manufacture is the salt (sodium chloride) that is usedto preserve the casings. Salt waste should be minimized, especially if the wastewater is ap-plied to land, as excessive application of sodium can damage soil structure.

F. Rendering

Rendering operations, which usually include blood processing, can account for 30% to45% of the total wastewater volume, and about 50% to 60% of the wastewater nitrogenloading from beef processing plants (Johns et al., 1995).


Figure 3 Pollutants resulting from the production of sausage casings from lamb small intestines.(MIRINZ, unpublished data.)


The quantity of raw materials and product lost to wastewater depends on how the rawmaterials are handled, the rendering technology, and the general standard of housekeeping.Rendering technologies have been reviewed by Swan (1991) and Fernando (1992).

The main sources of waste from rendering are discussed below.

1. Raw Material Conveyance and Storage

Fluid that drains from materials during conveyance to and storage at a rendering plant con-tains high concentrations of blood, fat, and other animal tissues. This significant source ofwastewater pollutant loading can be minimized by:

minimizing the addition of water to the materialsusing dry conveyance systems (e.g., screws and transport bins), which, unlike water-

chutes, pumping, and sometimes blow conveying, do not require the addition ofwater to aid conveyance

avoiding compression of the raw materialsdelaying size reduction until immediately before rendering

2. Condensate from Cookers and Meal Dryers

A large proportion of the volatile organic compounds in the hot gases released during cook-ing and drying processes ends up in the condensate that forms when these gas streams arecooled for heat recovery and odor control. The volume of condensate from dry renderingsystems depends largely on the amount of raw material and water loaded into the cookers.The condensate contributes a significant nitrogen and organic matter load to renderingwastewater. The non-condensable gas stream contains several hundred volatile organiccompounds (Luo et al., 1999) and normally requires further treatment to remove odor be-fore discharge to atmosphere.

3. Stickwater

In wet rendering and low temperature rendering processes, the meal, tallow, and aqueousphases are separated after the cooking stage by various decanting, pressing, centrifugation,and drying steps. The aqueous phase, often called stickwater, contains solubilized fat, pro-tein, and minerals and can be a major source of wastewater pollutant loading. For example,in the MIRINZ Low Temperature Rendering process, the stickwater separated from the tal-low by centrifugation contains high concentrations of pollutants (Table 11). Losses of fatand nonfat solids in this stream typically equate to about 2% and 6% of tallow and meat

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Table 11 Typical Characteristics ofStickwater from the MIRINZ LowTemperature Rendering Process

Concentration (g·m�3)

COD 30,000–80,000Total solids 20,000–60,000Fat 500–10,000TKN 1,500–4,000

Source: Compiled from Brown et al. 1993, Fernando1982, and unpublished data.


meal produced, respectively. pH adjustment of the influent and correct operation of the cen-trifuge are critical in minimizing this product loss to stickwater.

At some rendering plants the solids in stickwater are concentrated by evaporation orrecovered by physicochemical treatment, and incorporated into the meal product. Ultrafil-tration can recover a high proportion of the stickwater solids (Brown et al., 1993), but thistechnology has not yet been adopted by industry for this application.

4. Tallow Refining

Depending on the process, tallow may be further treated by washing and centrifugation toremove fines and other impurities. The wastewater from this process can be another sig-nificant source of organic loading.

G. Hide and Skin Processing

Soon after removal from a carcass, cattle hides and sheep skins are normally washed andcooled in water to remove blood and loose contaminating material. If they cannot be fur-ther processed quickly, they must be temporarily preserved (cured) to prevent deterioration.The most common preservative is salt (sodium chloride), as crystals or as brine. Fungicidesand other biocides may also be used in addition to the salt, and curing may also involve“fleshing,” which is the removal of flesh or adipose tissue remaining on the hide. The flesh-ings are normally rendered.

Salt and other chemicals lost to wastewater during curing, and washed from the hidesand skins during later processing, can represent a significant disposal problem. In theUnited States this source of waste is increasingly being avoided by processing the hides on-site so that curing is not necessary (Lollar, 1992). However, if further processing is doneon-site, this activity produces its own waste streams that must be managed.

For example, in New Zealand, sheepskins have for many years been processed (in de-partments called fellmongeries) immediately after slaughter. As opposed to simply saltingand exporting the whole skin with wool, fellmongering operations remove the wool fromthe skin with the aid of chemicals, and provide advanced preparation of the pelt for manu-facturing leather. After cooling and washing the pelts to remove blood and loose dirt, fell-mongery processing steps variously include the use of caustic lime and sulfide depilatorypaint, lime-sulfide solution, deliming chemicals, enzymes, sulfuric acid, and salt. Each canadd to the wastewater load.

The resulting wastewater contains high concentrations of COD and nitrogen (fromhydrolyzed wool, epidermal tissue, and adipose tissue), as well as sulfide, calcium, andother chemicals used in the process. Typical wastewater pollutant loadings from the majorwaste-producing operations in the fellmongering of ovine skins are given in Table 12. Forsheep and lamb slaughter plants, an on-site fellmongery can double the wastewater nitro-gen and increase the COD by 25%. Because of its high sulfide and total sulfur content, thewastewater is normally treated by chemical oxidation and/or aerobic biological treatment(not anaerobic) to oxidize the sulfides to sulfates, and thus avoid odor and toxicity prob-lems from sulfides, as discussed later.

The main opportunity to minimize the wastewater loading from skin processing is tomaximize the recovery of wool from the skins in the early stages of fellmongery process-ing, before the skins are immersed in a lime-sulfide solution that chemically degrades theresidual wool, adding to the wastewater loading. As wool is a saleable product, there is aneconomic incentive to maximize its recovery.



IV. WASTEWATER TREATMENT

Various treatment methods are commonly applied to meat processing wastewater, as dis-cussed below.

A. Primary Physical Treatment

The initial treatment of meat processing wastewater (termed primary treatment) almost al-ways involves a simple physical separation process to recover particulate solids. The mainphysical treatment processes are screening, sedimentation, and flotation.

1. Screening

Most meat processing plants use fine screens on one or more of their wastewater streams.Common screen types include brushed screens, rotating drum screens, inclined staticscreens and vibrating screens. Screen materials include woven wire, perforated plate, andwedge-wire. Static and rotating-drum wedge-wire screens are most popular (Fig. 4). Thewedge-shaped bars of such screens act to prevent the screen from blinding. The requiredscreen size depends on the flow rate, and the nature and quantity of the particulate solids inthe wastewater. Slots of 0.5 to 1.0 mm are suitable for most applications in meat process-ing. Narrower slots reduce the water flow rate through the screen whereas wider slots re-duce the efficiency of solids removal.

2. Gravity Separation

Primary sedimentation tanks (primary clarifiers, save-alls, interceptors, grease traps, ma-nure pits) have a long history in meat processing for the gravity-assisted removal of set-tleable solids and flotable matter from wastewater. The equipment usually consists of a pri-mary clarifier or save-all with top and bottom scrapers, which continuously remove thefloating grease and settled solids. For smaller applications and for stockyard waste streams,grease traps and manure pits are sometimes used, respectively, where solids are removedperiodically by various means.

The removal efficiency for suspended solids depends on the hydraulic retention timeand flow characteristics in the tank, and on how often the solids are removed. The hydraulic

650 van Oostrom

Table 12 Total Pollutants Discharged from the Liming,Deliming and Enzyme-Wash Steps of the FellmongeringProcess (Units are g·animal�1 unless indicated otherwise)

Pollutant Lambs Sheep

COD 168 210Soluble COD 143 173TKN 25 32Soluble TKN 23 30Fat 6 9Total solids 246 291Sulfide-S 6.2 7.7Thiosulfate-S 6.1 6.9Water use (L·animal�1) 25 32

Source: From Cooper and Russell, 1982.


retention time for primary clarifiers used in the industry is typically between 30 and 60minutes.

Gravity separation can generally remove a greater proportion of solids (particularlyfat) than screening, but has higher operating costs and a greater potential to generate odor.The trend has therefore been to replace gravity separators with screening or, increasingly,a combination of screening and dissolved air flotation technology.

3. Dissolved Air Flotation

In the dissolved air flotation (DAF) process, suspended solids in the wastewater are re-moved by flotation assisted by micrometer-sized air bubbles. The bubbles are produced bydissolving air in the wastewater or a recycle stream at 3 to 5 atmospheres. When the pres-surized air-saturated liquid enters the DAF tank, the decrease in pressure results in the re-lease of tiny air bubbles, in the same way that bubbles are released from solution whenchampagne is opened. The rising bubbles adhere to suspended solids in the wastewater andassist flotation. The floating solids are recovered with scrapers. DAF systems come in a va-riety of designs that vary in tank geometry, method of solids removal, and method of in-troducing the air-charged water into the flotation tank. In some systems a pressurized re-cycle stream is introduced together with the wastewater into the flotation tank, in othersthese streams enter the tank separately. A simplified flow scheme of the latter type is givenin Figure 5.

Although DAF has higher capital and operating costs than passive gravity separation,DAF works faster and produces a drier sludge. DAF systems are popular for recovering fatand protein from meat processing wastewater. The technology is commonly applied insteadof, or after, screening or settling, and often incorporates upstream chemical addition to aug-ment its effect, as discussed below.


Figure 4 Common types of wedge-wire screens.


B. Physicochemical Treatment

Simple physical processes will not remove soluble proteins, fat emulsions, or colloidal ma-terial from wastewater. However, by adjusting pH and dosing the wastewater with specificcoagulants and flocculants, some of this dissolved and finely dispersed organic matter canbe precipitated and agglomerated into larger particles (flocs) that can be recovered by aphysical process such as DAF or settling. Figure 6 shows a generalized scheme of physic-ochemical unit operations.

Without physicochemical treatment, many organic materials in the wastewater willnot agglomerate and settle because of their small mass and because they carry a surfacecharge. The repulsive forces between particles are greater than the forces (e.g., gravity) thatcause them to aggregate or settle. The mechanisms involved in chemical coagulation andflocculation involve reducing particle charge or overcoming the effects of the charge.

For example, dissolved protein molecules carry a net negative charge at pH valuesaround 7. Acidification of the solution reduces this charge, and at a specific pH for each

652 van Oostrom

Figure 5 Schematic of a dissolved air flotation (DAF) process.

Figure 6 Unit operations commonly involved in physicochemical treatment systems.


protein, known as the isoelectric point, the protein carries no net charge. At this point theelectrostatic repulsion between protein molecules is minimal, which allows some proteinsto flocculate. On the other hand, acidification to pH values below the isoelectric point willresult in the proteins carrying a net positive charge.

Adjustment of the pH of meat processing wastewater to between pH 4 and 5 willremove many proteins from solution. Within this range, many of the proteins are at ornear their isoelectric points and collectively they have minimal solubility. However, someproteins, including the blood protein hemoglobin, do not flocculate at their isoelectricpoint and cannot be removed from solution by simple pH adjustment (Cooper et al.,1983).

The use of cationic and anionic coagulants with pH adjustment can provide more ef-fective removal of protein and other organic material than pH adjustment alone.

The most common cationic coagulants used in wastewater treatment are iron (Fe3�)and aluminum (Al3�) salts. These ions interact with negatively charged proteins and col-loids, and for meat processing wastewater, they have maximum effect in the pH range 5.0to 5.5 (Russell and Cooper, 1981; Travers and Lovett, 1984). Synthetic polyelectrolytes areoften added to the wastewater after addition of the salts to assist in the formation of large,stable, dense flocs that are easily separable from the liquid and to aid subsequent dewater-ing. An advantage of using Fe3� and Al3� salts is that they also precipitate out much of thephosphorus from wastewater. A disadvantage is that these salts can make the recoveredsolids unsuitable for use as animal feed.

Anionic coagulants interact with positively charged proteins and, in contrast tocationic coagulants, are effective at pH levels on the acid side of the isoelectric point. Themost useful anions include sodium hexametaphosphate at pH 3.5 (Cooper et al., 1983), lig-nosulfonate at pH 3 (Foltz et al., 1974), and sodium alginate at pH 3.5–4.5 (Russell et al.,1984). These anionic coagulants remove hemoglobin, which can make up a large propor-tion of the soluble organic load in wastewater from meat processing. As well, these coag-ulants are nontoxic, so the recovered solids can be used in animal feeds.

Hemoglobin and other proteins can also be removed with a two-stage pH process(Cooper et al., 1982). The wastewater is acidified to pH 3, which precipitates some proteinsand irreversibly splits the hemoglobin molecule into globin subunits. The pH is then raisedwith calcium hydroxide, and the globin units precipitate at their isoelectric point of pH 6.5.To effect good floc formation, the pH is raised to between 8 and 9 and an anionic poly-electrolyte is added. Raising the pH to 9 with calcium hydroxide can also remove much ofthe phosphorus.

Generally, 40% to 70% of the TKN, 50% to 80% of the COD, and more than 80%of the fat can be removed from meat processing wastewater by the physicochemicaltreatment methods discussed. Depending on the chemicals used, a large proportion ofthe phosphorus can also be removed. Comparative performance data are given in Table13.

In the meat industry, physicochemical treatment is generally applied before biologi-cal treatment or discharge to a public sewer. It is particularly suitable where land area islimited and in situations where solids can be incorporated into a saleable product like meatand bone meal. A disadvantage of chemical treatment is the high cost of the chemicals.Also, the chemicals have the potential to create downstream problems. For example, theuse of sulfur-containing chemicals can increase the risk of odor and corrosion if thewastewater is to be treated anaerobically.



C. Anaerobic Treatment

1. Overview

In anaerobic treatment systems, organic matter in the wastewater is converted to methaneand carbon dioxide in the absence of oxygen. Three main groups of bacteria are involved:hydrolytic and fermentative bacteria (also known as acidogenic bacteria), acetogenic bac-teria, and methanogenic bacteria (Fig. 7).

The hydrolytic/fermentative bacteria hydrolyze fats, proteins and complex carbohy-drates into subunit fatty acids, amino acids, and simple sugars, and then ferment these toshorter chain fatty acids, acetic acid, formic acid, alcohols, hydrogen, and carbon dioxide.

654 van Oostrom

Table 13 Performance Comparison of Different Methods for Chemically Treating a Sample of MeatProcessing Wastewater (Units are g·m�3)

Effluent

Isoelectric pH Two-stage Lignosulfonate HexametaphosphatePollutant Influent 4.5 pH adjustment process process

COD 2240 950 890 750 580Soluble COD 1440 880 750 670 500TKN 165 100 95 70 60Soluble TKN 130 90 80 65 50NH3-N 10 10 10 10 10Fat 250 55 20 35 30

Source: Data are from Cooper et al., 1983.

Figure 7 The main transformations in the anaerobic microbial conversion of organic wastes tomethane and carbon dioxide.


The acetogenic bacteria convert fatty acids and alcohols to acetic acid, formic acid, and hy-drogen gas, whereas methanogenic bacteria use the acetic acid, formic acid, and hydrogengas as substrates for methane production.

Anaerobic treatment depends on complex interactions between bacterial activities. Forexample, the acetogens produce the acetic acid and hydrogen required by the methanogensand consume various fatty acids that are toxic to the methanogens. In return, the methanogensremove hydrogen, which is toxic to the acetogens. A balance between microbial populationsis essential for the stability and performance of an anaerobic treatment system. Anaerobictreatment performs best around pH 7, and when there is a high level of bicarbonate alkalin-ity to buffer the effects of organic acid production. Temperature is also important. The rateof anaerobic digestion at the normal temperature of meat processing wastewater (20° to35°C) is usually satisfactory, but digestion is more rapid at higher temperatures.

Meat processing wastewater is well suited to biological treatment, as it contains allthe nutrients required for microbes to grow. The fat and protein it contains are rapidly de-graded. Anaerobic treatment processes commonly achieve removal rates of 70% to 90% forCOD and BOD5.

The main advantage of anaerobic treatment is its low operating cost, due to lowsludge production and low energy requirements. For every unit of COD removed anaer-obically, only about 5% to 15% ends up as sludge, contrasting with about 40% to 60%for aerobic biological treatment and 100% for physical and physicochemical treatment.The waste sludge from anaerobic and aerobic treatment consists of bacterial biomass, aswell as any refractory and slowly biodegradable particulate material present in the pro-cess influent.

Anaerobic treatment requires little or no energy input and is a net producer of energyif the biogas is recovered as fuel. Methane yields of up to 0.23 kg per kg COD removedhave been reported for the anaerobic treatment of meat processing wastewater (Metznerand Temper, 1990; Borja et al., 1995), 92% of the theoretical maximum. This yield trans-lates to 12.8 MJ of energy per kg of wastewater COD removed.

Anaerobic treatment does not remove nitrogen or phosphorus. A further disadvan-tage is that it rapidly reduces organic forms of nitrogen and sulfur to ammonia and hydro-gen sulfide, which can be toxic to fish and other aquatic organisms. The hydrogen sulfidecan also cause an odor nuisance and corrosion of equipment. (In anaerobic treatment, sul-fides are also produced by bacterial reduction of sulfates in the wastewater.) Therefore,anaerobically treated meat processing wastewater usually requires further treatment beforedischarge to waterways.

Anaerobic treatment of meat processing wastewater is generally applied as a treat-ment step before discharge to a public sewer, aerobic biological treatment, or land applica-tion.

2. Anaerobic Lagoons

Anaerobic lagoons are a popular method of treating meat processing wastewater becauseof their simplicity, reliability, and low cost. They are typically between 3 and 6 m deep,with an operating volume that equates to a loading rate of 0.1 to 0.4 kg BOD5�m�3�day�1

(approx. 0.2 to 0.8 kg COD�m�3�day�1) or a hydraulic retention time of 5 to 15 days.Anaerobic lagoons are shaped to suit their site, but generally the greater the

length:width ratio—where the influent and effluent are at opposite ends—the better the per-formance because short-circuiting of flow is minimized. Sometimes several anaerobic la-goons are operated in parallel or in series.



A layer of sludge consisting of denser wastewater solids and anaerobic biomassforms at the bottom of the lagoon; fat and other floating solids form a scum (Fig. 8a).Gas belches from the sludge layer causing localized mixing of sludge with the super-natant, particularly near the inlet end of the lagoon, and this mixing aids in the removalof soluble BOD5 from the wastewater. Typically, sludge needs to be removed every 5 to10 years.

Anaerobic lagoons have the potential to cause an odor nuisance, but this risk can beminimized or eliminated by:

minimizing wastewater sulfur loading: changing to a low-sulfate water supply (Chit-tenden et al., 1977) and reducing the use of sulfur-containing chemicals in pro-cessing and wastewater treatment can reduce odor

promoting a stable scum layer on the lagoon: scum reduces odor production; the rateof scum development can be enhanced during lagoon commissioning by tem-porarily discharging an increased quantity of paunch contents and fat to the lagoon

covering the lagoon with a membrane: this allows the odorous biogas to be collectedand burned as fuel or simply flared (Chittenden et al., 1977; Dague et al., 1990)

making the lagoon as deep as possible, because reducing the surface area helps min-imize odor release

656 van Oostrom

Figure 8 Examples of anaerobic treatment technologies.


3. High-rate Anaerobic Systems

High-rate systems are characterized by high densities of anaerobic microorganisms (typi-cally 4000 to 8000 g�m�3 measured as suspended solids), allowing BOD and COD loadingrates typically 5 to 20 times greater than those of anaerobic lagoons. The relatively smallsize of high-rate systems makes them most suitable where land area is limited, and biogascollection and/or strict odor control are objectives.

In suspended-growth systems, the biomass in the reactor is maintained in suspensionas flocs or granules. Suspended-growth technologies differ in the way that they maintainhigh concentrations of suspended biomass in the digester. In one such technology, theanaerobic contact process (Fig. 8b), the digester contents are stirred. The biomass washedout with the effluent is recovered by gravity in a clarifier and some is returned to the di-gester. The solids-laden effluent from the digester must be degassed (e.g., by applying avacuum) to effect good biomass settling in the clarifier (Steffen and Bedker, 1962; Steboret al., 1990).

In the upflow anaerobic sludge blanket process (Fig. 8c), the wastewater passes up-ward through the sludge blanket at a rate that prevents washout of the biomass, and thusavoids the need for a separate clarifier tank.

In attached-growth systems, the biomass is immobilized on media that have a highsurface-to-volume ratio. These systems include anaerobic biofilters (Fig. 8d; Metzner andTemper, 1990) and fluidized bed reactors (Borja et al., 1995).

Anaerobic sequencing batch reactors work with repeated cycles of fill, react, settle,and decant. (The aerobic counterpart of this process is illustrated in Figure 10.) This rela-tively new process shows much promise for the treatment of meat processing wastewater(Morris et al., 1998).

Increasing concerns about odor from anaerobic lagoon systems will probably resultin an increased use of high-rate enclosed anaerobic systems in the meat industry. However,compared with lagoons, such high-rate systems have higher capital and operating costs, andtheir performance can be more sensitive to variations in organic loading.

D. Aerobic Treatment and Biological Nitrogen Removal

1. Overview

Before anaerobically treated wastewater is discharged to waterways, it is treated aerobi-cally to remove most residual BOD5 and suspended solids, and to oxidize ammonia and hy-drogen sulfide to less harmful nitrate and sulfate. Increasingly, aerobic treatment is beingcoupled with specialized anoxic treatment to biologically remove nitrogen as well. Aero-bic treatment is also commonly used to treat meat-processing wastewater before landapplication.

If the wastewater is not properly treated, the BOD5 and suspended solids can resultin oxygen depletion and produce turbidity and color in the receiving waters. Moreover, am-monia and hydrogen sulfide can deleteriously affect aquatic life. Discharges of nitrogenand phosphorus nutrients are increasingly being controlled because they contribute to algalblooms and other undesirable biological growths in waters.

Aerobic biological treatment systems can be designed for carbonaceous BOD5 re-duction only, but for meat processing effluent they are usually also used for ammonia oxi-dation (nitrification), and sometimes nitrogen removal by nitrate reduction (denitrifica-tion). Sulfide will be rapidly oxidized in these systems without need for special design.



2. Main Microbial Processes

a. Carbon Removal. During aerobic treatment, heterotrophic bacteria remove or-ganic matter (measured as COD and BOD5) from the wastewater by two main methods: bybiological oxidation to carbon dioxide and water in the presence of oxygen, and by incor-poration into cell biomass, which is subsequently removed as sludge.

About 60% to 70% of the COD taken up by the heterotrophic bacteria is incorporatedinto biomass, while the balance is respired to provide the energy for cell synthesis (Eq. 1).

Organic substrate � nutrients � O2

→ CO2 � bacterial cells � other end products(1)

As well as growth, there is biomass decay by respiration using cell biomass as an energysource. Decay of biomass under aerobic conditions is illustrated in Equation 2.

Bacterial cells � O2 → CO2 � H2O � NH4� � energy (2)

Taking growth and decay into account, the proportion of wastewater COD and BOD5 con-verted into cell biomass depends on how long the biomass is retained in the aerobic treat-ment system. The longer sludge stays in the system, the less sludge is produced due to celldecay processes. Because cell decay consumes oxygen, a cost of reducing sludge produc-tion is the price of supplying more oxygen.

b. Nitrification. Nitrification is carried out by specialized bacteria that sequen-tially oxidize ammonium to nitrite, and then to nitrate. Two different groups of nitrifyingbacteria are involved: ammonium oxidizers, and nitrite oxidizers (Eq. 3). These slow-grow-ing bacteria are autotrophs: they use the energy derived from the oxidation of inorganic ni-trogen compounds to fix inorganic carbon (carbon dioxide).

NH4� Ammoniumoxidizers→ NO2

� Nitrite oxidizers→ NO3� (3)

The stoichiometry for complete nitrification including cell synthesis is (USEPA, 1993):

NH4� � 1.89O2 � 0.081CO2

→ 0.016 C5H7O2N � 0.98NO3� � 0.95H2O � 1.98H�

(4)

where C5H7O2N represents the ratio of these elements in new bacterial cells.

On a weight basis, each gram of ammonium nitrogen removed requires 4.3 g of oxy-gen, produces about 0.13 g of nitrifying organisms, and consumes 7.1 g of alkalinity (mea-sured as CaCO3) through the production of hydrogen ions.

Nitrification occurs between 4° and 45°C, with an optimal temperature between 30°and 35°C. The optimal pH is between pH 6.5 and 8. Ammonia oxidation causes thewastewater pH to decline (by consuming alkalinity), which in turn can inhibit nitrification.To ensure all the ammonia in a meat processing wastewater is oxidized, it may be neces-sary to add alkalinity by dosing the wastewater with hydrated lime. Nitrification in meatprocessing wastewater is particularly sensitive to pH because of the high nitrogen contentof the wastewater. This sensitivity is due to the fact that high pH increases the concentra-tion of the un-ionized form of ammonia and low pH increases the concentration of un-ion-ized nitrous acid. Both un-ionized ammonia and nitrous acid can inhibit nitrification.

c. Denitrification. Biological denitrification reduces nitrate or nitrite primarily tonitrogen gas (N2) but also to nitrous oxide gas (N2O). A broad range of bacteria can ac-complish denitrification. Denitrifying bacteria are heterotrophic and therefore obtain theirenergy and carbon from organic compounds.

658 van Oostrom


Denitrification of nitrate to N2 is approximately represented by the following equa-tion (McCarty et al., 1969), where methanol is the illustrative carbon source:

NO3� � 1.08CH3OH � H�

→ 0.065C5H7O2N � 0.47N2 � 0.76CO2 � 2.44H2O(5)

On a weight basis, each gram of nitrate nitrogen removed consumes about 2.9 g of COD(net), and produces about 3.5 g of alkalinity and 0.5 g of denitrifying bacteria.

Denitrification is an anoxic process. In wastewater treatment, anoxic conditions aredefined as the absence of oxygen and, unlike anaerobic (fermentative) conditions, the pres-ence of an alternative terminal electron acceptor for respiration, such as nitrate or nitrite.However, denitrification is considered under aerobic treatment because it depends on theoxygen-dependent formation of nitrate (or nitrite, not shown) (Eq. 5).

3. Aerobic Treatment Technologies

a. Aerobic Lagoons. Naturally aerated lagoons, often called oxidation ponds, arepopular in the meat processing industry because of their low cost and good reliability (Fig.9a). The lagoons are typically 1 to 1.5 m deep and commonly follow an anaerobic treatmentstage. To maintain aerobic conditions throughout much of their depth, they must have a rel-atively low organic loading rate (60–120 kg BOD5�ha�1�day�1) and natural oxygenation byalgal photosynthesis and wind-aided diffusion from the atmosphere. In practice, oxygenconcentrations and pH in these lagoons fluctuate greatly with diurnal and season variationsin algal activity.

Oxidation ponds can reduce soluble BOD5 to low levels (�20 g�m�3); however, acommon problem is that they are able to support a high algal population, the biomass ofwhich can contribute to high levels of suspended solids and associated BOD5 in the dis-charged effluent.

Mechanically aerated lagoons (Fig. 9b) are also popular and can be used in variouscombinations with oxidation ponds. Mechanical aerators oxygenate the wastewater by ag-itation or by introducing fine air bubbles into the wastewater. Because these lagoons relylargely on mechanical oxygenation, they can be deeper than naturally aerated lagoons. Theenergy requirements is typically about 1 kWh per kg of oxygen supplied.

In one treatment configuration, high concentrations of suspended solids (TSS) dis-charged from a mechanically aerated lagoon are removed in an oxidation pond (Table 14).Aerated lagoons, when operated with a long hydraulic retention time to prevent washout ofnitrifying bacteria, can oxidize a large proportion of the ammonium in the wastewater. Ox-idation ponds can also effect some nitrification. However, conventional aerobic lagoon sys-tems seldom remove more than 40% of the influent total nitrogen, as conditions in such la-goons do not favor denitrification, which requires readily biodegradable carbon compounds(e.g., fatty acids) and the absence of oxygen.

b. High-rate Systems. Like high-rate anaerobic processes, high-rate aerobic treat-ment systems are characterized by high densities of microorganisms and can be dividedinto suspended growth systems (activated sludge processes) and attached growth systems.

Activated sludge treatment: The single-stage activated sludge process (Fig. 9c) is theaerobic counterpart of the anaerobic contact process, and consists of an aerated basin (atank or lagoon) and a clarifier. Oxygen is supplied by mechanical aerators or by blowingair through diffusers in the base of the basin. The sludge age (also called the mean cell res-idence time) in the system can be controlled by sludge wasting, to effect BOD5 removalonly or to achieve nitrification as well.



660 van Oostrom

Figure 9 Examples of some aerobic wastewater treatment technologies.

As with aerated lagoons, alkalinity dosing may be required to oxidize all of the am-monia in a meat processing wastewater.

Figure 9d shows a continuous-flow activated sludge process incorporating an anoxicbasin followed by an aerated basin. The process is designed to enhance nitrogen removalby recycling a proportion of the effluent from the aerated basin to the anoxic basin. In theanoxic basin, which is not aerated but gently mixed, the low-BOD, high-nitrate recyclestream mixes with the return sludge and a high-BOD influent. This creates ideal conditionsfor denitrification: nitrate, microorganisms, and a biodegradable carbon source are all pre-sent, and oxygen is absent.

The sequencing batch rector (SBR) is an alternative activated sludge process that isparticularly suitable for nitrogen removal (Fig. 10). Unlike continuous-flow activated



Table 14 Characteristics of the Effluent from an Anaerobic Lagoon, Aerated Lagoon, andan Oxidation Pond Serially Treating Beef Slaughterhouse Wastewater (Data are mean �standard deviation for 32 samples)

Anaerobic lagoon Aerated lagoon Oxidation pond~12 days HRT ~10 days HRT ~10 days HRT

BOD5 (g·m�3) n.d. n.d. 15 � 12COD (g·m�3) 555 � 115 380 � 68 145 � 54TSS (g·m�3) 175 � 61 230 � 86 30 � 17TKN (g·m�3) 210 � 22 75 � 30 50 � 22NO3-N (g·m�3) 0 100 � 38 83 � 39NO2-N (g·m�3) 0 6.1 � 12 2.4 � 2.8NH3-N (g·m�3) 190 � 24 60 � 29 45 � 21Alkalinity (g·m�3 as CaCO3) 910 � 63 115 � 225 111 � 150pH 6.9 � 1.4 5.9 � 1.2 6.3 � 1.8Total nitrogen removal (%) n.d. 13.6 � 9.2 26.5 � 14

HRT, hydraulic retention time.n.d. not determined.Averaged temperatures were 17.1�C for the anaerobic lagoon, 16.6�C for the aerated lagoon and 14.2�C forthe oxidation pond.Source: Data are from Russell and Cooper 1992.

Figure 10 Operation of a sequencing batch reactor for nitrogen removal.


sludge plants, which spatially separate the treatment steps, an SBR treats the effluent inbatches using one basin for all steps. The conditions within the basin are changed with time,and the sequence of steps is repeated in cycles. An SBR typically operates with 1 to 4 cy-cles per day. Flow balancing must be provided by an upstream equalization tank or lagoon,or by operating two or more SBRs in parallel. A continuously fed variant of the SBR(Young, 1988) avoids this requirement.

An important advantage SBRs have over continuous-flow systems is flexibility. Pro-cess changes can be made by simply adjusting the cycle conditions, whereas to make sim-ilar changes in a continuous process can require major equipment modifications such as re-sizing of basins.

In addition to nitrogen removal, an important benefit of denitrification in activatedsludge systems is the recovery of half the alkalinity lost during nitrification, so reducing oreliminating the need for alkalinity dosing. Denitrification also makes the process more en-ergy efficient: the oxygen in nitrate and nitrite is used to oxidize organic carbon and so isnot wasted.

Activated sludge systems can be used to treat primary-treated wastewater, but thistreatment option produces more sludge and requires more energy than if the wastewater isfirst treated anaerobically to remove much of the organic carbon. However, if total nitro-gen removal is an objective, thorough anaerobic pretreatment will not leave enoughbiodegradable organic carbon to support denitrification. Minimizing the organic load onthe activated sludge plant, while still supplying enough organic carbon, can be achieved bypartial anaerobic pretreatment (Subramaniam et al., 1994; Slaney and van Oostrom, 1997).

Activated sludge treatment can remove more than 90% of the nitrogen and COD inmeat processing wastewater, and can also be operated to remove the phosphorus biologi-cally (Subramaniam et al., 1994).

Attached-growth systems: A common form of attached-growth system is the trick-ling filter. Trickling filters consist of a 4 to 10 m deep bed of porous media such as rocksor plastic packing (Fig. 9e). The wastewater is applied to the surface of the bed and trick-les downwards through the media, to which the microorganisms are attached. In meatprocessing, trickling filters are sometimes used as highly loaded roughing filters for thepreliminary removal of BOD5 before activated sludge treatment (e.g., Frose and Kayser,1985).

c. Constructed Wetlands. Over the past two decades, wastewater treatment sys-tems involving wetland plants have become popular worldwide for secondary or tertiarywastewater treatment (Fig. 11). For the treatment of meat processing wastewater, wetlandscan be useful as a final effluent “polishing” step before discharge to surface waters.

Surface-flow wetlands consist of a shallow pond in which wetland plants grow eitherrooted in the soil base of the wetland or floating in a raft on the water surface. In subsur-face-flow wetlands, the wastewater flows through a bed of porous soil, sand, or gravel, inwhich the wetland plants are rooted.

Wetland systems reduce the pollutants in wastewater by a complex variety of bio-logical, chemical, and physical processes associated with the plants, microorganisms, sub-strates, and sediments. Generally, both aerobic and anoxic zones occur in wetlands. For thefinal treatment of meat processing effluent, wetlands can outperform tertiary wastewaterponds, producing effluents low in TSS and BOD5 (�20 g�m�3) (van Oostrom and Cooper,1990). Gravel bed subsurface-flow wetlands generally produce higher quality effluentsthan surface-flow wetlands, but the gravel is prone to blocking by solids, eventually forc-ing the wastewater to flow across the gravel surface.

662 van Oostrom


Decaying plant matter can provide a carbon source for denitrification in surface-flowwetlands, and nitrogen reductions of up 75% have been be achieved when the influent ni-trogen was predominantly nitrate (van Oostrom, 1995).

E. Disinfection

1. Infectious Microorganisms

An important route for the spread of infectious illness in humans is the fecal-oral route, bywhich a disease-causing microorganism that is shed in the feces of an infected animal orhuman is subsequently ingested, causing disease. A wide variety of microorganisms can beisolated from animal feces. Some of these are zoonotic, defined as harmful (pathogenic)microorganisms acquired from animals and that have the potential to cause disease inhumans.

Zoonoses that can be present in meat industry wastewaters include a number of bac-teria, such as species of Salmonella, toxigenic strains of Escherichia coli (including E. coliO157:H7), Campylobacter jejuni and C. coli, Yersinia enterocolitica, and Listeria mono-


Figure 11 Two types of constructed wastewater wetland.


cytogenes. Viral zoonoses have not been linked to meat industry wastes, but enteric para-sites such as Giardia and Cryptosporidium can be found from time to time in ruminant fe-ces. Wastes from slaughterhouses contain high concentrations of fecal matter and thereforecould contain some of these parasites. It is possible for healthy animals to be symptomlesscarriers and shed zoonoses (Acha and Szyfres, 1987; Donnison and Ross, 1999).

Although large numbers of microorganisms need to be ingested to cause some infec-tious diseases (for example �104 Salmonella cells), this is not always the case. Low-dosezoonoses include the bacteria C. jejuni and E. coli O157:H7, and cysts of the enteric para-sites Giardia and Cryptosporidium.

For a detailed account of human wastewater microbiology, which has many parallelswith meat processing wastewater microbiology, the reader is referred to Wastewater Mi-crobiology (Bitton, 1999). Many of the infectious microorganisms that are found in meatindustry wastes are also found in human wastewater.

2. Fecal Indicator Microorganisms

When meat-processing wastewater is discharged to the environment, the processor is oftenrequired to measure concentrations of fecal microorganisms in the wastewater and some-times in the receiving environment to ensure there is no health risk. As zoonoses may ormay not be present, indicator microorganisms are usually measured to determine the mi-crobiological risk that might result from the discharge. An indicator microorganism is anorganism that is consistently found in fecal wastes, so that its presence in an environmen-tal sample demonstrates that the environment has been fecally polluted.

Fecal coliform bacteria are widely used as an indicator. However, a more specific in-dicator of fecal pollution is E. coli, a component species of the fecal coliform group. In tem-perate climates, the recovery of E. coli from an environmental sample is almost certainproof that that the environment had received a recent input of fecal matter. For fresh watersused for recreation, the Environmental Protection Agency has set criteria based on con-centrations of E. coli (USEPA, 1986).

3. Disinfection Treatments

Untreated slaughterhouse wastewater can contain up to a hundred million fecal coliformsper 100 ml, and the majority of these are usually E. coli. Treatment in a series of lagoonswith a total residence time of 20 to 30 days typically reduces the fecal coliform concentra-tion to 10,000 per 100 ml. Such lagoons, and other biological treatments, rarely reduce fe-cal coliforms to below 1,000 per 100 ml. The reduction that does takes place is due to acombination of predation by other organisms and removal in sediments and sludge, but ul-traviolet (UV) radiation from sunlight in lagoon systems also plays a role in disinfection(Davies-Colley et al., 1999).

In some situations, natural disinfection processes are inadequate or too slow, and insuch cases effluents may be subjected to disinfection treatment. For light-colored effluentsthat are low in suspended solids, UV radiation at 256 nm inactivates fecal coliform bacte-ria but may not be as effective for all potential zoonoses, particularly enteric parasite cysts.Before UV irradiation, wastewater is often sand-filtered to improve the efficiency of theUV treatment, and with sufficient filtration cysts may be removed.

Chlorination is widely used to disinfect human sewage. However, chlorination is notusually recommended for meat processing effluents because even after treatment these ef-fluents still contain organic compounds that can react with chlorine to form toxicorganochlorines that present their own environmental risks (Donnison, 1996).

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F. Land Treatment

Where enough suitable land is available, irrigation of meat processing wastewater can bean attractive disposal option, because it avoids a point source discharge into waterways andcan increase the productivity of the land by providing nutrients and water for plants.

Pollutants in the wastewater are removed by plant uptake as well as by a range ofphysical, chemical, and microbial processes in the soil. In this role the soil is a biologicalfilter.

Irrigation of meat-processing effluent is not without risks. Wastewater irrigation sys-tems must be designed and managed to avoid such problems as groundwater contamina-tion, nuisance odors, aerosol drift, surface runoff into waterways, and degradation of soilstructure and quality.

Successful land treatment relies on balancing the amount of wastewater applied withthe nutrient uptake by the plants, and ensuring that any excesses do not detrimentally affectthe environment (Russell et al., 1991). For irrigation of meat processing wastewater, the an-nual application rate, and therefore the area required, is usually governed by the nitrogenloading. Nitrogen has the potential to cause nitrate contamination of groundwater if appliedin excess. Unlike ammonium and organic nitrogen, nitrate is highly mobile in soils. It maybe either formed in the soil from the oxidation of organic nitrogen and ammonium presentin the wastewater, or it may already be present in aerobically treated wastewater. Nitratethat is not taken up by plants eventually leaches to groundwater or is converted to nitrogengases by denitrification and returned to the atmosphere (Fig. 12).

Plant uptake of nitrogen followed by nitrogen removal in plant or animal products isthe main sink for the applied nitrogen and depends on the cover crop and how it is man-aged. For example, meat-processing wastewater can be applied to grazed pasture. Because


Figure 12 Scheme showing the main nitrogen inputs, transformations and outputs that occur atwastewater irrigation sites.


grazing animals recycle about 90% of their nitrogen intake back onto pasture, only 10% ofthe nitrogen taken up by plants is eventually removed from the site in the form of animalproducts. More nitrogen can be removed if the pasture or crop is not grazed but instead har-vested and taken off-site (e.g., as hay or silage). Harvesting allows higher wastewater ni-trogen application rates.

After only primary treatment, meat processing effluent can be irrigated without ad-verse effects on plant growth, and the high carbon content of such wastewater can promotehigh rates of denitrification in the soil (Russell et al., 1993). However, lagoon treatment andstorage of the wastewater is commonly provided before irrigation to avoid potential odorproblems and allow irrigation to be delayed during adverse conditions (e.g., during rain-fall). Lagoon pretreatment and storage also reduces the microbial risk associated with irri-gation of the wastewater (Donnison and Ross, 1992).

Various types of flood and spray irrigation systems have been used for applyingmeat processing wastewater to land. Factors important in selecting an irrigation systeminclude terrain and soil characteristics, crop type, capital and operating costs, aerosol pro-duction, and the precision and control of application. Spray irrigation systems providemore control over the application rate than flood systems but have a greater risk ofaerosol production.

The volume of wastewater applied during each irrigation event should be less thanthe water-holding capacity of the topsoil to minimize the quantity of nitrogen washedthrough the root-zone and out of reach of the plants. The wastewater should be distributedevenly and applied slowly to maximize wastewater renovation and avoid surface runoffinto waterways.

The environmental effects of wastewater components other than nitrogen also needto be considered. Many soils have the capacity to store large quantities of applied phos-phorus, and for such soils phosphorus leaching losses are very low. However, excess phos-phorus application will eventually saturate the soil and phosphate breakthrough to ground-water will then occur. This usually takes several decades at normal application rates formeat-processing wastewater.

Wastewater with high concentrations of sodium relative to calcium and magnesiumcan cause an accumulation of sodium in the soil and adversely effect soil structure and plantgrowth. Meat-processing wastewaters generally have favorable salt ratios. However, someprocesses (e.g., salt curing) produce effluents high in sodium. Where such effluent is irri-gated, the effect on soil structure should be closely monitored.

V. SOLID WASTE MANAGEMENT

As we have already seen, the main organic solid wastes associated with meat processingare animal tissue wastes, fecal and gut content solids, solids and slurries recovered from pri-mary and physicochemical wastewater treatment systems, and sludges produced from bio-logical wastewater treatment systems. In the past it was common practice to landfill at leastsome of these wastes, but with increased restrictions and costs associated with this practice,landfilling is now uncommon.

Obviously the wastes containing predominantly animal tissues should be rendered ifpossible, to recover some value from these materials and avoid disposal costs. Tissuewastes collected in the plant and recovered from wastewater screens can be transported di-rectly to rendering. Slurries recovered from physicochemical treatment systems, if suitablefor use as animal feed, generally need to be dewatered before they can be further processed(Fig. 6).

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Fecal wastes, gut contents and biological treatment sludges are unsuitable for ren-dering, but their high nutrient and organic content makes them useful as a fertilizer and soilconditioner. Such wastes are often applied directly onto land, either as a slurry or as dewa-tered solids. However, these solids generally have a high oxygen demand and can carrypathogens and weed seeds. Like wastewater application, the land application of suchwastes must be managed to avoid odor, leaching to groundwater, runoff, and adverse ef-fects on animal and human health.

Where it is not possible to avoid such impacts, or where direct land application is notpermitted, the solids are commonly treated by anaerobic digestion or composting. Thesetreatments biologically stabilize the solids, to reduce or eliminate odor generation and theattraction of flies and vermin.

Anaerobic digesters range from simple sludge lagoons to high-rate systems usingfully enclosed heated tanks. Anaerobic solids digestion has the potential to recover methanefor energy (Umstadter, 1986). An important disadvantage is that much of the nutrient con-tent from the solids is released into the liquid phase during digestion, creating a high-strength liquid waste requiring further treatment.

Composting is a popular method of stabilizing waste solids from meat processing. Inthe composting process, bacteria and fungi oxidize biodegradable compounds such as fatsand proteins into water and carbon dioxide, leaving a stable organic residue of mature com-post. The process is accelerated by high temperatures and moist, aerobic conditions withinthe composting mass. Aerobic conditions are also important for odor minimization. Thehigh temperatures (50° to 80°C) achievable during composting kill pathogens and help toensure a safe product.

A variety of composting methods can be used to stabilize meat processing wastes,from simple windrow composting techniques, to high-rate forced-aeration methods (Kee-ley and Skipper, 1988; van Oostrom, 1993).

Regardless of the type of composting process, the first step is to ensure that the wastehas a porous and open structure to assist aeration. This usually involves draining excess liq-uid from the waste and mixing the waste with a bulking agent, such as sawdust, choppedstraw, crushed pine bark, or recycled compost.

In windrow composting, the waste is formed into long piles. Oxygen is supplied bymechanically turning the pile, and by passive and convective aeration. In forced-aerationcomposting, the compost is placed in a pile or vessel over an air distribution system, andoxygen is supplied by forcing air through the composting mass using a fan. With thismethod, oxygen supply and compost temperatures can be controlled more effectively thanwith windrow composting, resulting in faster stabilization and a reduced potential for odorgeneration.

All organic waste solids from meat processing operations can be stabilized by com-posting. Even high-fat semi-liquid dissolved air flotation (DAF) solids can be successfullycomposted if they are mixed with enough bulking agent. Simple windrow techniques aremore suitable for composting paunch manure and fecal solids, although they can treat mostmeat processing wastes. Forced-aeration systems are particularly suited to stabilizingwastes containing animal tissues, which have a very high oxygen demand and a high po-tential for odor generation.

Several weeks or months are needed for composting to produce a mature product, de-pending on the process used. A variety of products can be made from meat-processingwaste compost, ranging from a basic soil conditioner to a high-quality potting medium (vanOostrom et al., 1988).



VI. CONCLUDING REMARKS

Although the meat processing industry has made major advances in waste reduction andby-product recovery, it remains a large producer of waste. To be economically competi-tive and to meet increasingly strict environmental standards, meat processors face the on-going challenge of reducing waste at source, finding new uses for waste materials, andselecting cost-effective methods for waste treatment and disposal. In this chapter we haveidentified the main sources and characteristics of meat processing wastes, and investi-gated some of the methods that can be used to reduce, recover, treat, and dispose of thesewastes.

ACKNOWLEDGMENTS

The author is grateful to his former employer, MIRINZ Food Technology and Research, forpermission to present unpublished MIRINZ data. He also thanks Drs. Andrea Donnisonand Patricia Johnstone for their valuable assistance in preparing the manuscript.

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