/ASTEWATER

REATMENT

WASTEWATER MICROBIOLOGY

Role of Microorganisms

Classification of Microorganisms

Some Microbes of Interest in Wastewater Treatment

Bacterial Biochemistry , I `

Decomposition of Waste

Population Dynamics

CHARACTERISTICS OF DOMESTIC VW\STEWATER

Physical Characteristics of Domestic Wastewater ,

Chemical Characteristics of Domestic Wastewater

Characteristics of Industrial Wastewater

ON~SITE DISPOSAL SYSTEMS

Without Water Carriage

With Water Carriage I

MUNICIPAL WASTEWATER TREATMENT SYSTEMS

Pretreatment of Industrial Wastes

UNIT OPERATIONS OF PRETREATMENT

Bar Racks '

Grit Chambers

Comminutors

Equalization

PRIMARY 'TREATMENT

UNIT PROCESSES UF SECONDARY TREATi\'lliN'l`

Overview

5-I WASTEWATER MICROBIOLOGY

Role of Microorganisms

The stabilization of organic matter is accomplished biologically using a variety of

microorganisms. The rttieroorganisms convert the colloidal and dissolved carbona-

ceous organic matter into various gases and into protoplasm. Because protoplasm

has ;t specihc gravity slightly greater than that of water, it can be removed from the

treated liquid by gravity settling.

lNTR()DUCTl()N TO ENVIRONMENTAL ENGINEEKINLJ

h roto lasm produced from the

It is important to note that unless t e p p

matter is removed from the solution, complete treatment will not be acer

because the protoplasm, which itself is organic, will be measured as BOD in

efiiuent, lf the protoplasm is not removed, the only treatment that will be

` ' t 'al conversion of a portion of the organic

is that associated with the bac eri

originally present to v

arious gaseous end products.

Classiiication of Microorganisms

` d 'nto five broad Groups based on

By kingdoms. Microorganisms are organize i D

structural and functional differences. The groups are called kingdoms The nve king.

doms are animals, plants, prozisra, fungi, and bacfe ria_ Representative examples and

‘ ` ` ` e 5- l _

characteristics of differentiation are shown in Figur

"""` EXAMPLES

By energy and carbon source. The relationship between the source of carbon and

the Source of energy for the microorganism is important. Carbon is the basic building

glock for cell synthesis. A source of energy must be obtained from outside the cell to

enable synthesis to proceed. Our goal in Wastewater treatment is to convert both the

¢;arbon and the energy in the wastewater in the cells of microorganisms, which we

can remove from the water by settling. Therefore, we wish to encourage the growth

gf organisms that use organic material for both their carbon and energy source.

lf the microorganism uses organic material as a supply of carbon. it is called

heterorrophic. Autotrop/is require only_CO3 to supply their carbon needs.

Organisms that rely only on the sun for energy are called phototroplzs.

Clzemotrophs extract energy from organic or inorganic oxidation/reduction reac-

tions. Organozrophs use organic materials, while lit/iofrophs oxidize inorganic com~

pounds.;

By their relationship to oxygen. Bacteria also are classitied by their ability or in-

ability to utilize oxygen as a terminal electron acceptor3 in oxidation/reduction reac-

tions. Obligate aerobes are microorganisms that Hlust have oxygen as the terminal

electron acceptor. When wastewater contains oxygen and can support obligate aer-

obes, it is called aerobic.

Obligate anaerobes are microorganisms that cannot survive in the presence of

oxygen. They cannot use oxygen as a terminal electron acceptor. Wastewater that is

devoid of oxygen is called anaerobic. Faculmlive rzriaembes can use oxygen as the

terminal electron acceptor and, under certain conditions, they can also grow in the

absence of oxygen.

Under anoxic conditions, a group of facultative anaerobes called de/zitrijiers

utilizes nitrites (NOQ ) and nitrates (NO§ ) as the terminal electron acceptor. Nitrate

nitrogen is converted to nitrogen gas in the absence of oxygen. This process is called

anoxic denitrificatiofi. `

By their preferred temperature regime. Each species of bacteria reproduces best

within a limited range of temperatures. Four temperature ranges are used to classify

ll. E. Bailey and D. F. Ollis, Biochemical Engineering Fundaizzertralr, New Yorltt McGraw-Hill, p. 222,

1977.

3An organic substrate is not directly oxidized to carbon dioxide and water in a single chemical step

because there is no energy-conserving mechanism that could trap so much energyi Thus. biological

oxidation occurs in small steps. Oxidation requires the transfer of an electron from the substance being

oxidized to some acceptor molecule that will subsequently bc reduced ln most biological Systctlls. each

step in the oxidation process involves the removal of two elections and the simultaneous loss of two

protons (H+ )_ The combination ofthe two losses is equivalent to the molecule having lost two hydrogen

atoms. The reaction is often referred to as de/zydmgerzatiom The electrons and protons are not released

into the cell, but are transferred to an acceptor molecule. The acceptor molecule will not accept the

protons until it has accepted the electrons and thus it is referred to as an electron acceptor. Since thc

net result of accepting an electron and proton is the same as accepting a hydrogen atom, such acceptors

are also called hydrogen acceptors. (C. P. L. Grady and H. C. Lim. Bmlogicul lllirrawriteri Trmzzzzienf,

`l`/ifvirv(uniA/1/1/u~<1r1'<1fz.\'. New York: Marcel Dekker. l‘)S().)

bacteria. Those that grow best at temperatures below 20°C are called psychrop/tiles

Mesophiles grow best at temperatures between 25 and 4O°C. Between 45 and 6006*

the thennophiles grow best. Above 60°C, stenorhermop/tiles grow best. The gfgwtfg

, gf .___

?°`

v _rw r

ran e of acttlzazive zlzermo./tiles extends from the thermo hilic ran e into r

g P P g the

mesophilic range. These ranges are qualitative and somewhat subjective. You will

note the gaps between 20 and 25°C and between 40 and 45°C. Don’t make the

rules just aren’t that hard and fast. Bacteria will grow over a range of temperaturég

and will survive at a very large range of temperatures. For example, Escherichig

coli, classified as mesophiles, will grow at temperatures between 20 and 50°C

will reproduce, albeit very slowly, at temperatures down to 0°C, If frozen rapidly,

they and many other microorganisms can be stored for years with no significant

death rate,

Some Microbes of Interest in Wastewater

'Ireatment

Bacteria. The highest population of microorganisms in a wastewater treatment

plant will belong to the bacteria. They are single~celled organisms which use solu-

ble food. Conditions in the treatment plant are adjusted so that chemoheterotrophs

predominate. No particular species is selected as “the best."

Fungi. Fungi are multicellular, nonphotosynthetic, heterotrophic organisms. Fungi

are obligate aerobes that reproduce by a variety of methods including fission, bud-

ding, and spore fomiation. Their cells require only half as much nitrogen as bacteria

so that in a nitrogen-dencient wastewater, they predominate over the bacteriafl

Algae. This group of microorganisms are photoautotrophs and may be either uni-

cellular or multicellular Because of the chlorophyll contained in most species, they

produce oxygen through photosynthesis. In the presence of sunlight, the photosyn-

thetic production of oxygen is greater than the amount used in respiration. At night

they use up oxygen in respiration. If the daylight hours exceed the night hours by a

reasonable amount, there is a net production of oxygen.

Protozoa. Protozoa are single-celled organisms that can reproduce by binary ]?s»

siorz (dividing in two). Most are aerobic chemoheterotrophs, and they often consume

bacteria. They are desirable in wastewater effluents because they act as polishers in

consuming the bacteria.

Rotifers and crustaceans. Both rotifers and crustaceans are animals-~-aerobic,

rnulticellular chemoheterotrophs The rotifer derives its name from the apparent

1L>,i.».~L xi ir if I 1

mistake of saying that an organism that grows well at 20.5°C is a mesophile T115 '%<

rotating motion of two sets of cilia on its head. The cilia provide mobility and a

mechanism for catching food. Rotifers consume bacteria and small particles of or»

ganic matter.

Crustaceans, a group that includes shrimp, lobsters, and barnacles, are charac~

terized by their shell structure. They are a source of food for fish and are not found

in wastewater treatment systems to any extent except in underloaded lagoons Their

presence is indicative of a high level of dissolved oxygen and a very low level of

organic matter.

Bacterial Biochemistry

Metabolism. The general term that describes all of the chemical activities perf

formed by a cell is metabolism. This in turn is divided into two parts: catabolism and

anabolism. Catabolism includes all the biochemical processes by which a substrate

is degraded to end products with the release of energy.5 In wastewater treatment,

the substrate is oxidized. The oxidation process releases energy that is transferred to

an energy carrier which stores it for future use by the bacterium (Figure 5-2). Some

chemical compounds released by catabolism are used by the bacterial cell for its life

functions.

Anabolism includes all the biochemical processes by which the bacterium syn-

thesizes new chemical compounds needed by the cells to live and reproduce. The

synthesis process is driven by the energy that was stored in the energy carrier.

Decomposition of Waste

The type of electron acceptor available for catabolism determines the type of de~

composition (that is, aerobic, anoxic, or anaerobic) used by a mixed culture of mi-

croorganisms. Each type of decomposition has peculiar characteristics which affect

its use in waste treatment.

Aerobic decomposition. From our discussion of bacterial metabolism you will re-

call that molecular oxygen (OZ) must be present as the terminal electron acceptor

for decomposition to proceed by aerobic oxidation. As in natural water bodies, the

oxygen is measured as DO. When oxygen is present, it is the only terminal electron

acceptor used. Hence, the chemical end products of decomposition are primarily car-

bon dioxide, water, and new cell material (Table 5-l). Odiferous gaseous end prod-

ucts are kept to a minimum. In healthy natural water systems, aerobic decomposition

is the principal means of self-purification.

A wider spectrum of organic material can be oxidized aerobically than by any

other type of decomposition. This fact, coupled with the fact that the final end prod-

ucts are oxidized to a very low energy level, results in a more stable end prod-

uct (that is, one that can be disposed of without damage to the environment and

without creating a nuisance condition) than can be achieved by the other oxidation

systems.

Because of the large amount of energy released in aerobic oxidation, most aer-

obic organisrns are capable of high growth rates. Consequently, there is a relatively

large production of new cells in comparison with the other oxidation systems. This

means that more biological sludge is generated in aerobic oxidation than in the other

oxidation systems.

Aerobic decomposition is the method of choice for large quantities of dilute

wastewater (BOD; less than 500 mg/L) because decomposition is rapid, efficient,

and has a low odor potential. For high-strength wastewater (BOD5 is greater than

1,000 mg/L), aerobic decomposition is not suitable because of the difficulty in sup~

plying enough oxygen and because of the large amount of biological sludge pro-

duced. In small communities and ini special industrial applications where aerated

lagoons (see Section 5-7) are used, wastewaters with BOD5 up to 3,000 mg/L may

be treated satisfactorily by aerobic decomposition.

Anoxic decomposition. Some microorganisms can use nitrate (NO§ ) as the termi-

nal electron acceptor in the absence of molecular oxygen. Oxidation by this route is

called denitrification.

The end products from denitrilication are nitrogen gas, carbon dioxide, water,

and new cell material. The amount of energy made available to the cell during deni-

trifrcation is about the same as that made available during aerobic decomposition. As

a consequence, the rate of production of new cells, although not as high as in aerobic

decomposition, is relatively high.

Denitrification is of importance in wastewater treatment where nitrogen must

be removed to protect the receiving body. ln this case, a special treatment step is

added to the conventional process for removal of carbonaceous material. Denitrifif

cation will be discussed in detail later.

One other important aspect of denitrification is in relation to final clarification

ofthe treated wastewater. If the environment of the final clariiier becomes anoxic,

the formation of nitrogen gas will cause large globs of sludge to float to the surface

and escape from the treatment plant into the receiving water. Thus, it is necessary to

ensure that zinoxic conditions do not develop in the Final clarifier.

nerobic decomposition. In order to achieve anaerobic decomposition, molecu-

>xygen and nitrate must not be present as terminal electron acceptors. Sulfate

2' ), carbon dioxide, and organic compounds that can be reduced serve as termi-

electron acceptors. The reduction of sulfate results in the production of hydrogen

ide (HZS) and a group of equally odiferous organic sulfur compounds called mer-

fans.

The anaerobic decomposition (fermentation) of organic matter generally is

sidered to be a two-step process ln the first step, complex organic compounds are

dented to low-molecular-weight fatty acids (volatile acids). ln the second step,

organic acids are converted to methane. Carbon dioxide serves as the electron

:ptor_

Anaerobic decomposition yields carbon dioxide, methane, and water as the

or end products. Additional end products include ammonia, hydrogen sulfide,

mercaptans. As a consequence of these last three compounds, anaerobic decom-

.tion is characterized by an unbelievably horrid stench!

Because only small amounts of energy are released duii ng anaerobic oxidation,

amount of cell production is low. Thus, sludge production is low. We make use

his fact in wastewater treatment by using anaerobic decomposition to stabilize

lges produced during aerobic and anoxic decomposition.

Direct anaerobic decomposition of wastewater generally is not feasible for di-

waste.6 The optimum growth temperature for the anaerobic bacteria is at the

er end of the mesophilic range. Thus, to get reasonable biodegradation, we must

'ate the temperature of the culture. For dilute wastewater, this is not practical. For

centrated wastes (BGD5 greater than LOO0 mgds), anaerobic digestion is quite

ropriate_

)ulation Dynamics

:terial growth requirements. In the discussion of the behavior of bacterial cul-

»s which follows, there is the inherent assumption that all the requirements for

wth are initially present. Since these requirements are fairly extensive and strin-

t, it is worth taking a moment to recapitulate them, The following list summarizes

major requirements that must be satisfied:

¢\ terminal electron acceptor

Wacronutrients

1. Carbon to build cells

J. Nitrogen to build cells

:. Phosphorus for ATP (energy carrier) and DNA

3. Micronutrients

a. Trace metals

b. Vitamins are required by some bacteria

4. Appropriate environment

a. Moisture

b. Temperature

c. pH

Growth in pure cultures. As an illustration, let us examine a hypothetical situation

in which l,4OO bacteria of a single species are introduced into a synthetic liquid

medium. Initially nothing appears to happen. The bacteria must adjust to their new

environment and begin to synthesize new protoplasrn. On a plot of bacterial growth

versus time (Figure 58), this phase of growth is called the lag phase.

At the end ofthe lag phase the bacteria begin to divide_ Since all ofthe organ-

isms do not divide at the same time, there is a gradual increase in population. This

phase is labeled accelerated growl/1 on the growth plot.

At the end of the accelerated growth phase, the population of organisms is

large enough and the differences in generation time are small enough that the cells

appear to divide at a regular rate Since reproduction is by binary fission (each cell

divides producing two new cells) the increase in population follows in geometric

progression; l -»> 2 -> 4 -> 8 A 16 ee 32, and so forth_The populationofbacteda

INTRODUCTIUN TO ENVIRONMENTAL ENGINEERING

(P) after the nth generation is given by the following expression:

P I Po(2)" (5~ i)

where P0 is the initial population at the end of the accelerated growth phase. If we

take the log of both sides of Equation 5-l, we obtain the following:

logP = log P0 + n log2 (5-2)

This means that if we plot bacterial population on a logarithmic scale, this phase of

growth would plot as a straight line of slope n and intercept P0 at I0 equal to the end

of the accelerated growth phase. Thus, this phase of growth is called the log growth

Jr exponential growth phase.

The log growth phase tapers off as the substrate becomes exhausted or as toxic

>y»products build up_ Thus, at some point the population becomes constant either as

1 result of cessation of fission or a balance in death and reproduction rates. This is

lepicted by the stationary phase on the growth curve_

Following the stationary phase, the bacteria begin to die faster than they repro~

luce. This death phase is due to a variety of causes that are basically an extension

if those which lead to the stationary phase.

lrowth in mixed cultures. In wastewater treatment, as in nature, pure cultures

f microorganisms do not exist. Rather, a mixture of species compete and survive

/ithin the limits set by the environment. Population dynamics is the term used to

escribe the time~varying success ofthe various species in competition. It is exe

ressed quantitatively in terms of relative mass of microorganisms.7

The prime factor goveming the dynamics of the various microbial populations

the competition for food. The second most important factor is the predator-prey

zlationship. . i

The relative success of a pair of species competing for the same substrate is a

tnction ofthe ability of the species to metabolize the substrate. The more successful

aecies will be the one that tnetabolizes the substrate more completely. In so doing, it

ill obtain more energy for synthesis and consequently will achieve a greater mass.

Because of their relatively smaller size and, thus, larger surface area per unit

ass, which allows a more rapid uptake of substrate, bacteria will predominate over

ngi. For the same reason, the fungi predominate over the protozoa.

When the supply of soluble organic substrate becomes exhausted, the bacterial

pulation is less successful in reproduction and the predator populations increase.

a closed system with an initial inoculum of mixed microorganisms and substrate,

2 populations will cycle as the bacteria give way to higher level organisms which

turn die for lack of food and are then decomposed by a different set of bacteria

igure 5~4). In an open system, such as a wastewater treatment plant or a river, FIGURE 5-4

Pbpulation dynamics in a closed system. (Source: Curds, “A Theoretical Study of Factors influencing the Mitr »

bial

Population Dynamics ofthe Activated Sludge Process-I." Water Resources, vol. 7, p. 1269, l‘973_)

with a continuous inflow of new substrate, the predominant populations will change

through the length of the plant (Figure 5-5). This condition is known as dy/zaniif

equilibrium. It is a highly sensitive state, and changes in influent characteristics must

be regulated closely to maintain the proper balance of the various populations.

The Monod equation. For the large numbers and mixed cultures of mieroorgan~

isrns found in waste treatment systems, it is convenient to measure biomass rather

a wi lim we

JJU lwltiuut/\,\|ui v eu ~a.\U,..~,t,. .____._,t.

than numbers of organisms? in the log»growth phase, the rate expression for

increase is

dX

4 Z X _

dr 'L (5 3)

dX .

where E? 1 growth rate of the biomass, mg/L - t

it 1 growth rate constant, Fl

X 1 concentration of biomass, mgfls

Because of the difficulty of direct measurement of ,tt in mixed cultures,

Monodq developed a model equation that assumes that the rate of food utiliza~

tion, and therefore the rate of biomass production, is limited by the rate of enzyme

reactions involving the food compound that is in shortest supply relative to its need.

The Monod equation is

_ ii _

*L K, + 5 (5 4)

...l

where um 1 maximum growth rate constant, t

S 1 concentration of limiting food in solution, mg/L

K, 1 half saturation constant, mg/L

1 concentration of limiting food when /L 1 O.5;at,,,

The growth rate of biomass follows a hyperbolic function as shown in Fig-

ure 5-6.

Two limiting cases are of interest in the application of Equation 5~4 to waste-

water treatment systems. In those cases where there is an excess of the limiting

food, then S >> K, and the growth rate constant, pt, is approximately equal to pnm.

Equation 5-3 then becomes tirst»order in biomass. At the other extreme, when S <<

K,, the system is food-limited and the growth rate becomes zero-order with respect

to biomass, that is, it is independent of the biomass.

Equation 5~4 assumes only growth of microorganismsand does not take into

account natural die-off. It is generally assumed that the death or decay of the micro~

bial mass is a tirst-order expression in biomass and hence Equations 5-3 and 5-4 are

expanded to ,

dX _ ,u,,,SX

E ' rim “X (56)

where kd 1 endogenous decay rate constant, t`l.

“Frequently this is done by measuring suspended solids or volatile suspended solids (those that burn at

550 i 5O°C). When the wastewater contains only soluble organic matter, the volatile suspended solids

test is reasonably representative. The presence of organic particles (which is often the case in municipal

wastewater) confuses the issue completely.

QI. Monod, "The Growth of Bacterial Cultures," Ari/ina! Review nfM

FIGURE 5-6

Monod growth rate constant as a function of limiting food concentration.

If all ofthe food in the system were converted to biomass, the rate of food

utilization (HTS/dz) would equal the rate of biomass production. Because of the inefh-

ciency ofthe conversion process, the rate of food utilization will be greater than the

rate of biomass'utilization, so

as dx

-- I i* (5-6)

. dr Y dt

where Y 1 decimal fraction of food mass converted to biomass

Z , _?__m_

`eld coefhcent mg/L biomass

Y’ ’ mgfic food utilized

Combining Equafions 5-3, 5-4, and 5-6,

dS 1 ,USX

M 5

"airiii W)

Equations 5-5 and 5-7 are a fundamental part of the development of the design

equations for wastewater treatment processes.

5-2 CHARACTERISTICS OF DOMESTIC

WASTEWATER . I

Physical Characteristics of Domestic

Wastewater

Fresh. aerobic, domestic wastewater has been said to have the odor of kerosene or

freshly tumed earth. Aged, septic sewage is considerably more offensive to the ol-

factory nerves_ The characteristic rotten-egg odor of hydrogen sullide and the mer-

captans is indicative of septic sewage. Fresh sewage is typically gray in color. Septic

sewage is black.

352 iurnooucriorr TO ENVIRONMENTAL Euoiusenimo

Wastewater temperatures normally range between l0 and 20°C. ln general,

the temperature of the wastewater will be higher than that of the water supply. This

is because of the addition of warm water from households and heating within the

plumbing system ofthe structure.

One cubic meter of wastewater weighs approximately l,000,000 grams. lt will

contain about 500 grams of solids. One-half of the solids will be dissolved solids

such as calcium, sodium, and soluble organic compounds. The remaining 250 grams

will be insoluble. The insoluble fraction consists of about l25 grains of material that

will settle out of the liquid fraction in 30 minutes under quiescent conditions. The

remaining l25 grams will remain in suspension for a very long time. The result is

that wastewater is highly turbid. '

Chemical Characteristics of Domestic

Wastewater

Because the number of chemical compounds found in wastewater is almost limitless,

we normally restrict our consideration to a few general classes of compounds. These

classes often are better known by the name of the test used to measure them than by

what is included in the class. The biochemical oxygen demand (BOD5) test, which

we discussed in Chapter 4, is a case in point. Another closely related test is the

chemical oxygen demand (COD) test. C

The COD test is used to determine the oxygen equivalent of the organic matter

that can be oxidized by a strong chemical oxidizing agent (potassium dichromate)

in an acid medium. The COD of a waste, in general, will be greater than the BOD5

because more compounds can be oxidized chemically than can be oxidized biologi-

cally, and because BOD5 does not equal ultimate BOD.

The COD test can be conducted in about three hours. lf it can be correlated with

BOD5, it can be used to aid in the operation and control of the wastewater treatment

plant (WWTP).

Total Kjeldahl nitrogen (TKN) is a measure of the total organic and ammonia

nitrogen in the wastewater.” TKN gives a measure of the availability of nitrogen for

building cells, as well as the potential nitro genous oxygen demand that will have to

be satisfied. )

Phosphorus may appear in many forms in 'wastewater Among the forms

found are the orthophosphates, polyphosphates, and organic phosphate. For our

purpose, we will lump all of these together under the heading “Total Phosphorus

(as P).” `

Three typical compositions of untreated domestic wastewater are summarized

in Table 5-2. The pH for all of these wastes will be in the range of 6_5 to 8.5, with a

majority being slightly on the alkaline side of 7.0.

composition of untreated domestic wastewater

Weak Medium Strong

t (all mg/L except settleable solids)

(as CaCo3)" 50 100 200

Oz) 100 200 300

30 50 100

(as OZ) 250 500 L000

solids (SS) 100 200 350

solids, mL/L 5 10 20

Total dissolved solids (TDS) 200 500 1,000

Total Kjeldahl nitrogen (TKN) (as N) 20 40 80

Total organic carbon (TOC) (as C) 75 150 300

phosphorus (as P) 5 10 20

"To be added to amount in domestic water supply, Chloride is exclusive of contribu-

tion from water-softener backv/ash_

Characteristics of Industrial Wastewater

Industrial processes generate a wide variety of wastewater pollutants. The charac

teristics and levels of pollutants vary significantly from industry to industry The

Environmental Protection Agency has grouped the pollutants into three categories:

conventional pollutants, nonconventional pollutants, and priority pollutants. The

conventional and nonconventional pollutants are listed in Table 5-3. The priority

pollutants are listed in Table 1-3.

Because of the wide variety of industries and levels of pollutants, we can only

present a snapshot view of the characteristics. A sampling of a few industries for two

conventional pollutants is shown in Table 5»4.

A similar sampling for nonconventional pollutants is shown in Table 5 5

TABLE 5-3

EPA’s conventional and nonconventional pollutant categories

Conventional . Noncanventional

Biochemical oxygen demand (BOD5)

Total suspended solids (TSS)

Oil and grease

Oil (animal, vegetable)

Oil (mineral)

pH

Ammonia (as N)

Chromium V1 (hexavalent)

Chemical oxygen demand (COD)

COD/BOD7

Fluoride

Manganese

Nitrate (as N)

Organic nitrogen (as N)

Pesticide active ingredients (FAI)

Phenols, total

Phosphorus, total (as P)

Total organic carbon (TO

Without Water Carriage

The pit privy. Although most modem environmental engineering texts would skip

this subject, the mere existence of l0,000 of these or their modern equivalent in the

United States is just too much for us to ignore. Furthermore, the facts ofthe matter are

thatjunior engineers are the most likely candidates for designing, erecting, operating,

dismantling, and closing the beasts. '

Figure 5~7 provides most of the information you will ever want to know about

the construction of an outhouse. The slab is usually poured over Hat ground on top

of rooting paper. The riser hole is formed using l2~gauge galvanized iron. Once the

slab has set, it is lifted into place over the pit. The concrete is a l:2:3 mix, that is,

one part l’ortI;md cement, two parts sand, and three parts gravel less than 25 mm in

diameter.

'l`hc |>rinciple ofoperation of the pit privy is that the liquid materials percolate

into the soil tlwough the cribbing and the solids “dry out." A pit ofthe dimensions

shown in l"l}\lll`L5 S -7 should last a family of four about ten years Rainwater is to

be prcvuntutl from entering the pit_ A cup of kerosene at weekly intervals discour~

ages n|<>s<|ni|o hut-t-<ling, and odors can he reduced by the use of a cup of hydrated

Q Q

ig’fateftight vault. A special truck (fondly called a “honey wagon”) is used to pump ,

icipient anaerobic decomposition, vault toilets are much more odiferous than the 3

ld pit privies. Many masking agents (perfumes) and disinfectants are available to

mitigate the stench. Unfortunately, most of them have unpleasant odors themselveS_

/aste, will perform near-miracles in odor reduction. y

The chemical toilet. The airplane toilet, the coach-bus toilet, and the self-contained

oilets of recreation vehicles are all versions of the chemical toilet. The essence of

he system is a strong disinfectant chemical used to carry the waste to a holding

ank and render it inoffensive until it can be pumped from the holding tank. While

hese vehicular systems are quite effective, the chemical must be selected with an

:ye toward its impact on the treatment system which ultimately must receive it. The

:hemical toilet has not found wide acceptance in permanent installations. This is due

.o the cost of the chemical and to the impracticality of maintenance.

With Water Carriage

Septic tanks and tile fields. A typical septic tank and tile field arrangement for a

residential dwelling is illustrated in Figure 5-8. The septic tank and tile field are a

unit. Neither part will function as intended without the other.

The main function of the septic tank is to remove large particles and grease

which would otherwise clog the tile held. Heavy solids settle to the bottom where

they undergo anaerobic decomposition. Grease iioats to the surface and is trapped.

It is only slightly decomposed.

Since the septic tank is not heated, little reduction in BOD5 occurs. Rather, the

solid organic material which settles out is liquefied. It then passes to the tile field.

Since not all of the solid material can be liquefied, the tank must be pumped at peri-

odic intervals. The time interval between pumping depends on the amount of use and

the objects which find their way to the tank. Toilet paper is easily degraded; how-

ever, plastic-lined disposable diapers cannot be degraded within a reasonable time.

A family of four with young children can expect to have their septic tank pumped

every two years. A household of two may not have to have its septic tank pumped in

five or ten years of use. Grease accumulation is often the major factor in determining

the frequency of cleaning.

ln the past, the volume of the septic tank has been a function of the number of

bedrooms in the dwelling. Current practice suggests that a 24-hour hydraulic deten-

tion time at design tiow be used. ln any case, the tank should not be less than 4.0 m3

in volume.

ln the tile field, the waste Hows out of the joints between the tiles and through

the gravel layer. The gravel serves to trap some of the solids that escaped from the

septic tank. lt also provides a storage area for holding the liquid while it seeps into

the soil. Bacteria on the gravel degrade some of the trapped particulate matter. Bac-

teria in the soil aerobically degrade the liquefied organic material. The treated water

percolates into the groundwater system.

ut the vault at regular intervals. Because of the liquefying action of the bacteria and .

felectricity is at hand, an ozone generator, set to vent into the gas space above the

FIGURE 5-8

Schematic layout of a septic tank and tile tield.

A septic tank and tile field can be used only when soil conditions are favorable.

One method used to determine whether or not a tile system may be installed is the

soil percolation test, better known as the perc test (or sometimes perk). In simple

terms, the test is performed by digging a hole of prescribed size, filling it with water,

and measuring the rate at which the water percolates into the soil. An alternative,

and preferred, method for determining the suitability of the soil is to dig a trench

in the area proposed for the tile field and visually inspect it_ The inspector looks

for unsuitable soil (clay, for example) and the presence of mottled (discolored) soil.

Mottled soil indicates that the groundwater table has, at some point in time, risen to

a level which would interfere with the operation of the tile field and, perhaps more

important, bring the groundwater into direct Contact with sewage. The information

in Table 5-6 is then used to determine the size ofthe tile field.

Further limitations on the use of a septic-tank tile~f1eld system usually include

the following;

1. The tile field must be located more than 30111 from any well, surface water, footing

drain, or storm drain.

2. The tile held must be located in least 71 in from any property line.

ABLE 5-6

Maximum acceptable application rates for tile fields

Percolation rate Maximum acceptable

l-~ application rate

Soil texture and structure mm/h min/mm (m3 of vol/ml area)

Coarse and medium sand 2 150 <0.40 0.04

Fine and loamy sand 75~150 0.404180 0.03

Sandy loam 50-75 0.80~1.20 0.02

Loam and sandy clay 35~50 1 .20-1.71 0.01

Loams <35 >1.7l Not pemiitted

Clays, silts, muck, peat, mari <<35 >> 1.71 Not permitted

3. The minimum distance between the bottom of the absorption trench and the

groundwater table or any impermeable layer must not be less than 1.25 rn. i

4. The earth cover placed over the absorption tile muSI not be less than 0.3 m noi;

more than 0.6 rn deep.

5. A clean aggregate graded between 12 and 36 mm must be placed around the tile

pipe. It must be a minimum of 50 mm above the pipe and 150 mm below the

pipe, with a total depth of not less than 300 mm.

Most states limit septic tank/tile held installation to facilities producing less

than 40 m3/d of wastewater. This limits their use to single family residences, small

apartments, freeway rest areas, parks, and isolated commercial establishments.

Example 5-1. John and Mary Jones are considering the purchase of a plot of land on

which to build a retirement home. Based on their water bills for the past live years,

their average daily water consumption is about 0.4 m3. What size septic tank and tile

field should they expect to put on the lot if it perks at 1.00 min/mm?

Solution. lf the septic tank must provide a detention time of 24 h, then its volume

should be

V =0.<1m3/d ><1d :0.4m3

However, the minimum recommended volume is 4.0 m3 _ Good septic tank design prac-

tice calls for length to width (1/W) ratios greater than 2 to 1 and a minimum liquid depth

of about 1.2 rn. Using these criteria and a 4.0 ni3 volume, the liquid surface area would

be

40 . ,

A, I T3 a 3.33m

If we choose a width of 1.15 rn and a length of 3 m, we will have a well-sized tank of

4.14 m3 and a l/w ratio of 2.61 to l.

From Table 5-6 we find that a perk rate of 1.00 min/mm will allow an application

rate of 0.02 m3/m2 of trench. The bottom area of trench should then be about

_/yeli' r§;\ ~ » Ni

Barriered-landscape water-renovation system (BLWRS). In the' summer 4

1969, Dr. A. Earl Erickson demonstrated the eflicacy of utilizing a BLWRS (pn

fiigunced “blowers," like “iiowers”) to denitiify water containing l00 mg/L of nitrat

Vgubgequently, he and his associates demonstrated that the BLWRS could be used

iienovate both dairy cow and swine feedlot wastewater (Table 5-7).” The system i

of course, equally applicable to domestic wastewater.

The BLWRS consists of a mound of soil underlain by an impervious wat

barrier (Figures 5-9a and 5-911). As the renovated water passes beyond the edge

the barrier, it may be collected in drains or be allowed to recharge the aquifer. T

mound is constructed of a fine sand. The dimensions of the BLWRS depend on t'

soil texture and expected wastewater application rates (Table 5~8). A 0.15 rn lay

of topsoil is used to cover the sand. A water-hardy grass (quack grass or volunte

weed cover) must be established on the surface and banks to maintain the soi

permeability and stability. A carbon source is installed to penetrate the anoxic Zo

that forms along the barrier. The carbon source is a mixture of one part corn and l

parts peat.

TABLE 5-7

BLWRS wastewater renovation efficiencies

Average iniiuent Average effluent

concentration concentration Efficiency

(mg/L) (mg/L) (%)

Swine waste” ’

BOD5 l,l3l l8.9 98.3

P 18 0.02 99.9

SS 3.000 NIL ~l00.0

TKN 937 l87_4 80.0

Dairy waste”

BOD5 1,637.0 l8.9 98.8

P 38.5 0.23 99.4

SS 4,400.0 NIL ~ l00.0

TKN 917.0 27.5 97.0

“Average application rate of 15 mm/d for 503 d.

"Average application rate of 8.8 mm/d for -150 cl

Source' See Erickson, et al _ footnote ll.

"A. E. Erickson, B. G. Ellis, J. M. Tiedje, A. R_ V\/olcolt, C. M. Hansen, F. R. Peabody. E. C. M

and J. W. Thomas, S011 l`10ljU7CLlIi()l1)@)!' Denirrzfcafiun and Phosphate Reduction of Feed./ot ll

(Environmental Protection Agency Report No. EPA-660/2-74-057), Washington, DC: U.S, Govern:

Printing C¢l`licc_ 1974,

secondary treatment is to remove the soluble BOD5 that escapes the primary process

and to provide added removal of suspended solids. Secondary treatment is typically

achieved by using biological processes. These provide the same biological reactions

that would occur in the receiving water if it had adequate capacity to assimilate

the wastewater. The secondary treatment processes are designed to speed up these

natural processes so that the breakdown ofthe degradable organic pollutants can

66 achieved in relatively short time periods. Although secondary treatment may rt

Qmove more than 85 percent of the BOD5 and suspended solids. it does not remo\

fgighiticant amounts of nitrogen, phosphorus, or heavy metals, nor does it complete!

eslremove pathogenic bacteria and viruses.

_ _

A In cases where secondary levels of treatment are not adequate, additional trea

processes are applied to the secondary eftluent to provide advanced wastewati

Qiffeatment (AWT). These processes may involve chemical treatment and filtration ~

wastewaterémuch like adding a typical water treatment plant to the tail er

iff; _ .

a secondary plant~or they may involve applying the secondary effluent to tl

land in carefully designed irrigation systems where the pollutants are removed by

af? ,

t50il»crop system. Some of these processes can remove as much as 99 percent of tl

§§VVBOD5. phosphorus, suspended solids and bacteria, and 95 percent ofthe nitroge

They can produce a sparkling clean, colorless. odorless effluent indistinguishable

appearance from a high»quality drinking water. Although these processes and lar

treatment systems are often applied to secondary effluent for advanced treatiner

they have also been used in place of conventional secondary treatment processes.

Most of the impurities removed from the wastewater do not simply vanis

t Some organics are broken down into harmless carbon dioitide and water. Most

~ the impurities are removed from the wastewater as a solid, that is. sludge. Becau

most ofthe impurities removed from the wastewater are present in the sludge. slud

~ handling and disposal must be carried out carefully to achieve satisfactory pollutii

° control,

Pretreatment of Industrial Wastes

In municipalities, industrial wastewaters can pose serious hazards because wastevx

ter collection and treatment systems have not been designed to treat them. The wasl

can damage sewers and interfere with the operation of treatment plants. They m

pass through the WWTP?untreated or they may concentrate in the sludge, renderi

it a hazardous waste.

The Clean Water Act gives the Environmental Protection Agency (EPA) t

authority to establish and enforce pretreatment standards for discharge of industr

wastewaters into municipal treatment systems. Specific objectives of the pretre

ment program are:

0 To prevent the introduction to the WWTPS of pollutants that will interff

with the operation of a WWTP, including interference with its use or dispo

Y of municipal sludge.

° To prevent the introduction to WVVTPs of pollutants that will pass throu

the treatment works or otherwise be incompatible with such works.

° To improve opportunities to recycle and reclaim municipal and industi

wastewaters and sludge.

EPA has established “prohibited discharge standards" (40 CFR 403.53 t

apply to all nondomestic discharges to the WWTP and "categorical pretreatm

standards" that are applicable to specilic industries (40 CFR 405-471). Congr

assigned the primary responsibility for enforcing these standards to local WW'l`l

Velocity controlled. This type of grit chamber, also known as a horizonfaljiow

grit chamber, can be analyzed by means of the classical laws of sedimentation for

discrete, nonflocculating particles (Type l sedimentation).

Stokes’ law (See Section 3-6) may be used for the analysis and design of

horizontal-flow grit chambers if the horizontal liquid velocity is maintained at about

0.3 rn/s. Liquid velocity control is achieved by placing a specially designed Weir at

the end of the channel. A minimum of two channels must be employed so that one

can be out of service without shutting down the treatment plant. Cleaning may be

either by mechanical devices or by hand. Mechanical cleaning is favored for plants

having average flows over 0.04 m3/s. Theoretical detention times are set at about

one minute for average flows. Washing facilities are normally provided to remove

organic material from the grit.

. Y Y ~ _-`_._V ‘Mr

Example 5-2. Will a grit particle with a radius of 0.10 mm and a specitic gravity of

2.65 be collected in a horizontal grit chamber that is 13.5 m in length if the average

grit-chamber flow is 0. l5 m3/s, the width ofthe chamber is 0.56 rn, and the horizontal

velocity is 0.25 m/s? The wastewater temperature 22°C

Solution. Before we can calculate the terminal settling velocity of the particle, we

must gather some information from Table A-l in Appendix A. At a wastewater tem»

perature of 22°C, we find the water density to be 997.774 kg/m3_ We will use 1,000

kg/m3 as a sufficiently close approximation. Since the particle radius is given to only

two significant figures, this approximation is reasonable. From the same table, we hnd

the viscosity to be 0.995 mPa - s. As noted in the footnote, we must multiply this by

l0`3 to obtain the viscosity in units of Pa ~ s. Using a particle diameter of O. 20 ><

l0`3 m, we can calculate the terminal settling velocity using Equation 3~98.

U Z s(/15 ~ />)d2

I l8,u.

U I 2.80(2,650 - ioomtogzo >< iojf (in/S2><i<g/m3>(mQ

X l8(0.000995)(l<g ~ m/m3 - s2)(s)

U, = 3.61 >< l0`2 m/s or about 36 mm/s

Note that the product of the specitic gravity of the particle (2.65) and the density of

water is the density ofthe particle (ps). The Reynolds number for this settling velocity

(3.61 >< l0'Z mls) and particle size is 7.54. This is within the laminar range and Stokes’

law is valid.

With a flow of0. l5 m3/s and a horizontal velocity of0.25 in/s, the cross~sectional

area of How may be estimated to be

0. I5 _ »

A I wi- = _ > ~

C O25 0 6( m

The depth of flow is then estimated by dividing the cross-sectional area by the

width of the channel.

0.60

/ I ea* 2 _)7

z O56 I( m

7\’AS"l`l~§WAl`ER TREATMENT

If the grit particle in question enters the grit chamber at the liquid surface.

take h/U, seconds to reach the bottom.

1.07 W

Z ~t_.- 1 _)_ ~

’ 0,0361 6”

Since the chamber is 135 m in length and the horizontal velocity is 0.2

the liquid remains in the chamber.

‘5 S

Thus, the particle will be captured in the grit chamber.

Aerated grit chambers. The spiral roll of the aerated grit chamber liquid “dr

the grit into a hopper which is located under the air diffuser assembly (Figure 5

The shearing action of the air bubbles is supposed to strip the inert grit of mu

the organic material that adheres to its surface.

Aerated grit chamber performance is a function of the roll velocity and dt

tion time. The roll velocity is controlled hy adjusting the air feed rate. Nominz

flow values are in the range of 0.15 to 0445 cubic meters per minute of air per

ter of tank length (ml/min - m), Liquid detention times are usually set to be a

three minutes at maximum flow. Length-to-width ratios range from 2.511 to 5: l

depths on the order of 2 to 5 m.

Grit accumulation in the chamber varies greatly, depending on whether

sewer system is a combined type or a separate type, and on the efficiency ol

chamber. For combined systems, 90 m3 of grit per million cubic meters of sev

300 iminuuo-ci.\,.` .c._,_, V

(m3/106 m3) is not uncommon. In separate systems you might expect something less

than 30 m3/106 m3. Normally the grit is buried in a sanitary landfill. _

Comminutors

Devices that are used to macerate wastewater solid (rags, paper, plastic, and other

materials) by revolving cutting bars are called Comminutors (Figure 5-l3). These

devices are placed downstream of the grit chambers to protect the cutting bars from

abrasion. They are used as a replacement for the downstream bar rack but must be

installed with a hand-cleaned rack in parallel in case they fail.

Equalization

Flow equalization is not a treatment process per se, but a technique that can be used

to improve the effectiveness of both secondary and advanced wastewater treatment

a-;..». 1 ,rg gf ~-13; v 3* if

miiita.

V

processes.” Wastewater does not flow into a municipal wastewater treatment planl

a constant rate (see Figure l~4); the liow rate varies from hour to hour, reflecting i

living habits of the area served. In most towns, the pattern of daily activities sets i

pattern of sewage flow and strength. Above-average sewage flows and strength oct

in mid-moming. The constantly changing amount and strength of wastewater toi

treated makes efficient process operation difficult. Also, many treatment units ni

be designed for the maximum flow conditions encountered, which actually resi

in their being oversized for average conditions. The purpose of fiow equalizatioi

to dampen these variations so that the wastewater can be treated at a nearly consi

liow rate. Flow equalization can significantly improve the performance of an cxisd

plant and increase its useful capacity. In new plants, iiow equalization can redi

the size and cost of the treatment units. i

Flow equalization is usually achieved by constructing large basins that col;

and store the wastewater flow and from which the wastewater is pumped to the tri

ment plant at a constant rate. These basins are normally located near the head

of the treatment works, preferably downstream of pretreatment facilities such as

screens, comminutors, and grit chambers. Adequate aeration and mixing must

provided to prevent odors and solids deposition. The required volume of an eqi

ization basin is estimated from a mass balance of the How into the treatment pi

with the average flow the plant is designed to treat. The theoretical basis is the sq

as that used to size reservoirs (see Section 2-4). N

l

_f __ __ ___,_,_ ______ l

T

Example 5-3. Design an equalization basin for the following cyclic tiow pattern. l

vide a 25 percent excess capacity for equipment, unexpected How variations, and st

accumulation. Eva

is the first flow rate greater than the average after the sequence of nighttime low tlowg,

then the last row ofthe computation should result in a storage value of zero.

The first step then is to calculate the average flow. In this case it is

0.05657 m3/s. Next, the flows are arranged in order beginning with the time and

flow that first exceeds the average. ln this case it is at 0900 h with a flow of .0631

m3/s. The tabular arrangement is shown on the following page. An explanation ofthe

calculations for each column follows.

The third column converts the flows to volumes using the time interval between

flow measurements: A

if ; (0,063l ml/S><i h)(3600s/li) = 227,16 tn3

The fourth column is the average volume that leaves the equalization basin.

V = (005657 in;/s)(l h)(3600 s/h) 1 203.655 m3

The fifth column is the difference between the inflow volume and the outflow

volume. V

as = vu, vw, 1 227.16 m3 ~ 203.655 m3 f 23.505 m3

The sixth column is the cumulative sum of the difference between the inflow and out-

llow. For the second time interval, it is

stmge i iris < 37.55 m3 + 23.51 m3 1 61.06 m3

Note that the last value for the cumulative storage is 0. l 2 m3. It is not zero because of

round-off truncation in the computations. At this point the equalization basin is empty

and ready to begin the next day`s cycle.

The required volume for the equalization basin is the maximum cumulative stor~

age. With the requirement for 25 percent excess, the volume would then be

Storage volume I (86374 m3)(l.25) I l, 079.68 or L080 m3 ‘

The mass of BOD5 into the equalization basin is the product of the inflow (Q),

the concentration of BOD5 (So), and the integration time (Ar):

A/[BOD~in :

The mass of BOD; out of the equalization basin is the product of the average

outliovv (Qavg), the average concentration (Savg) in the basin, and the integration time

(Al):

MBOD-out : (Qa\'g)(Savg)(-Af)

The average concentration is determined as

S g (Vi)(So) + (Vs)(5pre\')

* ]\'i1 " _"_

v,+vg

where V, : volume of inflow during time interval At. m3

Si, 1- average BOD5 concentration during time interval At, g/m3

V; >f volume of wastewater in the basin at the end ofthe previous time inter-

val, m`

Sl ,,_.\, concentration of BOD; in the basin at the end of the previous time in-

Icrval

_ (previous.S`J,,g), g/ml

Noting that 1 mg/L 1 l g/m3. the first row (the 090011 time) computations are

1wBOD.,,, = (00631 m3/s)(l40 g/m3)(l h)(3,600 s/h)(lO`3 kg/g)

= 31.8 kg

S ; (22716 m3)(l40 g/niiglig

~ W 227.16 m3 + 0

1 140 mg/L

MBODM = (o.o5657 m3/s)(l40 g/m3><1 h><3,6o0 s/h)(l0`3 kg/g)

= 28.5 ke

Note that the zero values in the computation of Savg are valid only at startup of an'

empty basin. Also note that in this case MBODM, and /VIBOUM dilfer only because of

the difference in liow rates. For the second row (1000-h), the computations are

MBOf,.,, 1 (0.0670ml/s)(150 g/mlyti h><R,6t>o sm>(1o‘“ kg/g>

1 36.2 kg

S Z (241.2O`rn5)(l§0 g/ml) m3)(V140 g/mfl)

avg 241.20 ml + 23_5l in;

I 149.11 mg/L

MBOD.0u( 2 (ODS657 ml/s)(149.ll g/tniltl h)l'3,6OO s/h)(l()"5 kg/g)

; 30.37 kg

Note that VS is the volume of V1/3S{CW3[Cl' in the basin at the end ofthe previous

time interval. Therefore, it equals the accumulated dS. The concentration of BOD5

fSp,¢,,) is the average concentration at the end of previous interval (Sdyg) and not the

nliuent concentration for the previous interval (SO ).

For the third row (1100 h), the concentration of BOD; is

S g (24552 m3)(l55 g/ni3) + (61.06 m3)(l49.ll g/m3)

avg ` `

» 245521113 + 61.06 rn3

= 153.83 mg/L

PRIMARY TREATMENT

he screening completed and the grit removed, the wastewater still contains

tganic suspended solids, some of which can be removed from the sewage by

' in a sedimentation tank. These tanks can be round or rectangular. The mass

ed solids is called raw sludge. The sludge is removed from the sedimentation

/ mechanical scrapers and pumps (Figure 5-14). Floating materials, such as

and oil, rise to the surface ofthe sedimentation tank, where they are collected

rface skimming system and removed from the tank for further processing.

rimary sedimentation basins ( [)l'll1'l(1l'y tau/fs) are characterized by Type II

int settling. The Stokes equation cannot be used because the iiocculating par-

re continually changing in size, shape, and, when water is entrapped in the

ecitic gravity. There is no adequate mathematical relationship that can be

describe Type ll settling. Laboratory tests with settling columns are used to

> design data (see Chap. 3).

ectangular tanks with common-wall construction are frequently chosen be-

iey are advantageous for sites with space constraints. Typically, these tanks

Hopper

FIGURE 5-14

Primary settl ing tank.

range from 15 to 100 in in length and 3 to 24 in in width. Common length»to-width

ratios for the design of new facilities range from 3:1 to 511. Existing plants have

length~to-width ratios ranging from 1.531 to 15: 1. The width is often controlled by

the availability of sludge collection equipment. Side water depths range from 2 to

5 m. Typically the depth is about 3.5 m.

Circular tanks have diameters from 3 to 90 m. Side water depths range from

2.4 to 5 rn.

As in water treatment clarifier design, overllow rate is the controlling parame-

ter for the design of primary settling tanks. At average How, overflow rates typically

range from 25 to 60 3/ml ~ d (or 25 to 60 m/d). When waste-activated sludge is re-

tumed to the primary tank, a lower range of overflow rates is chosen (25 to 35 m/d).

Under peak flow conditions, overflow rates may be in the range of 80 to 120 m/d.

Hydraulic detention time in the sedimentation basin ranges from 1.5 to

2.5 hours under average flow conditions. A 2.0-hour detention time is typical.

The Great Lakes-Upper Mississippi River Board of State Sanitary Engineers

(GLUMRB) recommends that weir loading (hydraulic How over the effluent weir)

rates not exceed 120 m3/d of flow per m of weir length (rn3/d - rn) for plants with

average flows less than 0.04 m3/s. For larger flows, the recommended rate is 190

m3/d < rn.” If the side water depths exceed 3.5 m, the Weir loading rates have little

effect on performance.

Two different approaches have been used to place the weirs. Some designers

believe in the “long” approach and place the weirs to cover 33 to 50 percent of374 inraooucttow TO Enviaowmawrat ENGINEERING

the length of the tank. Those ofthe “short school” assume the weir length is less

important and place it across the width of the end of the tank as show-n in Figure

5~l4. The spacing may vary from 2.5 to 6 rn between weirs.

As mentioned previously, approximately 50 to 60 percent of the raw sewage

suspended solids and as much as 30 to 35 percent of the raw sewage BGD5 may be,

removed in the primary tank.

Example 5-4. Evaluate the following primary tank design with respect to detention

time, overfiow rate, and weir loading.

Design dam: Effective Length Weir

Flow = 0.150m3/s g

lniluent SS = 280 mg/L 1; 1 1 Y ` 0 T A1 5»~~

Sludge concentration I 6.0% \ Hs

_ g ~>| H? \ `**l

Efhciency - 60% [mst Sludge Omg(

Length I 40.0 m (effective) Zone L*-*W “me

Width = 10.0 m

Liquid depth 2 2.0 m

Weir length I 750m

The detention time is simply the volume of the tank divided by the How;

iv’ 40.0 >< 10.0 >< 2_0

[i@;gTi§“"

2 5333.33 sor 1.5 h

This is a reasonable detention time.

The overflow rate is the flow divided by the surface areai

0.150

U0 = -+-

40.0 X 10.0

= 3,75 >< 10” m/s >< 86,400 s/d 1 32 m/d

This is an acceptable overflow rate.

The Weir loading is calculated in the same fashion:

0.150

We Z in

= O.0O2Ol`l13/S`1T1 >< 86,400 s/d = l72.S or 173 m3/d - m

'l`lf\i<~ in Qnrql-\lQ “mir I/\nA§n"

ingredients needed for conventional aerobic secondary biologic treatment are the

‘availability of many microorganisms, good Contact between these organisms and the

-organic material, the availability of oxygen, and the maintenance of other favorable

environmental conditions (for example, favorable temperature and sufficient time

for the organisms to work). A variety of approaches have been used in the past to

meet these basic needs. The most common approaches are (l) trickling filters, (2)

'activated sludge, and (3) oxidation ponds (or lagoons).

` A process that does not fit precisely into either the trickling filter or the ac-

tivated sludge category but does employ principles common to both is the rotating

biological contactor (RBC).

Trickling Filters

A trickling filter consists of a bed of coarse material, such as stones, slats, or plastic

materials (media), over which wastewater is applied Trickling filters have been a

popular biologic treatment process.” The most widely used design for many years

was simply a bed of stones from l to 3 m deep through which the wastewater passed.

The wastewater is typically distributed over the surface ofthe rocks by a rotating arm

(Figure 5-15). Rock filter diameters may range up to 60 in.

As the wastewater trickles through the bed, a microbial growth establishes it-

lfon the surface ofthe stone or packing in a fixed film. The wastewater passes over

3 stationary microbial population, providing contact between the microorganisms

d the organics.

Trickling filters are not primarily a filtering or straining process as the name

iplies. The rocks in a rock filter are 25 to 100 mm in diameter, and hence have

tenings too large to strain out solids_ They are a means of providing large amounts

surface area where the microorganisms cling and grow in a slime on the rocks as

ey feed on the organic matter

Excess growths of microorganisms wash from the rock media and would cause

,desirably high levels of suspended solids in the plant effluent if not removed.

ius, the flow from the tilter is passed through a sedimentation basin to allow these

lids to settle out, This sedimentation basin is referred to as a secondary clargieg

fna! clarzjier to differentiate it from the sedimentation basin used for primary

ttling

Although rock trickling filters have performed well for years, they have certain

nitations. Under high organic loadings, the slime growths can be so proline that

ey plug the void spaces between the rocks, causing flooding and failure of the

stem. Also, the volume of void spaces is limited in a rock filter, which restricts

2 circulation of air and the amount of oxygen available for the microbes. This

nitation, in turn, restricts the amount of wastewater that can be processed.

To overcome these limitations, other materials have become popular for filling

3 triclrling nlter. These materials include modules of corrugated plastic sheets (Fig-

e 5~ l6) and plastic rings, These media offer larger surface areas for slime growths

'pically 90 square meters of surface area per cubic meter of bulk volume, as com»

red to 40 to 60 square meters per cubic meter for 75 mm rocks) and greatly increase

id ratios for increased air flow. The materials are also much lighter than rock (by

iactor of about 30), so that the trickling lilters can be much taller without facing

uctural problems. Vl/hile rock in filters is usually not more than 3 m deep, syntheticN inclmii ng recirculation.

media depths may reach l2 m, thus reducing the overall space requirements for the

tricl<ling-filter portion of the treatment plant.

Tricl-cling filters are classified according to the applied hydraulic and organic

load. The hydraulic load may be expressed as cubic meters of Wastewater applied

per day per square meter of bulk filter surface area (m3/d - m3) or, preferably, as the

depth of water applied per unit of time (mm/s or m/d). Organic loading is expressed

as kilograms of BOD5 per day per cubic meter of bulk filter volume (kg/d - m3),

Common hydraulic and organic loadings for the the various filter classihcations are

summarized in Table 5-9.

An important element in trickling filter design is the provision for 1‘€tu1Tl of a

portion of the efiiuent to iiow through the filter. This practice is called recirculation.

The ratio of the returned Flow to the incoming flow is called the recirculation ratio

(r). Recirculation is practiced in stone tilters for the following reasons:

L To increase contact efficiency by bringing the waste into contact more than once

with active biological material.

2. To dampen variations in loadings over a 24-hour period. The strength of the rc-

circulated tiow lags behind that ofthe incoming wastewater. Thus, recirculation

dilutes strong iniluent and supplements Weak influent

3. To raise the DC) oi’ the iniiuent.

4. To improve distribution over the surface, thus reducing the tendency lo clog and

also reduce filter iiies.

5. To prevent the biological slimcs l`rom drying out and dying during llli‘,\\lli|\\t‘ po

riods when flows may be too low to keep the iilter wet continuously.

Waste

Sludge

ZURE 5-17

o-stage trickling»filter plant. (Courtesy of Dow Chemical Company.)

circulation may or may not improve treatment efhciency. The more dilute the

:oming wastewater, the less likely it is that recirculation will improve efficiency.

Recirculation is practiced for plastic media to provide the desired wetting rate

keep the microorganisms alive. Generally, increasing the hydraulic loading above

: minimum wetting rate does not increase BOD; removal. The minimum wetting

e normally falls in the range of 25 to 60 m/d.

Two-stage trickling filters (Figure 5-l7) provide a means ofimproving the per-

mance of filters. The second stage acts as a polishing step for the effluent from

1 primary stage by providing additional contact time between the waste and the

croorganisms. Both stages may use the same media or each stage may have dif-

'ent media as shown in Figure 5-l7. The designer will select the types of media

d their arrangement based on the desired treatment efficiencies and an economic

alysis of the alternatives.

:sign formulas. Numerous investigators have attempted to correlate operating

ta with the bulk design parameters of trickling hlters. Rather than attempt a com-

:hensive review of these formulations, we have selected the National Research

suncil (NRC) equations” and Schulze’s equation" as illustrations. A thoroughreview of several of the more iinpox tant equations is given in the Water Environment

Federation's publication on wastewater treatment plant design.”

During World War II, the NRC made an extensive study of the operation of

trickling filters serving military installations. From this study, empirical equations

were developed to predict the efhciency of the filters based on the BOD load, the

volume of the filter media, and the recirculation. For a single-stage hlter or the iirst

stage of a two~stage hlter, the efficiency is

1

El Z ”" _ 0,5 (58)

1+ 4_i2<QQ?>

W

where E1 1 fraction of BOD5 removal for first stage at ZOOC, including recircula-

tion and sedimentation

Q = wastewater How rate, m3/s

Cin 1 influent BOD5, rngflc

V = volume of filter media, m5

F 1 recirculation factor

The recirculation factor is

` l + R

~ - ~~~» <5-<>>

f _

(l +O.lR)2

where R 1 recirculation ratio = Q,/Q

Q, = recirculation flow rate, m3/s

Q = Wastewater flow rate, m3/s

The recirculation factor represents the average number of passes of the raw

wastewater BOD through the filter. The factor O. lR is to account for the empirical

observation that the biodegradability of the organic matter decreases as the number

of passes increases. For the second stage filter, the efficiency is

E2 = ~- 1 0_5 (5-10)

1 i ii

l 4 E l ‘sf F

where E3 = fraction of BOD5 removal for second stage filter at 2O°C, including

f recirculation and sedimentation, %

E, = fraction of BOD5 removed in first stage

Ce = effluent BOD5 from first stage, mg/L

The effect of temperature on the efficiency may be estimated from the followa

ing equation: .

ET = E20 0<T'20> (5-ll)

where a value of 1.035 is used for 9.

Some care should be used in applying the NRC equations Military wastewater

during this period (World War ll) had a higher strength than domestic wastewater

today. The filter media was rock. Clarifiers associated with the triclding filters were

shallower and carried higher hydraulic loads than current practice would permit. The

second stage filter is assumed to be preceded by an intermediate settling tank ( see

Figure 5~ l7)_

Example 5-S. Using the NRC equations, determine the BOD5 of the effluent from

a single-stage, low~rate trickling filter that has a filter volume of 1,443 m3, a hy~

draulic loading of l,9OO m3/d, and a recirculation factor of 2.78. The inliuent BOD5 is

150 mg/L.

Solution. To use the NRC equation, the hydraulic loading must first be converted to

the correct units.

Q I (1,900 mi/d) l-` 2 0.022 mi/s

86,400 s/d

The efficiency of a single-stage filter is

1+4_l2 (0.02-)(1>0)

(l,443)(2178)

The concentration of BOD5 in the efliuent is then

Ce = (l - 0.8943)(l50) = 15.8 mg/L

Schulze” proposed that the time of wastewater CODIHCI with the biological

mass in the filter is directly proportional to the depth of the filter and inversely pro-

portional to the hydraulic loading rate:

CD

where I = contact time, d

C = mean active film per unit volume

D = Iilter depth, m

Q

A

zz = empirical constant based on filter media

= hydraulic loading, mi/d

= filter area over which wastewater is applied, mi

The mean active film per unit volume may be approximated by

l

C l W

where m is an empirical constant that is an indicator of biological slime dis

It is normally assumed that the distribution is uniform and that m : 0.

is 1.0. _

Schulze combined his relationship with Velz’s‘9 first-order equation

removal

2 _ ex g KD

So P . <Q/A>~

where K is an empirical rate constant with the units of

(ni/d)"

m

The temperature correction for K may be computed with Equation _

is substituted for ET and KZ() is substituted for Eg).

Example 5-6. Determine the BOD; of the eflluent from a low-rate tricltlii

has a diameter of 35.0 rn and a depth of l_5 ni if the hydraulic loading is

and the inlluent BOD5 is 150.0 mg/L. Assume the rate constant is 2.3 (n

n = 0.67. _ ~

Solution. We begin by computing the area of the lilter.

1r(35.0)2

A 1 ,_,_.

4

` = 962.11 ml

This area is then used to compute the loading rate.

Q _ 1,900 m3/d

A ` 962.llm2

= 1.97 m3/d - ml --

Now we can compute the efliuent BOD using Equation 5-14.

S, = (l50)exp

= 16.3 mg/L .

tivated Sludge i H »

: activated sludge process is a biological wastewater treatment technique in which

ixture of Wastewater and biological sludge (microorganisms) is agitated and aer_

l. The biological solids are subsequently separated from the treated wastewater

returned to the aeration process as needed.

The activated sludge process derives its name from the biological mass founeff

rn air is continuously injected into the wastewater. ln this process, microorgané

s are mixed thoroughly with the organics under conditions that stimulate their

Nth through use ofthe organics as food. As the microorganisms grow and arg

ed by the agitation ofthe air, the individual organisms clump together (floccu-

) to form an active mass of microbes (biologic tloc) called activated sludge.

In practice, wastewater Hows continuously into an aeration tank (Figure 5-18)

:re air is injected to mix the activated sludge with the wastewater and to Sup;

the oxygen needed for the organisms to break down the organics The mixture

ctivated sludge and wastewater in the aeration tank is called mixed liquor The

ed liquor flows from the aeration tank to a secondary clarifier where the activated

lge is settled out_ Most of the settled sludge is returned to the aeration tank (and

:e is called return sludge) to maintain the high population of microbes that per-

; rapid breakdown of the organics. Because more activated sludge is produced

1 is desirable in the process, some ofthe return sludge is diverted or wasted to the

lge handling system for treatment and disposal. ln conventional activated sludge

ems, the wastewater is typically aerated for six to eight hours in long, rectangu-

ieration basins. About 8 rnl of air is provided for each m3 of wastewater treated.

icient air is provided to keep the sludge in suspension (Figure 5-l9). The air

Qected near the bottom of the aeration tank through a system of diffusers (Fig-

5-20). The volume of sludge returned to the aeration basin is typically 20 to 30

:ent of the wastewater flow.

The activated sludge process is controlled by wasting a portion of the mi-

irganisms each day in order to maintain the proper amount of microorganisms to

n .` _

fficiently degrade the BOD5. “Wasting” means that a portion of the microorganisms

is discarded from the process. The discarded microorganisms are called waste acti-

vated sludge (WAS). A balance is then achieved between growth of new organisms

and their removal by wasting. If too much sludge is wasted, the concentration of

microorganisms in the mixed liquor will become too low for effective treatment. If

too little sludge is wasted, a large concentration of microorganisms will accumulate

and, ultimately, overflow the secondary tank and flow into the receiving stream.

The mean cell residence time 96, also called solids retention time (SRT) or

sludge age, is denned as the average amount of time that microorganisms are kept

in the system.

Many modifications of the conventional activated sludge process have been

developed to address specitic treatment problems. A brief description of these is

given in Table 5- lO. We have selected the completely mixed and conventional plug-

liow processes for further discussion.

Completely mixed activated sludge process. The design formulas for the coni-

pletely mixed activated sludge process are a mass-balance application of the equa~

tions used to describe the kinetics of microbial growth. A mass-balance diagram for

the completely mixed system (CSTR) is shown in Figure 521. The mass-balance

equations are written for thc system boundary shown by the dashed line. Two mass

balances are required to define the design of the reactor: one for biomass and one for

food (soluble BOl`);)_

process modification

Description

Conventional plug»tiow

Complete~mix

Tapered aeration

Step»feed aeration

Modified aeration

Contact stabilization

Extended aeration

High-rate aeration

Settled wastewater and recycled activated sludge enter the h

of the aeration tank and are mixed by diffused»air or mechar

aeration. Air application is generally uniform throughout tan

During the aeration period, adsorption, iiocculation, and oxit

of organic matter occur, Activated sludge solids are separate

secondary settling tank.

Process is an application ofthe How regime of a continuous-

stirred-tank reactor. Settled wastewater and recycled activat

are introduced typically at several points in the aeration tanl

organic load on the aeration tank and the oxygen demand ar

throughout the tank length.

Tapered aeration is a modification ofthe conventional plug-

process. Varying aeration rates are applied over the tank len

depending on the oxygen demand. Greater amounts of air a

to the head end of the aeration tank, and the amounts dimin

mixed liquor approaches the effluent end. Tapered aeration

achieved by using different spacing of the air diffusers over

length,

Step feed is a modification of the conventional plug-dow pi

in which the settled wastewater is introduced at several poi

the aeration tank to equalize the F/M ratio, thus lowering p

oxygen demand. Generally three or more parallel channels

Flexibility of operation is one of the important features of t

Modified aeration is similar to the conventional plug-How |

except that shorter aeration times and higher F/M ratios ar

BOD removal efficiency is lower than other activated slud

processes.

Contact stabilization uses two separate tanks or compartm<

treatment of the wastewater and stabilization ofthe activat

The stabilized activated sludge is mixed with the intluent <

or settled) wastewater in a contact tank. The mixed liquor

in a secondary settling tank and return sludge is aerated se

in a reaeration basin to stabilize the organic matter. Aerati

requirements are typically 50 percent less than conventior

Extended aeration process is similar to the conventional p

process except that it operates in the endogenous respirati<

ofthe growth curve, which requires a low organic loading

aeration time. Process is used extensively for prefabricate

plants for small communities. '

High-rate aeration is a process modification in which higi

concentrations are combined with high volumetric loadin;

combination allows high F/M ratios and long mean celi»r

times with relatively short hydraulic detention times. Adi

mixing is very important.

TABLE 5-10

(continued )

:§`

» are an

,1

Nga

Process or

process modification

Kraus process

High-purity oxygen

Oxidation ditch

Sequencing batch reactor

Deep-shaft reactor

Single-stage nitrification

Separate stage nitrification

mia -Q

"ef wg -~

ws; ~

Description i

. Q.,

'

Kraus process is a variation of the step aeration process used 10 gem ‘

Wastewater with low nitrogen levels. Digester supematant is added .

a nutrient source to a portion of the retum sludge in a separate aeratigif

tank designed to nitrify. The resulting mixed liquor is then added to

the main plug»flow aeration system.

High-purity oxygen is used instead of air in the activated sludge

process. Oxygen is difused into covered aeration tanks and is

recirculated. A portion of the gas is wasted to reduce the concentratioifgi

of carbon dioxide. pH adjustment may also be required. The amount

oxygen added is about four times greater than the amount that can '

added by conventional aeration systems.

The oxidation ditch consists of a ring- or oval~shaped channel and

is equipped with mechanical aeration devices. Screened wastewater

enters the ditch, is aerated, and circulates at about 0.25 to 0.35 mfs,

Oxidation ditches typically operate in an extended aeration mode with

long detention and solids retention times. Secondary sedimentation

tanks are used for most applications,

The sequencing batch reactor is a fill-and-draw type reactor system »

we

'

involving a single complete-mix reactor in which all steps of the

activated sludge process occur. Mixed liquor remains in the reactor

during all cycles, thereby eliminating the need for separate secondary i

,wo

-as

ffm?

M5

sedimentation tanks. .V

The deep vertical-shaft reactor is a form of the activated sludge '

process. A vertical shaft about l2O to 150 m deep replaces the primary Y

clarifiers and aeration basin. The shaft is lined with a steel shell and

and air are forced down the center ofthe shaft and allowed to rise i

upward through the annulus.

In single-stage nitrification, both BOD and ammonia reduction occur »

in a single biological stage. Reactor conhgurations can be either a

series of complete-mix reactors or plug flow.

In separate stage nitrification, a separate reactor is used for

nitrihcation, operating on a feed waste from a preceding biological

treatment unit. The advantage of this system is that operation can be

optimized to conform to the nitritication needs.

"Source: Metcalf & Eddy, Wastewater Engineering: Treatment, Disposal and Reuse, New York: McGra\v~Hill, l99l.

Under steady-state conditions, the mass balance for biomass may be Written as:

Biomass in Biomass _ Biomass in Biomass

. + - +

iniluent accumulated effluent wasted

(5-15)

The biomass in the inliuent is the product of the concentration of microorgan-

isms in the inliuent (XO) and the How rate of wastewater (Q). The concentration of

microorganisms in the inlluent (Xu) is measured as suspended solids (mg/L). The

biomass that accumulates in the aeration tank is the product of the voluinc of the

IGURE 5-21

Completely mixed biological reactor with solids recycle.

tank (V) and the Monod expression for growth of microbial mass (Equation 5-5)

SX

if Elia/<X 'sa

()KX+S (1 (l

The biomass in the efliuent is the product of How rate of treated wastewa

leaving the plant (Q - QW) and the concentration of microorganisms that does 1

settle in the secondary clarifier (Xe). The flow rate of wastewater leaving the pl.

does not equal the How rate into the plant because some of the microorganisms m

be wasted. The flow rate of wasting (QW) is deducted from the flow exiting the plz

The biomass that is wasted is the product of concentration of microorganismf

the WAS flow (X,) and the WAS flow rate (Qr). The narrative mass-balance equat

may be rewritten as 5

PLIIISX A _ _ _

QX0 + (V) KX Jr S /MX “ (Q Qw)Xe + QwXf (5

The variable are summarized as follows:

Q = wastewater How rate into the aeration tank, m3/d

1 microorganism concentration (volatile suspended solids or VSS)2°

tering aeration tank, mgfl.

V 1 volume of aeration tank, m3

/t,,, = maximum growth rate constant, d

Xa

ml

2°Suspended solids means that the material will he retained on a Glter, unlike dissolved solids su(

NaCl. The amount ofthe suspended solids that volatilizes at 500 t 50°C is taken to be a measure ofai

biomass concentration. The presence of nonliving organic particles in the inljuent wastewater will c

some error (usually small) in the use of volatile suspended solids as ll measure of biomass.

S '= soluble BOD; in aeration tank and effluent, mg/L

X = microorganism concentration (mixed-liquor volatile suspended solids

or MLVSS)21 in the aeration tank, mg/L

K, = half velocity constant

I soluble BOD5 concentration at one-half the maximum growth rate,

mg/L

kd = decay rate of microorganisms, d`1

QW I How rate of liquid containing microorganisms to be Wasted, m3/d

XE I microorganism concentration (VSS) in effluent from secondary Sep

tling tank, mg/L

X, = microorganism concentration (VSS) in sludge being wasted, mg/L

At steady-state, the mass~balance equation for food (soluble BOD5) may be

written

Food in + Food _ Food in + Food in

influent consumed _ effluent WAS (5-18)

The food in the influent is the product ofthe concentration of soluble BOD5 in

the intluent (SO) and the flow rate of wastewater (Q). The food that is consumed in

the aeration tank is the product of the volume of the tank (V) and the expression for

rate of food utilization (Equation 5-7)

/.L,nSX

“V” aero “gl

The food in the effluent is the product of flow rate of treated wastewater leaving

the plant (Q e QW) and the concentration of soluble BOD5 in the effluent (S). The

concentration of soluble BOD5 in the effluent (S) is the same as that in the aeration

tank because we have assumed that the aeration tank is completely mixed. Since

the BOD5 is soluble, the secondary settling tank will not change the concentration.

Thus, the effluent concentration from the secondary settling tank is the same as the

influent concentration.

The food in the waste activated sludge flow is the product of the concentration

of soluble BOD; in the influent (S) and the WAS flow rate (Q,). The nanative mass-

balance equation for steady-state conditions may be rewritten as

_ MISX _ _ _

QSO (V) WE _ (Q QWDS 'P' QWS (320)

where Y 1 yield coefficient (see Equation 5-6).

The intiuent and effluent biomass concentrations are negligible compared to tl

in the reactor_

The influent food (S 0) is immediately diluted to the reactor concentration in 1

cordance with the definition of a CSTR.

1

2.

3, All reactions occur in the CSTR.

from the First assumption we may eliminate the following tenns from Equati

5-l7t QX0, and (Q - Qw)Xe because XO, and Xe, are negligible compared to X, Eqi

tion 5-17 may be simplified to

,tt,,,SX

(V) -T - kdX = +Q,,,.X, (5-1

K,+S

For convenience, we may rearrange Equation 5-21 in terms ofthe Monod eq

tion

/-'L/115 : QȢfXr + kd (5_

Kg, + S VX

Equation 5-20 may also be rearranged in terms of the Monod equation

MIYIS Q Y

_____ Z __ ) ~ 5,

<K,+S> ’v'X(S{ S) (

Noting that the left side of Equations 5-22 and 5-23 are the same, we set the rig

hand side of these equations equal and rearrange to give:

QM/Xl" Q Y

_T ; _E _ 2 5_

VX V X(5a S) /Q1 (

Two parts of this equation have physical significance in the design of a comple

mixed activated sludge system. The inverse of Q/V is the hydraulic detention z

(6) ofthe reactor: '

5 - Q <5-

Q

The inverse ofthe left side of Equation 5-24 dehnes the mean cell-reside

time (QC): _

vx

4- = at 5

QWX,~ <

The mean cell-residence time expressed in Equation 5-26 must be modihed il

effiuent biomass concentration is not negligible. Equation 5-27 accounts for efli

49 1 ___._.___; _ ~

“ <a»+@~omm> “UW

From Equation 5-22, it can be seen that once 65 is selected, the concentratifiii

of soluble BOD5 in the effluent (S) is nxed: S A

K5(l + kd/(96)

S = _--_-7 _ '

mmmeeww , 63%

Typical values of the microbial growth constants are given in Table 5-l

that the concentration of soluble BOD5 leaving the system (S) is affected onlyvby

the mean cell~residence time and not by the amount of BOD5 entering the aeratioii

tank or by the hydraulic detention time. lt is also important to reemphasize that`§§

is the soluble BOD; and not the total BOD; Some fraction of the suspended solid§

that do not settle in the secondary settling tank also contributes to the BOD5 load

the receiving body. To achieve a desired effluent quality both the soluble and insol

uble fractions of BGD5 must be considered. Thus, to use Equation 5~28 to achiei/E

a desired effluent quality (S) by solving for 49, some estimate of the BGD5 of the

suspended solids must be made first. This value is then subtracted from the total

§

;1ug_f1ovv system is difficult to develop from basic mass-balance equations.

~ , .'== i.

simplifying assumptions, Lawrence and McCarty/22 have developed a u

i uation. The assumptions are:

egg

The concentration of microorganisms in the intluent to the aeration tank is ap

imately the same as that in the effluent from the aeration tank. This assurr

-applies if 95/6 is greater than 5.

':.=`~»f =

rate of soluble BOD 5 utilization as the waste passes through the aeratio

is given by '

M /lm S X av g

fu 1 1 -"”

*T where Xavg is the average concentration of microoorganisms in the aeratioi

The design equation is

1 Y;Lm(SU 1 5) A

9. T <50 1 S) + qi + a>1<,in<si/S) T 'd

where a 1 recycle ratio, Q,/Q

ln 1 logarithm to base e

L, Si 1 influent concentration to aeration tank after dilution with

flow, mg/L

T l + a

Other terms are the same as those defined previously.

Example 5-7. The town of Gatesville has been directed to upgrade its primarj

to a secondary pl`ant that can meet an eflluent standard of 30.0 mgfl, B4

30.0 mg/L suspended solids (SS). They have selected a completely mixed

sludge system

Assuming that the BOD5 of the SS may be estimated as equal to 63 r

the SS concentration, estimate the required volume of the aeration tank. The `

data are available from the existing primary plant.

Existing plant effluent characteristics

Flow 1 0.150 m3/s

BOD5 1 84.0 mg/L

Assume the following values for the growth constants; KX 1 l00 mg

,um 1 2.5 d"; kd 1 0.050 d"‘; Y 1 0.50 mg VSS/mg BOD5 removed.

6

gr A low rate of wasting causes a tow F/M ratio, which yields organisms that are

This results in more complete degradation ofthe waste.

A long 65 (low F/M) is not always used, however, because ofcertain trade-offs

0 a larger and more costly aeration tanl<_ lt

at must be considered. A long C means g

means a higher requirement for oxygen and, thus, higher power costs. Problems

“ ` ` " ` ` b tered if6 is too

“th oor sludge settleability in the final clariher may e encoun C

P t

gpg, However, because the waste is more completely degraded to final end products

3

if

*ii

less of the waste is converted into microbial cells when the microorganisms are

iiéarved at a low F/M, there is less sludge to handle.

I,@;l= . . .

ef Because both the F/M ratio and the cell-detention time are controlled by

3

Q

2

X

d A l `<fh F/M eorres onds to a short 9,

§§fastinU of organisms, they are interrelate _ ug ‘ p

D

l U 6* Ff‘\/I values ty tcally range from Osl to

a low F/M corresponds to a ong L- t p

mg/mg~d for the various modilications of the activated sludge process.

ows

Tank A ls settled onte eath day and h tlt the liquid is remox ed with care not to

2* disturb the sludge that settles to the bottom, This liquid is replaced with fresh settled

Example 5'8. Two "till and draw" batch-operated sludge tanks are operated

Sludge return. The purpose of sludge return is to maintain a sufficient concentra-

tion of activated sludge in the reactor basin. One method used to control the rate of

sludge return to the reactor basin is based on the empirical measurement known as

the sludge voiume index (SVI).

SVI is determined from a standard laboratory testi” The procedure involves

measuring the MLSS and sludge settleability A one-liter sample of mixed liquor

is obtained from the aeration tank at the discharge end. The sludge settleability is

measured by hlling a standard one-liter graduated cylinder to the 1.0 liter mark,

allowing undisturbed settling for 30 minutes, and then reading the volume occupied

by the settled sludge. The MLSS is determined by Hltering, drying, and weighing

a second portion of the mixed liquor. The SVl, which is dedned as the volume in

rnilliliters occupied by l g of activated sludge after the aerated liquor has settled 30

min, is calculated as follows:

sv

svi 1 >< 1,000 mg/g (5-34)

where SVl 1 sludge volume index, ml,/g

SV = volume of settled solids in one~liter graduated cylinder after 30 min

settling, mL/L

MLSS = mixed liquor suspended solids, mg/L

PW" '“'M9W€’f Tuff. Jw

3-22

§f;;§ypotlietical relationship between settled sludge volume from SVI test and re-

sludge How, (Source: M. J. Hammer, Water and Wastewater Technology,

Vgision, New York: Wiley, l9`/7. Reprinted by permission.)

s.

Conceptually, SVI can be related to the quantity and solids concentrz

:the secondary settling tank as we have depicted in Figure 5-22. In the followi

eibussion and mathematical relationships, the secondary tank is assumed to r

4/-.

iidentically to the graduated cylinder used in the SVI test. This assumption is t

gdinary to say the least. In fact, Vesilind has shown that for MLSS concentra

less than 5,000 mg/L, the settling rates are I0 to 20 percent greater than rr

' expected in a final claritierm Nonetheless, environmental engineers have de'

a large body of empirical data based on it.

The SVI can be used as an indication of the settling characteri:

“ the sludge, thereby impacting on return rates and MLSS. Typical values

for activated sludge plants operating with an MLSS concentration of 2

I 3,500 mg/L range from 80 to 150 mL/g. As the sludge concentration is incrc

the 3,000 to 5,000 mg/L range, there is a higher solids loading on the settlin

and as a consequence, a lower SVI or larger settling basin is required to ai

loss of solids caused by “washout” or hydraulic displacement.

The SVI is a key factor in the system design. Indirectly, it limits the

basin l\/ILSS concentration and, in turn, the MLVSS that can be achieved, be

controls the settling tank underflow concentration. Thus, for a given SVI an

sludge rate, the maximum l\/ILSS and MLVSS are fixed Within narrow limi

Most activated sludge plants are designed to permit variable sludge rett

I0 to 100 percent of the raw waste How. This range of return sludge tlow g

operator reasonable flexibility to adjust the MLSS to the desired concentr;

general, the return sludge ratio should be limited to or below I00 percent

particularly true if the SVI is higher than 150 mL/g and there is no provi

additional lloor area in the hnal clarihcation step.

Without operating data, the Joint Task Force suggests that MLSS be li

5,000 mg/I. (lower at temperatures of less than 20°C), even though the SVIf

very low.” Design values over 5,000 mg/'L generally will lead to inordina

FIGURE 5-23

Design MLSS versus SVI and return sludge ratio, (Source: loint Task Force of

the Water Environment Federation and the American Society of Civil Engineers,

Design of Municipal Wastewater Treatment Plants Vol. I, Manual of Practice

No, 8, Chapters l-ll, Alexandria( VA( l99l_ Reprinted by permission.)

detention times that are more subject to washout unless surge control is planned.

Design MLSS values should not be any higher than needed, since the final settling

basin operations become critical at high MLSS levels.

The mixed liquor concentration as a function ofthe SVI and the return sludge

ratio (Q,/Q) is shown in Figure 5-23. The return sludge pumping rate may be de-

termined froni a mass balance around the settling tank in Figure 5-21. Assuming

that the amount of sludge in the secondary settling tank remains constant (steady-

state conditions) and that the effluent suspended solids (Xe) are negligible, the mass

balance is

accumulation = inflow e outflow (5-35)

0 = (Q + Qf)(X') r (QfX,f + QWXD (5436)

where Q I wastewater flow rate, m3/d

Q, = return sludge liow rate, m3/d

X' I mixed liquor suspended solids (MLSS), g/m3

X; 1 maximum return sludge concentration, g/m3

QW = sludge wasting flow rate, ml/d

Solving for the return sludge flow rate:

ix' e Q.,x,i

Qi- 1 &QT'jjYr' (537)

/,_

Frequently, the ussuinption that the effluent suspended solids are negligible is not

\f;ilid_ It the ellliieiit suspended solids are significant, the mass balance may then be

expressed as `

U (C) l Q,->lX'l r (Q, X1 i' (ji\».\',i l (Q e Q.,~l/Ya) (5 38)

Solving for the return sludge [low rate:

._ QX' - QNX; - <Q - Qi->Xi -V

Q, - X; _ X, o 39>

Note that X; and X' include both the volatile and inert fractions. Thus, they differ

from X, and X by a constant factor. With the volume of the tank and the mean cell-

'rresidence time, the sludge Wasting How rate can be determined with Equation 5-26

if the maximum return sludge concentration (Xl) can be determined. The maximum

return sludge concentration is related to the SVI as follows:

U L/L 6

X; I 1,000 mg/g(l,000m > 2 10 mg/L (540)

SVI ' SVI D

Figure 5-23 has been constructed on the basis of rapid sludge removal and uses the

concentration achieved in the 30-minute settling test as the settling basin underliow

concentration. Practice has shown this to be a relatively valid approach.

The maximum achievable undertiow concentration is also a function oftemper-

ature, Temperature affects zone settling velocity, as well as the SVI. In cold weather,

the SVI increases because of poor settling. The Joint Task Force’s recommended

mixed-liquor design concentration as a function of the minimum design reactor-basin

temperature for several SVI values is shown in Figure 5-24. (The SVI is taken at the

temperature of the reactor basin contents.)

Example 5-10. In the continuing saga of the Gatesville plant expansion, we now wish

to consider the question of the return sludge design. Based on the aeration tank design

(Example 5-7) and an informed, reliable source, we have the following data:

Design data:

Flow 1 0.150 ml/s

MLVSS(X) 1 2,000mg/L

MLSS(X') = L43 (MLVSS)

Efiiuent suspended solids 2 30 mg/L

Wastewater temperature 2 l8.0°C

Solution. We begin by computing the anticipated concentration of the MISS.

MLSS = l_43(2,000)

MLSS = 2,860 nig/L

We caii`t really predict SVI but Figure 5-24 gives us a reasonable range to assume

a value. Alternatively, we could assume a return sludge concentration. Using Fig-

ure 5-24_ we select an SVI ot 175 based on the calculated MLSS and the reactor basin

temperature.

Now, using Equation 5-40, we can determine the return sludge concentration.

IO6

“' 175

Y,

Xf 1 5,714.29 or 5.700 nu:/l,

Sludge production. The activated sludge process removes substrate, which exerts

an oxygen demand by convening the food into new cell material and degrading

this cell material while generating energy. This cell material ultimately becomes

Sludge, which must be disposed of. Despite the problems in doing so, researchers

have attempted to develop enough basic information on sludge production to permit

a reliable design basis. Heukelekian and Sawyer both reported that a net yield of

0_5 kg MLVSS/kg BOD5 removed could be expected for a completely soluble or~

ganic substrate.” Most researchers agree that, depending on the inert solids in the

System and the SRT, 0.40 to 0.60 kg MLVSS/kg BOD5 removed will normally be

Observed.

The amount of sludge that must be wasted each day is the difference between

the amount of increase in sludge mass and the suspended solids (SS) lost in the

eflluenli

Mass to be wasted = increase in MLSS e SS lost in effluent (5-Jil )

The net activated sludge produced each day is determined by; 0

Y

Y Q 1 #Wi » ir.g 4.2

°”~ i + kde, "

and

Pi 1 YOb5Q(S0 e S>(10"`3 kg/ga <s»-is

where PX 1 net waste activated sludge produced each day in terms of VSS, kg/d

Yobs 2 observed yield, kg MLVSS/kg BOD5 removed

Other terms are as defined previously.

The increase in MLSS may be estimated by assuming that VSS is some fractior

of MLSS. lt is generally assumed that VSS is 60 to 80 percent of MLVSS. Thus, thc

increase in MLSS intEquation 5-43 may be estimated by dividing P, by a factor

of 0.6 to 0.8 (or multiplying by 1.25 to l_667). The mass of suspended solids los

in the effluent is the product of the How rate (Q e QW) and the suspended solid:

concentration (XR).

Example 5-ll. Estimate the mass of sludge to be wasted each day from the new acti

vated sludge plant at Gatesville (Examples 5-7 and 5-10).

Solution. Using the data from Example 5»7, calculate Yum;

0.50 kg \/SS/kg BOD; removed

y N 1 ..._,,,_ .... , . _._.,..__....,,

“"' 1 + |ro.o>oa ‘><S tn]

li 1A\L».v \/QQ/L»n IUWD- r.~\ni1\\/.>1|

The net waste activated sludge produced each day is ~

P. - (O.4O)(0.150 m3/s)(84_0 g/m3 -s ii.: g/m3)(86,400 s/d)(lO" kg/g) 5

= 377.9 kg/d of VSS

The total mass produced includes inert materials. Using the relationship betwegu

MLSS and MLVSS in Example 5-10,

Increase in MLSS = (l.43)(377.9 kg/d) 2 540.4 kg/d

The mass of solids (both volatile and inert) lost in the effluent is

(Q - Q,.>(Xf> - (O.l5O m3/s - 0.0011 m3/s)(~30 g/m3)(86,4O0 S/d><l(_)f3 kg/gy a

1 385.9 kg/d

The mass to be wasted is then

Mass to be wasted = 540.4 A 385.9 I 154.5 kg/d

Note that this mass is calculated as dry solids. Because the sludge is mostly water, the

actual mass will be considerably larger. This is discussed further in Section 5-1 l.

xygen demand. Oxygen is used in those reactions required to degrade the sub-

‘ate to produce the high-energy compounds required for cell synthesis and for res-

ration. For long SRT systems, the oxygen needed for cell maintenance can be of

3 same order of magnitude as substrate metabolism. A minimum residual of/0_5 to

ng/L DO is usually maintained in the reactor basin to prevent oxygen deficiencies

>m limiting the rate of substrate removal.

' An estimate of the oxygen requirements may be made from the BOD5 of the

iste and amount of activated sludge Wasted each day. If We assume all ofthe BOD5

converted to end products, the total oxygen demand can be computed by converting

DD5 to BOD L. Since a portion of waste is converted to new cells that are wasted,

3 BODL of the wasted cells must be subtracted from the total oxygen demand. An

proximation ofthe oxygen demand ofthe wasted cells may be made by assuming

ll oxidation can be described by the following reaction:

C5Hil\lO2 +502 2 5CO2 + 2H2O + NH3 + energy (5-4-4)

CC S

ie ratio of gram molecular weights is

5(32)

-_ = L42

ll3

tus the oxygen demand of the waste activated sludge may be estimated as

12 (PX).

The mass of oxygen required may be estimated as:

Q(Sf, - S)(l0`3 l<“/0)

Mm: - --4»7__2i - l.42(P‘,f) <5-45)

Where Q = wastewater flow rate into the aeration tank, m3/d

S0 = iniiuent soluble BOD5, mg/L

S 1 effluent soluble BOD5, mg/L

f = conversion factor for converting BOD5 to ultimate BODL

PX I waste activated sludge produced (see Equation 5-43)

The volume of air to be supplied must take into account the percent of air that

is oxygen and the transfer efficiency of the dissolution of oxygen into the waste-

§§2

’

M

,L ,

e

gt.

£1

és*

W 816 [_

Example 542. Estimate the volume of air to be supplied (m3/d) forthe new activated

sludge plant at Gatesville (Examples 5»7 and 5»l())_ Assume that BOD; is 68 percent

ofthe ultimate BOD and that the oxygen transfer efticiency is ZS percent.

Solution. Using the data from Examples 5-7 and 5-ll

(O.l50 m3/s)(84.O g/m3 - ll.l g/ni5)(86,4OO s/d)(lO"'3 kg/g)

MU! 1 g __.t`~%§`~..g -..._. 7-.. set- .E

- l.42(377.9 kg/d of \/SS)

1 l,389.4 f 536.6 I 852.8 kg/d of oxygen

From Table A~5 in Appendix A, air has a density of l.l85 kg/ni’ at standard

conditions. By mass, air contains about 23.2 percent oxygen. At 100 percent transfer

efficiency, the volume of air required is

852.8 kv/d

Z3 Z 3' A “ 3

(~ l85 kg/m3)(0_232) lOl 99 or ’»,l0()ni /d

At 8 percent transfer efnciency

3,101.99 m3/d

0.08 = 38,774.9 or 38,000 mi/d

Process design considerations. The SRT (i.e., 66) selected for design is a func-

tion ofthe degree of treatment required. A high SRT (or older sludge age) results

in a higher quantity of solids being carried in the system and a higher degree of

treatment being obtained. A long SRT also results in the production of less waste

sludge.

SRT values for design of carbonaceous BOD5 removal as a function of the

minimum temperature at which the reactor basin will be operated are depicted in

Figure 5-25. The SRT values given are those for normal domestic wastewater. lt is

expected that the soluble BOD5 in the efliuent from the aeration system will be 4 to

8 mg/L.

If industrial wastes are discharged tothe municipal system, several additional

concerns must be addressed. Municipal wastewater generally contains sullicient nif

trogen and phosphorus to support biological growth. The pmsoiitzo of large volumes

of industrial wastewater that is deficient in either of these nutrients will result in

poor removal efficiencies. Addition of supplemental nitrogen and nliosnhorns may

uired. The ratio of nitrogen to BUD; should be l:32. The ratio of phosphorus

D5 should be l:l50.

Xlthough toxic metals and organics may be at low enough levels that they do

erfere with the operation ofthe plant, two other untoward effects may result if

"e not excluded in a pretreatment program. Volatile organics may be stripped

nlution into the atmosphere in the aeration tank. Thus, the WWTP may become

:e of air pollution. The toxic metals may precipitate into the waste sludges.

Jtherwise nonhazardous sludges may be rendered hazardous.

)il and grease that pass through the primary treatment system will form grease

n the surface of the aeration tank, The microorganisms cannot degrade this

il because it is not in the water where they can physically come in contact

Special consideration should be given to the surface skimming equipment in

Jndary clariher to handle the grease balls.

lary clarifier design considerations. Although the secondary settling tank

5-26) is an integral part of both the trickling filter and the activated sludge

, environmental engineers have focused particular attention on the secondary

' used after the activated sludge process. A secondary clarifier is important

s ofthe high solids loading and fluffy nature ofthe activated sludge biological

so. it is highly desirable that sludge recycle be well thickened.

:condary settling tanks for activated sludge are generally characterized as

Type Ill settling. Some authors would argue that Types l and ll also occur.

ie following guidance has been excerpted from the Joint Task Force of the

ollution Control Federation and the American Society of Civil Engineers.”

T

Return Sludge Line System

FIGURE 5-Z6

Rendering and cross~sectional diagram of secondary settling tank tfouitesy of FMC Corporation.)

The design factors discussed here are the result of the experiences of investigators,

plant superintendents, and equipment manufacturers. The criteria primarily apply

to circular (or square) center-fed basins, which comprise the majority of activated

sludge secondary settling units designed in the last 25 years.

An overfiow rate between 20 and 34 m/d for the average How in a conventional

process can be expected to result in good separation of liquid and SS. The design

engineer also must check the peak hydraulic rates that will be imposed on the settling

basin.

Suggested secondary settling tank side water depths (SWD) and solids loading

rates are shown in Table 5~ l2 and Figure 527, respectively.

The GLUMRB has set maximum recommended weir loadings for secondary

settling basins at 125 to 250 nr;/d per in of weir length (in-l/d - rn). This criterion is

based on effluent quality of operating units. lt appears that the settling basin design

may have had much to do with this limitation. Also, IHOSI ofthe observations apply

to rectangular settling basins.

One of the continuing difficulties with the design of secondary settling tanks is

the prediction of effiuent suspended solids concentrations as a function of common

design and operating parameters. Little theoretical work has been conducted, and

empirical conelations have been less than satisfactorylg

Sludge problems. A bulking sludge is one that has poor settling characteristics and

poor compactability There are two principai types of sludge bulking. The hrst is

caused by the growth of tilamentous organisms, and the second is caused by wit

ter trapped in the bacterial floc, thus reducing the density of thc aggloinerate and

resulting in poor settling.

Filamentous bacteria have been blamed for much ot’ the bulking problem in

activated sludge. Although hlamentous organisms are effective in removing organic

matter, they have poor liocforming and settling Cl\2\I`L\C[Cl`lSllCS. Bulking may also he

cttused by a number of other factors, including long, slow-moving collectiowsystein

transport; low available ammonia nitrogen when the organic load is high; low pH,

which could favor acid-t`ztvoring fungi; and the lack of m;1cronutrients_ which stint

ulates predomination ofthe tilnmentous ztctinomycetes over the normal fioc-i`ornting

bacteria. The luck of nitrogen also invors slime-producing bncterin, which httvc tt

lo\v specific gravity, even though they are not iilamentous. The multicellulnr tun-

gi cannot compete with thc liztctcriii normally but can contpctc under Sl)Ct,`lllt`

tmvironmental conditions, such us low pll, low nitrogen, low oxygen, and ltigxh

bohydrates. As the pH decreases below 6.0, the fungi are less affected than

bacteria and tend to predominate. As the nitrogen concentrations drop below 3

'DS I N ratio of 20: l , the fungi, which have a lower protein level than the bacteria,

able to produce normal protoplasm, while the bacteria produce nitrogen-dehciem

toplasm_ `

g A sludge that iloats to the surface after apparently good settling is called 3

f'18_Sll1dge. Rising sludge results from denitnfication, that is, reduction of nitrates

i nitritesto nitrogen gas in the sludge blanket (layer). Much of this gas remains

pped in the sludge blanket, causing globs of sludge to rise to the surface and Heat

:r the weirs into the receiving stream.

Rising-sludge problems can be overcome by increasing the rate of retum

dge l:l0W (Qf), by increasing the speed of the sludge-collecting mechanism, by

zreasing the mean cell residence time, and, if possible, by decreasing the flow

m the aeration tank to the offending tank.

ridation Ponds

3aU1`1€llt ponds have been used to treat wastewater for many years, particularly as

tstewater treatment systems for small communities.” Many terms have been used

describe the different types of systems employed in wastewater treatment. For

3U\Pl€, in recent years, oxidation pond has been widely used as a collective term

'all types of ponds. Uriginally, an oxidation pond was a pond that received partially

ated wastewater, whereas a pond that received raw wastewater was known as a

it-'age lagoon Waszc smbiligarion pond has been used as an all-inclusive term that

'ers to a pond or lagoon used to treat organic waste by biological and physical

Jcesses. These processes would commonly be referred to as self-purification if

:y took place in a stream_ To avoid confusion, the classification to be employed in

,s discussion will be as follows;3°

Aelobic ponds: Shallow ponds, less than l m in depth, where dissolved oxy-

gen lS maintained throughout the entire depth, mainly by the action of photosyn-

thesis.

Facultative ponds: Ponds l to 2.5 m deep, which have an anaerobic lower zone,

a facultative middle zone, and an aerobic upper zone maintained by photosyn-

thesis and surface reaeration_

Arraerobic ponds: Deep ponds that receive high organic loadings such that anaer-

obic conditions prevail throughout the entire pond depth.

_ Maturalion or tertiary ponds: Ponds used for polishing effluents from othtzr br-

ological processes. Dissolved oxygen is fumished through pliotosynthcsis and

surface reaeration. This type of pond is also known as a polls/1in_r; [mm/_

5_ Aerated lagoons: Ponds oxygenated through the action of surface or diffused air

aeration.

;\erobic ponds. The aerobic pond is a shallow pond in which light penetrates to

ihe bottom, thereby maintaining active algal photosynthesis throughout the entire

System. During the daylight hours, large amounts of oxygen are supplied by the

'photosynthesis process; during the hours of darkness, wind mixing of the shallow

water mass generally provides a high degree of surface reaeration. Stabilization of

the organic material entering an aerobic pond is accomplished mainly through the

'action of aerobic bacteria.

iAnaerobic ponds. The magnitude of the organic loading and the availability of dis-

solved oxygen determine whether the biological activity in atreatment pond will oc-

cur under aerobic or anaerobic conditions. A pond may be maintained in an anaerobic

condition by applying a BOD5 load that exceeds oxygen production from photosyn-

thesis. Photosynthesis can be reduced by decreasing the surface area and increasing

the depth. Anaerobic ponds become turbid from the presence of reduced metal sul-

tides. This restricts light penetration to the point that algal growth becomes negligi-

ble. Anaerobic treatment of complex wastes involves two distinct stages. ln the first

stage (known as acid fermentation), complex organic materials are broken down

mainly to short-chain acids and alcohols. In the second stage (known as methane

fermentation), these materials are converted to gases, primarily methane and carbon

dioxide. The proper design of anaerobic ponds must result in environmental condi-

tions favorable to methane fermentation.

Anaerobic ponds are used primarily as a pretreatment process and are partic-

ularly suited for the treatment of high-temperature, high-strength wastewaters.

However, they have been used successfully to treat municipal wastewaters as

well.

-Facultative ponds. Of the five general classes of lagoons and ponds, facultative

ponds are by far the most common type selected as wastewater treatment systems

for small communities. Approximately 25 percent of the municipal wastewater treat-

ment plants in this country are ponds and about 90 percent of these ponds are located

in communities of 5,000 people or fewer. Facultative ponds are popular for such

treatment situations because long retention times facilitate the management of large

fluctuations in wastewater flow and strength with no significant effect on eftiuent

quality Also capital, operating, and maintenance costs are less than those of other

biological systems that provide equivalent treatment.

A schematic representation of a facultative pond operation is given in Fig-

ure 5-28. Raw wastewater enters at the center of the pond. Suspended solids

contained in the wastewater settle to the pond bottom, where an anaerobic layer

develops. l\/licroorganisms occupying this region do not require molecular oxygen

as an electron acceptor in energy metabolism, but rather use some other chemi-

cal species. Both acid fermentation and methane fermentation occur in the bottom

E S-28

iatic diagram of facultative pond relationships.

The facultative zone exists just above the anaerobic zone. This means that

Iiolecular oxygen will not be available in the region at all times. Generally, the zone

.s aerobic during the daylight hours and anaerobic during the hours of darkness.

Above the facultative zone, there exists an aerobic zone that has molecular

Jxygen present at all times. The oxygen is supplied from two sources. A limited

amount is supplied from diffusion across the pond surface. However, the majority is

;upplied through the action of algal photosynthesis.

Two rules of thumb commonly used in Michigan in evaluating the design of

facultative lagoons are as follows:

l. The BOD5 loading rate should not exceed 22 kg/ha - d on the smallest lagoon

cell. ~

Z. The detention time in the lagoon (considering the total volume of all cells but

excluding the bottom 0.6 min the volume calculation) should be six months.

The first criterion is to prevent the pond from becoming anaerobic. The second

:riterion is to provide enough storage to hold the wastewater during winter months

when the receiving stream may be frozen or during the summer when the flow in the

;tream might be too low to absorb even a small amount of BOD.

Solution. To compute the BUD loading, we must nrst compute the mass of BOD;

entering each day.

BOD5 mass 1 (122 mg/L)(l,9O0 m3)(l,0OO L/m3)(l >< 10"6 mg/kg) = 231.8 kg/d

Then, we must convert the area into hectares. Using only one cell,

4 Area = (l15,000 mZ)(l >< 10” ha/m2)

= 11.5 ha each

Now we can compute the loading.

` 231.8 kv/d

BOD; loading 1 ~~i3flf;~ = 20.2 kg/ha - d

This loading rate is acceptable.

The detention time is simply the working volume between the minimum and

maximum operating levels divided by the average daily flow.

D mmm I ti ls (ll5,()()O m3)(3 lagoons)(l.5 4 0,6 ni)

e 1 I1 1 -~_i~l---_-\»--A-

l,9UO IH]/d

LT 165 .4 days

This is less than the desired 180 days,

We have ignored the slope ofthe lagoon walls in this calculation, For large la-

goons, this is probably acceptable. ln small lagoons, the slope should be considered.

Rotating Biological Contactors (RBCS)

The RBC process consists of a series of closely spaced discs (3 to 3.5 m in diame-

ter) mounted on a horizontal shaft and rotated, while about one-half of their surface

area is immersed in wastewater (Figure 5 -29). The discs are typically constructed of

lightweight plastic. The speed of rotation ofthe discs is adjustable.

When the process is placed in operation, the microbes i11 the wastewater begin

to adhere to the rotating surfaces and grow there until the entire surface area of the

discs is covered with a 1- to 3-mm layer of biological slime. As the discs rotate, they

carry a film of wastewater into the air; this wastewater trickles down the surface of

the discs, absorbing oxygen. As the discs complete their rotation, the film of water

mixes with the reservoir of wastewater, adding to the oxygen in the reservoir and

mixing the treated and partially treated wastewater. As the attached microbes pass

through the reservoir, they absorb other organics for breakdown. The excess growth

of microbes is sheared from the discs as they move through the reservoir. These

dislodged organisms are kept in suspension by the moving discs. Thus, the discs

serve several purposes:

1. They provide media forthe buildup of attached inicrohial growth.

2. They bring the growth into contact with the Wtlstcwztlttti

The attached growths are similar in concept to a trickling filter, except the

nicrobes are passed through the wastewater rather than the wastewater passing over

he microbes, Some of the advantages of both the trickling filter and activated sludge

>rocesses are realized. ‘

As the treated wastewater flows from the reservoir below the discs, it carries

he suspended growths out to a downstream settling basin for removal. The process

:an achieve secondary effluent quality or better. By placing several sets of discs in

;eries, it is possible to achieve even higher degrees of treatment, including biological V

:onversion of ammonia to nitrates. I

W i

DISINEECTION

Ei iixiie last treatment step in a secondary plant is the addition of a disinfectant to the

3 -gated Wastewater.” The addition of chlorine gas or some other form of chlorine is

rocess most commonly used for wastewater disinfection in the United States

5 p

ffijlorine is injected into the wastewater by automated feeding systems. Wastewater

flows into a basin, where it is held for about 15 minutes to allow the chlorine to

with the pathogens.

There is concern that wastewater disinfection ma do more harm than Good.

'ly U_S_ Environmental Protection Agency rules calling for disinfection to achieve

fecal coliforrns er 100 mL of wastewater have been modified to a re uirenient

fdisinfection onl during the summer season when eo le ma come into contact

y U P P y

contaminated water. There were three reasons for this change. The first was that

use of chlorine and, perhaps, ozone causes the formation of organic compounds

are carcinogenic. The second was the findin that the disinfection rocess was

effective in killing the predators to cysts and viruses than it was in killing the

%‘§f52lfl10g€HS themselves_ The net result was that the pathogens* survived longer in the

ii l v' nment because there were fewer predators The third reason was that

f-it . =

Qpatura en iro

is toxic to nsh_

259 ADVANCED WASTEWATER

Ellthough secondary treatment processes, when coupled with disinfection, inay re-

iinove over 85 percent of the BOD and suspended solids and nearly all pathogens,

*only minor removal of some pollutants, such as nitrogen, phosphorus, soluble COD,

isand heavy metals, is achieved. In some circumstances, these pollutants may be of

concern. In these cases, processes capable of removing pollutants not ade-

fguately removed by secondary treatment are used in what is called tertiary waste-

treatment, or advanced wastewater treatment (AWT). The following sections

available AWT processes. In addition to solving tough pollution problems,

processes improve the effluent quality to the point that it is adequate for many

feuse purposes, and may convert what was originally a wastewater into a valuable

tgresource too good to throw away. »

,

ew ,ew : .~

/5_1 , W

“wife -

?Filtration

§__Secondary treatment processes, such as the activated-sludge process, are highly et-

§§§fiCient for removal of biodegradable colloidal and soluble organics. However, the

efliuent contains a much higher BOD5 than one would expect from theory.

typical BOD is approximately 20 to 50 mgfL. This is principally because the

-

M 7

econdary clariflers are not perfectly efficient at settling out the microorganisms from

he biological treatment processes. These organisms contribute both to the suspended

Lolids and to the BOD5 because the process of biological decay of dead cells exerts

in oxygen demand.

By using a filtration process similar to that used in water treatment plants, it is

iossible to remove the residual suspended solids, including the unsettled microor-

ganisms. Removing the microorganisms also reduces the residual BOD5. Conven-

ional sand filters identical to those used in water treatment can 'be used, but they

iften clog quickly, thus requiring frequent backvvashing. To lengthen filter runs and

'educe backwashing, it is desirable to have the larger Glter grain sizes at the top

if the filter. This arrangement allows some of the larger particles of biological fioc

0 be trapped at the surface without plugging the hlter. Multimedia filters accom-

>lish this by using low-density coal for the large grain sizes, medium-density sand

'or intermediate sizes, and high-density garnet for the smallest size hlter grains.

fhus, during backwashing, the greater density offsets the smaller diameter so that

he coal remains on top, the sand remains in the middle, and the garnet remains on the

iottom.

Typically, plain filtration can reduce activated sludge effluent suspended solids

'rom 25 to 10 mg/L_ Plain nltration is not as effective on trickling filter effluents

>ecause trickling filter effluents contain more dispersed growth. However, the use

>f coagulation and sedimentation followed by filtration can yield suspended solids

:oncentrations that are virtually zero. Typically, filtration can achieve 80 percent

uspended solids reduction for activated sludge effluent and 70 percent reduction

or triclding filter effluent.

farbon Adsorption

Even after secondary treatment, coagulation, sedimentation, and filtration, soluble

'rganic materials that are resistant to biological breakdown will persist in the effiu-

Vnt. The persistent materials are often referred to as refractory organics. Refractory

»rganics can be detected in the effiuent as soluble COD. Secondary effluent COD

'alues are often 30 to 60 mg/L.

The most practical available method for removing refractory organics is by

rdsorbing them on activated carbon.” Adsorption is the accumulation of materials

,t an interface. The interface, in the case of wastewater and activated carbon, is

he liquid/solid boundary layer. Organic materials accumulate at the interface be-

ause of physical binding of the molecules to the solid surface. Carbon is activated

»y heating in the absence of oxygen. The activation process results in the forma~

lon of many pores within each carbon particle. Since adsorption is a surface phe-

omenon, the greater the surface area of the carbon, the greater its capacity to hold

rganic material. The vast areas of the walls within these pores account for most

of the total surface area of the carbon, which makes it so effective in removing

organics.

After the adsorption capacity of the carbon has been exhausted, it can be re-

gtored by heating it in a fumace at a temperature sufhciently high to drive off the

adsorbed organics. Keeping oxygen at very low levels in the furnace prevents car~

bon from burning. The organics are passed through an afterbumer to prevent air

pollution. In small plants where the cost of an on~site regeneration furnace can~

not be justihed, the spent carbon is shipped to a central regeneration facility for

processing,

Phosphorus Removal

All the polyphosphates (molecularly dehydrated phosphates) gradually hydrolyze in

aqueous solution and revert to the ortho form (POff) from which they were derived.

Phosphorus is typically found as mono-hydrogen phosphate (HPO§') in wastewater.

The removal of phosphorus to prevent or reduce eutrophication is typically

accomplished by chemical precipitation using one of three compounds. The precip-

itation reactions for each are shown below.

Using ferric chloride:

Feel; + Hi>o§r : FePOil + ii* + 3ci <5»46>

Using alum:

Al;(SC).i)3 + 2HPO§f 1 2AlPO4l + ZH* + 3SO§' (547)

Using lime:

5Ca(OH);`+ 3HPOff 2 Ca5(P()4)3OHt + 3H;O + 6OH' (5-48)

You should note that ferric chloride and alum reduce the pH while lime increases

it. The effective range of pH for alum and ferric chloride is between 5.5 and 7.0. If

there is not enough naturally occurring alkalinity to buffer the system to this range,

then lime must be added to counteract the formation of H+.

The precipitation of phosphorus requires a reaction basinand a settling tank

to remove the precipitate. When ferric chloride and alum are used, the chemicals

may be added directly to the aeration tank in the activated sludge system. Thus, the

aeration tank serves as a reaction basin. The precipitate is then removed in the sec-

ondary clarifier. This is not possible with lime since the high pl-I required to form

the precipitate is detrimental to the activated sludge organisms. In some wastewater

treatment plants, the FeCl3 (or alum) is added before the wastewater enters the pri-

mary sedimentation tank. This improves the efficiency of the primary tank, but may

deprive the biological processes of needed nutrients.

Solution. From Equation 546, we see that one mole of ferric chloride is required fgf

each mole of phosphorus to be removed. The pertinent gram molecular weights are 35

follows:

FeCl3 = 162,21 g

P 2 30.97 g

With a PO4-P of4.00 mg/L, the theoretical amount of ferric chloride would be

` 62.21

4.00 >< ; 1 20.95 or2l_0mg/L

30.97

Because of side reactions, solubility product limitations. and day-to~day variations,

the actual amount of chemical to be added must be determined by jar tests on the

wastewater. You can expect that the actual ferric chloride dose will he 1_5 to 3 times

the theoretically calculated amount. Likewise, the actual alum dose will be 1.25 to 2.5

times the theoretical amount.

Nitrogen Control

Nitrogen in any soluble form (NH3, NHI, NOQ, and NOQ. but not N; gas) is a nu~

trient and may need to be removed from wastewater to help control algal growth in

the receiving body. In addition, nitrogen in the form of ammonia exerts an oxygen

demand and can be toxic to Fish. Removal of nitrogen can be accomplished either bio-

logically or chemically. The biological process is called nifrzjication/dezzizrgdcariort

The chemical process is called ammonia stripping.

Nitrification/denitrification. The natural nitrification process can be forced to

occur in the activated-sludge system by maintaining a cell detention time (HC) of

I5 days in moderate climates and over 20 days in cold climates. The nitrification

step is expressed in chemical terms as follows:

NH; + 202 i NO§ + H20 + ZHT (5-49)

Of course, bacteria must be present to cause the reaction to occur, This step satisfies

the oxygen demand ofthe ammonium ion. If the nitrogen level is not of concern

for the receiving body, the wastewater can be discharged after settling. If nitrogen

is of concern, the nitrification step must be followed by anoxic denitrihcation by

bacteria: '

2NO§ + organic matter -+ N3 + CO3 + H30 (5-50)

As indicated by the chemical reaction, organic matter is required for denitrilication.

Organic matter serves as an energy source for the bacteria. The organic matter may

be obtained from within or outside the cell. In multistage nitrogen»removal systems,

because the concentration of BOD; inthe flow to the denitrihcation process is usually

quite low, a supplemental organic carbon source is required for rapid denitrification

(BGD5 concentration is low because the wastewater previously has underg

bonaceous BOD removal and the nitritication process.) The organic matter

either raw, settled sewage or a synthetic material such as methanol (CH3Ol

settled sewage may adversely affect the effluent quality by increasing the B4

ammonia content.

Ammonia stripping. Nitrogen in the form of ammonia can be removed ch

from water by raising the pH to convert the ammonium ion into ammonia, w

then be stripped from the water by passing large quantities of air through tl

The process has no ‘effect on nitrate, so the activated sludge process must

ated at a short cell-detention time to prevent tiitriticntion The ammonia stri;

action is

NH; + one 1 NH; + iilo

The hydroxide is usually supplied by adding lime. The lime also reacts wit

the air and water to form a calcium carbonate scale. which IUUSI be remo'

odically. Low temperatures cause problems with icing and reduced strippin

The reduced stripping ability is caused by the increased solubility of am

cold water.

5-10 LAND TREATMENT

This discussion on land treatment follows two EPA publications; Envin

Control Alternatives: Municipal l/Wzstewater and Land Treatment of Af

Wastewater Ejyflnents, Design Factors 1.3

An alternative to the previously discussed AWT processes for produci

tremely high-quality effluent is offered by an approach called land treatmt

treatment is the application of effluents, usually following secondary treat

the land by one of the several available conventional irrigation methods.

proach uses wastewater, and often the nutrients it contains, as a resource ra

considering it as a disposal problem. Treatment is provided by natural pro

the effluent moves through the natural filter provided by soil and plants. P

wastewater is lost by evapotranspiration, while the remainder returns to tl

logic cycle through overland How or the groundwater system, Most of tht

water eventually returns, directly or indirectly, to the surface water systerr

Land treatment of wastewaters can provide moisture and nutrients i

for crop growth, In semiarid areas, insufficient moisture for peak crop gr

limited water supplies make this water especially valuable. The primary

(nitrogen, phosphorus, and potassium) are reduced only slightly in con

secondary treatment processes, so that most of these elements are still t

4 .

by losses through soil erosion may be replaced by the application of wastewater.

Land application is the oldest method used for treatment and disposal of wastes;

Cities have used this method for more than 400 years. Several major cities, including

Berlin, Melbourne, and Paris, have used “sewage farms” for at least 60 years for

waste treatment and disposal. Approximately 600 communities in the United States

reuse municipal wastewater treatment plant effluent in surface irrigation.

Land treatment systems use one of the three basic approaches:

1. Slow rate

2. Overland Flow

3. Rapid infiltration

Each method, shown schematically in Figure 5-30, can produce renovated wa-

ter of different quality, can be adapted to different site conditions, and can satisfy

different overall objectives.

Slow Rate

Irrigation, the predominant land application method in use today, involves the

application of effluent to the land for treatment and for meeting the growth needs

of plants. The applied effluent is treated by physical, chemical, and biological

means as it seeps into the soil.” Efduent can be applied to crops or vegetation

(including forestland) either by sprinkling or by surface techniques, for purposes

such as:

1. Avoidance of surface discharge of nutrients

2. Economic return from use of water and nutrients to produce marketable crops

3. Water conservation by exchange when lawns, parks, or golf courses are irrigated

4. Preservation and enlargement of greenbelts and open space

Where water for irrigation is valuable, crops can be irrigated at consumptive

use rates (3.5 to l0 mm/d, depending on the crop), and the economic return from the

sale of the crop can be balanced against the increased cost of the land and distribution

system. On the other hand, where water for irrigation is of little value, hydraulic

loadings can be maximized (provided that renovated water quality criteria are met),

thereby minimizing system costs. Under high-rate irrigation (10 to l5 mm/d), water-

tolerant grasses with high nutrient uptake become the crop of choice.

How is essentially a biologica trea me p

ver the upper reaches of sloped terraces and allowed to flow across the v¢g_

rface to runoff collection ditches. Renovation is accomplished by physical

, and biological means as the wastewater flows in a thin sheet down the

l t nt rocess in which wastewater is

/ impervious slope. _ V

l rid How can be used as a secondary treatment process where dischafgé

er a

lied effluent low in BOD is acceptable or as an advanced wastewater treat;

cess. The latter objective will allow higher rates of application (l8 mm/dbf

d' <1 on the degree of advanced wastewater treatment required. Whgre

epen ing g

2 discharge is prohibited, runoff can be recycled or applied to the land in

1 or inhltration-percolation systems.

[nfiltration _

'ation-percolation systems, effluent is applied to the soil at higher rates by

ig in basins or by sprinkling. Treatment occurs as the water passes through

matrix. System objectives can include:

if ‘Q

indwater recharge

d ‘ hd wal or the use of underdrains for {_}§§

,ral treatment followed by pumpe wit ra

very

iral treatment where re

recharges a surface watercourse

novated water moves vertically and laterally in the soil

'here groundwater quality is being degraded by salinity intrusion, groundwa-

` ` ' = ‘ ` ' d te

arue can reverse the hydraulic gradient and protect the existing groun wa r. .ff

D

existing groundwater quality is not compatible with expected renovated qual-

/here existing water rights control the discharge location, a return of renovated'

' ' d ` or

3 surface water can be designed, using pumped withdrawal, under rains,

drainage. At Phoenix, Arizona, for example, the native groundwater quality

b ' hdrawn b umping, with discharge into

and the renovated water is to e wit y p

;ation canal. _

SLUDGE TREATMENT

process of purifying the wastewater, another problem is created; sludge. The

the degree of wastewater treatment, the larger the residue of sludge that must

` l

dl d. The exce tions to this rule are where land applications or polishing a-

C P

are used. Satisfactory treatment and disposal of the sludge can be the single

omplex and costly operation in a municip

al \VLlS[C\VZ1{Cl` [I`C2\UI1Cll[ SyST€l'l\.36

ri

‘ff ¢ Sludge is made of materials settled from the raw Wastewztter and of solids gen»

grated in the wastewater treatment processes.

The quantities of sludge involved are significant. lior primary treatment, they

iatay be 0.25 to 0.35 percent by volume of wastewater treated. When treatment is

pgfaded to activated sludge, the quantities increase to l.5 to 2.0 percent of this

`3}0lurne of water treated. Use of chemicals for phosphorus removal can add another

L0 percent. The sludges Withdrawn from the treatment processes are still largely

Svater, as much as 97 percent. Sludge treatment processes, then, are concerned with

fégparating the large amounts of water from the solid residues. The separated water

returned to the Wastewater plant for processing.

The basic processes for sludge treatment are as follows;

\

1, Thicr/ceriing: Separating as much water as possible by gravity or llotation.

rf _ Stabilization: Converting the organic solids to more refractory (inert) forms so

that they can be handled or used as soil conditioners without causing a nuisance

or health hazard through processes referred to as "digestion" (These are biochem-

'tif ical oxidation processes.)

bb

Conditioning: Treating the sludge with chemicals or heat so that the water can

be readily separated.

Dewatering: Separating water by subjecting the sludge to vacuum, pressure, or

F

drying.

Reduction: Converting the solids to a stable form by wet oxidation or incineration.

(These are chemical oxidation processes; they decrease the volume of sludge,

hence the term reduction.)

Although a large number of alternative combinations of equipment and pro-

cesses are used for treating sludges, the basic alternatives are fairly limited. The

ultimate depository of the materials contained in the sludge must either be land, air,

or water. Current policies discourage practices such as ocean dumping of sludge.

Air pollution considerations necessitate air pollution control facilities as part of the

sludge incineration process.

The following sections discuss the processes commonly used. The basic al-

ternative routes by which these processes may be employed are shown in Fig-

ure 5-31.

Sources and Characteristics of Various Sludges

Before we begin the discussion ofthe various treatment processes, it is worthwhile

to recapitulate the sources and nature of the sludges that must be treated.

i

Grit. The sand, broken glass, nuts, bolts, and other dense material that is collected

in the giit chamber is not true sludge in the sense that it is not fluid. However, it

still requires disposal. Because grit can be drained of water easily and is relatively

stable in terms of biological activity (it is not biodegradable), it is generally trucked

directlv to a landfill without further treatment.

mary or raw sludge. Sludge from the bottom ofthe primary clarihers contains

11 3 to 8 percent solids (l percent solids 2 l g solids/l00 mL sludge volume),

ich is approximately 70 percent organic. This sludge rapidly becomes anaerobic

l is highly odiferous.

zondary sludge. This sludge consists of microorganisms and inert materials that

e been wasted from the secondary treatment processes. Thus, the solids are about

percent organic. When the supply of air is removed, this sludge also becomes

erobic, creating noxious conditions if not treated before disposal. The solids con-

. depends on the source. Wasted activated sludge is typically 0.5 to 2 percent

ds, while trickling filter sludge contains 2 to 5 percent solids. In some cases,

mdary sludges contain large quantities of chemical precipitates because the aer-

n tank is used as the reaction basin for the addition of chemicals to remove phos-

rus.

tiary sludges. The characteristics of sludges from the tertiary treatment pro-

;es depend on the nature of the process. For example, phosphorus removal re-

s in a chemical sludge that is difhcult to handle andtreat. When phosphorus

oval occurs in the activated sludge process, the chemical sludge is combined

1 the biological sludge, making the latter more difhcult to treat. Nitrogen removal

lenitrification results in a biological sludge with properties very similar to those

'aste activated sludge.

ids Computations

ime-mass relationships. Since most WWTP sludges are primarily water, the

me ofthe sludge is primarily a function ofthe water content. Thus, if we know

aercent solids and the specific gravity ofthe solids we can estimatethe volume

ie sludge. The solid matter in wastewater sludge is composed of hxed (mineral)

50lidS and volatile (organic) solids. The volume of the total mass of solids may be

'expressed as

M _

Vsolids : _L (5“-32)

SSP

where M, = mass of solids, kg

S, = specitic gravity of solids

p 2 density of water 1 1,000 kg/m3

Since the total mass is composed of fixed and volatile fractions, Equation 5-52 may

be rewritten as:

A/Ii = & + £3 (5-53)

Sm Sfp Sup

where Mf = mass of lixed solids, kg

MU I mass of volatile solids, kg

Sf = specific gravity of fixed solids

SU = specific gravity of volatile solids

The specihc gravity of tltc solids may be tzxptessetl in terms oi the specific gravities

ofthe fixed and solid l`i';it‘tio|is by solving liqtizition 5 5) for 51,3

0 s -su

f (5~54)

st : M, _+~_-

V Mflgv -ir M5Sf`

The specific gravity of sludge (SKI) may be estimated by recognizing that, in a

similar fashion to the fractions of solids, the sludge is composed of solids and water

so that

&L Z .i. (555)

Sup S;/I Awp

where M51 = mass of sludge, kg

MW = mass of water, kg

SS, 2 specific gravity of sludge

SW = specific gravity of water ~

lt is customary to report solids concentrations as percent solids, where the fraction

of solids (P,) is computed as

M_

Pt 2 W; -56

t Mot. + Mt.. (5 )

and the fraction of water (PW) is computed as

MW

PW I; M__,I+..M._

8 H

Thus, it is more convenient to solve Equation 54-52 in terms of percent solids. lf we

divide each term in Equation 555 by (M, 9 MW) and recognize that M_,, 1 M5 +11/IW,

§;¢Mass balance. Barring black holes" and the uae, we an unueisuinu into me pnys-

Qical, chemical, and biological processes of wastewater treatment neither create nor

ii destroy matter. This fact allows us to employ Equation l-3 in anew context.

7 Z Min 'T Mont

df

where Min and /tim refer to the mass of dissolved chemicals. solids. or gas entering

and leaving a process or group of processes. lt` we assume steady-state conditions,

then dS/dt = O and Equation 5-61 reduces to the following:

Min Z Moot (5'6/2)

Several interrelated processes are examined together in the Flowsheet shown

5* in Figure 5-32. When labeled with mass flows, the iiowsheet may be called a quan-

titative flow diagram (QFD). The solids mass balance can be an important aid to a

designer in predicting long-term average solids loadings on sludge treatment com-

` ponents. This allows the designer to establish such factors as operating costs and

’ quantities of sludge for ultimate disposal. However, it does not establish the solids

loadiri that each e ui ment item must be ca able of rocessin _ A articular com-

8 Cl P P P g P

ponent should be sized to handle the XTIOSI rigorous loading conditions it is expected

to encounter. This loading is usuall not determined b a l ing stead -state mod-

D Y Y PP Y D Y

` els because of storage and plant scheduling considerations. Thus, the rate of solids

reaching any particular piece of equipment does not usually rise and fall in direct

proportion to the rate of solids arriving at the plant headworl<s_

The mass balance calculation is carried out in a step-by-step procedure:

l.. Draw the flowsheet (as in Figure 5-32).

2. Identify all streams. For example, Stream A contains raw sewage solids plus

chemical solids generated by dosing the sewage with chemicals. Let the mass

flow rata of solids in Stream A be equal to A kg pci' day.

For each processing unit, identify the relationship of entering and leaving streams

to one another in terms of mass. For example, for the primary sedimentation tank,

let the ratio of solids in the tank underiiow (E) to entering solids (A + M) be equal

to 115. 715 is actually an indicator of solids separation efficiency. The general form

in which such relationships are expressed is:

mass of solids in stream i

1 ~ = _ . .

7' mass of solids entering the unit

(5-63)

For example,

_Y P - _ - J

"P K + HW/ Y E

The processing unit’s performance is specified when a value is assigned to 17,

Combine the mass balance relationships so as to reduce them to one equation

describing a specific stream in terms of given or known quantities, or ones which

can be calculated from a knowledge of the process behavior.

Example S-I7. Using Figure 532 and assuming that A, 115, 171, ny, Typ, and 17” are

known or can be determined from a knowledge ol' water chemistry and an understand»

The example just worked was relatively simple. A more complex system

illustrated in Figure 533. Mass balance equations for this system are summarized

in Table 544. For this llowsheet the following information HIUSI be specified: i

A_

X_

175 770- TI/~ 77/\/» Tl/ei 231141 UT

770

$224

lntiuent solids

Effluent solids, that is, overall suspended/5%

solids removal must be specified

straightforward assumptions about the degreepfg

of solids removal, addition, or destruction

describes the net solids destruction reductioqiiié;

or the net solids synthesis in the biologicalffféé

system, and must be estimated from yield(

data. A positive 171; signines net solids def”

¥1E`&“

'ti "Q

aces

' 1-(ei

struction. A negative 170 signifies net

growth. ln this example, 8 percent of

solids entering the biological process are asf

sunied destroyed, that is, converted to gas or,

liquified

Note that alternative processing schemes can be evaluated simply by nizinipu- '”t‘ 1”

lating appropriate variables. For example:

Filtrtition can be eliminated by setting 17/Q to zero.

”l`liicl<ening can be eliminated by setting 775 to zero.

Digestion can be eliminated by setting T); to Zero.

Dewatering can be eliminated by setting Up to Zero.

g wi

tQ!0p of the liquid Uiofarionl or are allowed to settle to the bottom (gravizy I/If()/(Clif/ly).

a owsheet tor a tomplex WWTP (Source: U.S_ Environmental Protection Agency, I’rvce.»'.»‘

:gn Manual Sludge Treatment and Disposal.)

A system without primary sedimentation can be simulated by setting 175 equal

exactly zero since division by 175 produces indeterminate solutions when

eomputtng E

A set of different mass balance equations must be derived it’ flow paths between

tocessnxg units are altered. For example, the equations ot`Tahle 5- l 4 do not describe

perations in which the dilute stream from the thickener (Stream G) is returned to

secondary reactor instead of the primary sedimentation tank.

Thiekening

Thickening is usually accomplished in one of two ways; the solids are floated to the

F

#f

;_GURE 5-33

és' - ‘

to approximately zero, for example, l >< l0'8. 175 cannotvbe set equal to

Y A

Ti

FIGURE 5-34

5 Air notation thickener.

mechanism for further processing. The process typically increases the soli

tent of activated sludge from 0.541 percent to 3~6 percent. Flotation is est

effective on activated sludge, which is difficult to thicken by gravity.

Gravity thickening. Gravity thickening is a simple and inexpensive proci

has been used widely on primary sludgcs for many years. lt is essentially

mentation process similar to that which occurs in all settling tanks. Sludg

into a tank that is very similar in appearance to the circular clariliers used in 1

and secondary sedimentation (Figure 5-35); the solids are allowed to settle to

tom where a heavy-duty mechanism scrapes them to a hopper from which t

withdrawn for further processing. The type of sludge being thickened has a rr

fect on performance. The best results are obtained with purely primary slud

the proportion of activated sludge increases, the thickness of settled sludg

decreases. Purely primary sludges can be thickened from l~3 percent to

cent solids. The current trend is toward using gravity thickening for primary

Ri-irlcve

FIGURE 5-34

5 Air notation thickener.

mechanism for further processing. The process typically increases the soli

tent of activated sludge from 0.541 percent to 3~6 percent. Flotation is est

effective on activated sludge, which is difficult to thicken by gravity.

Gravity thickening. Gravity thickening is a simple and inexpensive proci

has been used widely on primary sludgcs for many years. lt is essentially

mentation process similar to that which occurs in all settling tanks. Sludg

into a tank that is very similar in appearance to the circular clariliers used in 1

and secondary sedimentation (Figure 5-35); the solids are allowed to settle to

tom where a heavy-duty mechanism scrapes them to a hopper from which t

withdrawn for further processing. The type of sludge being thickened has a rr

fect on performance. The best results are obtained with purely primary slud

the proportion of activated sludge increases, the thickness of settled sludg

decreases. Purely primary sludges can be thickened from l~3 percent to

cent solids. The current trend is toward using gravity thickening for primary

Ri-irlcve

and flotation thickening for activated sludges, and then blending the thickened

sludges for further processing.

Dick” has described a graphical procedure for sizing gravity thickencrs using

a batch flux curve. Flux is the term used to describe the rate of settling of solids. It

is defined as the mass of solids which pass through a horizontal unit area per unit of

time (kg/d - m2). This may be expressed mathematically as follows:

/"5 1 (CHXU) (5 64)

I (C,)(zone settling velocity)

where F, = solids flux, kg/mg < d

Cx = suspended solids concentration, kg/nil

C,, = concentration of solids in underflow, that is, sludge withdrawal pipe,

kg/m3

U = underflow velocity, m/d

The sizing procedure begins with a batch settling curve such as that shown in

Figure 5-36. Data from the batch settling curve are used to construct a batch flux

curve (Figure 5-37). Knowing the desired underflow concentration, a line through

the desired concentration and tangent to the batch flux curve is constructed. The

extension of this line to the axis ofordinates yields the design flux. From this flux

and the inflow solids concentration, the surface area may be determined.

Example 5-18. A gravity thickener is to be designed to thicken the sludge from the

primary tank described in Example 5- 16. The thickened sludge should have an under-

flow solids concentration of 10.0 percent. Assume that the sludge yields a batch settling

curve such as that shown in Figure 5~36.

Solution. First we must compute the solids flux for several arbitrarily selected sus-

pended solids concentrations.

SS, kg/nr' v, m/d F., kg/d ~ ntl SS, kg/nl’ v, m/d F,, kg/d - mz

100 0.125 12,5 20 5.30 106

80 0.175 14.0 34,0 340

60 0.30 18. 62.0 310

D0 0.44 22. 68.0 272

40 0.78 31. 76.0 228

T10 1,70 51. 33.0 166

“R. 1, Dick, "'l`1iit'1\ening," .»1¢/\~mir'¢'_\ zu 114110/' Quality /lll]II'0\'Cl!ll'Il[--P/1_\’5f(,`[1] mu/ C/l£'IlllL`£l[ [’m»

'u_t,r¢'s, E. F. Gloyna and W, W Fckenfizldcr, t-ds._ Austin, TX: University of Texas Press. p. 358, 1970.

l`heorigin;|1 tlevelopincnt of this inethotl was by N. Yoshioka and others. See “Continuous Thickening

it Honiogenous lflocculated S1UI`l`1CS,“(v/l(’Illl(`z1/EIl_§'ff1£.'(’I'ill`Q. 21, Tokyo 1957 (in Japanese).

rincipal purposes of sludge stabilization are to break down the organic solids

:mically so that they are more stable (less odorous and less putrescible) and

iewaterable, and to reduce the mass of sludge.” If the sludge is to be dewa~

and burned, stabilization is not used. There are two basic stabilization pr0_

in use. One is carried out in closed tanks devoid of oxygen and is called

Qbic digestion, The other approach injects air into the sludge to accomplish

?c digestion.

sic digestion. The aerobic digestion of biological sltxdges is nothing more than

,nuation ofthe activated sludge process. When a culture of aerobic heterotrophs

led in an environment containing a source of organic material, the microorgan-

ernove and utilize most of this material. A fraction ofthe organic material

ed will be used forthe synthesis of new biomass. The remaining material will

inneled into energy metabolism and oxidized to carbon dioxide, water, and

e inert material to provide energy for both synthesis and maintenance (lifea

rt) functions. Once the external source of organic material is exhausted, how»

he microorganisms enter into endogenous respiration. where cellular material

lized to satisfy the energy of maintenance (that is. energy for life~support re-

ients). If this condition is continued over an extended period of time, the total

ty of biomass will be considerably reduced. Furthermore, that portion remain-

ll exist at such a low energy state that it can be considered biologically sta-

d suitable for disposal in the environment. This forms the basic principle of

c digestion.

aerobic digestion is accomplished by aerating the organic sludges in an open

:sembling an activated sludge aeration tank. Like the activated sludge aeration

he aerobic digestor must be followed by a settling tank unless the sludge is

iisposed of on land in liquid form. Unlike the activated sludge process, the

it (supernatant) from the claritier is recycled back to the head end ofthe plant.

i because the supernatant is high in suspended solids (lOO to 300 mg/L), BOD5

) mg/L), TKN (to ZGO mg/L), and total P (to lOO mg/L).

3ecause the fraction of volatile matter is reduced, the specific gravity of

gested sludge solids will be higher than it was before digestion. Thus, the

settles to a more compact mass, and the clariher underllow concentration

: expected to reach 3 percent. Beyond this, its dewatering properties are

>

'obic digestion. The anaerobic treatment of complex wastes involves two dis»

iages. In the hrst stage, complex waste components. including fats, proteins,

and polysaccharides, are hydrolyzed to their component subunits. This is accorn~

plished by a heterogeneous group of facultative and anaerobic bacteria. These bac»

teria then subject the products of hydrolysis (triglycerides, fatty acids, amino acids,

and sugars) to fermentation and other metabolic processes leading to the formation

of simple organic compounds. These compounds are mainly short-chain (volatile)

acids and alcohols. The first stage is commonly referred to as acid fermentation. In

this stage, organic material is simply converted to organic acids, alcohols, and new

bacterial cells, so that little stabilization of BOD or COD is realized. In the second

stage, the end products of the first stage are converted to gases (mainly methane and

carbon dioxide) by several different species of strictly anaerobic bacteria. Thus, it

is here that true stabilization of the organic material occurs. This stage is generally

referred to as methane fermentation. The two stages of anaerobic waste treatment

are illustrated in Figure 5-38. You must understand that even though the anaerobic

process is presented as being sequential in nature, both stages take place sirnulta'

neously and synergistically. The primary acids produced during acid fermentation

are propionic and acetic. The significance of these acids as precursors for methane

formation is illustrated in Figure 5-38.

The bacteria responsible for acid fermentation are relatively tolerant to changes

in pH and temperature and have a much higher rate of growth than the bacteria

responsible for methane fermentation. As a result, methane fermentation is generally

assumed to be the rate-controlling step in anaerobic waste treatment processes.

Considering 35°C as the optimum temperature for anaerobic waste treatment,

Lawrence proposes that, in the range of 20 to 35°C, the kinetics of methane fermenta-

tion of long- and shortachain fatty acids will adequately describe the overall kinetics

of anaerobic treatment.” Thus, the kinetic equations we presented to describe the

completely mixed activated sludge process are equally applicable to the anaerobic

process.

There are essentially two types of anaerobic digestion processes used today:

the standard-rate process and the high-rate process,

The standard-rate process does not employ sludge mixing, but rather the di»

gester contents are allowed to stratify into zones, as illustrated in Figure 539. Sludge

feeding and withdrawal are intermittent rather than continuous. The digester is gen-

erally heated to increase the rate of fermentation and therefore decrease the required

retention time. Retention time ranges between 30 and 60 days for heated digestersi

The organic loading rate for a standardfate digester is between 0.48 and 1.6 kg total

volatile solids per m3 of digester volume per day.

The major disadvantage of the standardfrate process is the large tank voli

required because of long retention times, low loading rates, and thick scumalz

formation. Only about one-third of the tank volume is utilized in the digestion

cess. The remaining two~thirds of the tank volume contains the scum layer, stabil

solids, and the supernatant. Because of this limitation, systems of this type are ;

erally used only at treatment plants having a capacity of 0.04 m3/s or less.

The high»rate system evolved as a result of continuing efforts to improve

standard»rate unit. ln this process, two digesters operating in series separate the fi

tions of fermentation and solids/liquid separation (see Figure 540). The cont

of the hrst~stage, high-rate unit are thoroughly mixed and the sludge is heate

increase the rate of fermentation. Because the contents are thoroughly mixed, i

perature distribution is more uniform throughout the tank volume. Sludge fee

and withdrawal are continuous or nearly so. The retention time required for the i

stage unit is normally between l0 and l5 days. Organic loading rates vary betx

l.6 and 8.0 kg total volatile solids per m3 of digester per day.

The primary functions of the second-stage digester are solids/liquid separ:

and residual gas extraction. First-stage digesters may be equipped with fixed or i

ing covers. Second-stage digester covers are often of the floating type (Figure 5

Second-stage units are generally not heated.

The tirstfstage digester of a high-rate system approximates a completely ir

reactor without solids recycle. Hence, the biological solids retention time (SRT

the hydraulic retention time are equal for this system. As with the aerobic diges

the most important operating parameters affecting VSS reduction are solids retea

time and digestion temperature.

The BOD remaining at the end of digestion is still quite high. Likewise

suspended solids may be as high as l2,000 mg/L, while the TKN may be on the 4

Sludge Conditioning

Chemical conditioning. Several methods of conditioning sludge to facilitate 1

geparation of the liquid and solids are available. One of the most commonly ur

is the addition of coagulants such as ferric chloride, lime, or organic polymers. A

from incinerated sludge has also found use as a conditioning agent. As happens wl

coagulants are added to turbid water, chemical coagulants act to clump the solids

gether so that they are more easily separated from the water. ln recent years, orga

polymers have become increasingly popular for sludge conditioning. Polymers

easy to handle, require little storage space, and are very effective. The condition

chemicals are injected into the sludge just before the dewatering process and

mixed with the sludge.

Heat treatment. Another conditioning approach is to heat the sludge at high tt

peratures (175 to 230°C) and pressures (1,000 to 2,000 kPa). Under these cor

tions, much like those of a pressure cooker, water that is bound up in the solid

released, improving the devvatering characteristics of the sludge. Heat treatment

the advantage of producing a sludge that dewaters better than chemically conditio

sludge. The process has the disadvantages of relatively complex operation and mx

tenance and the creation of highly polluted cooking liquors that when recycled to

treatment plant impose a significant added treatment burden.

Sludge Dewatering

Sludge drying beds. The most popular method of sludge dewatering in the i

has been the use of sludge drying beds. These beds are especially popular in sr

plants because of their simplicity of operation and maintenance. ln 1977, two-th

of all United States Wastewater treatment plants utilized drying beds; one-half o

the municipal sludge pioduced in the United States was dewatered by this metl

Although the use of drying beds might be expected in the wanner, sunny regi

they are also used in several large facilities in northern climates.

Operational procedures common to all types of drying beds involve the foll

ing steps: ‘

l. Pump 0.20 to 0.30 m of stabilized liquid sludge onto the drying bed surface.

2. Add chemical conditioners continuously, if conditioners are used, by injec

into the sludge as it is pumped onto the bed.

3. When the bed is filled to the desired level, allow the sludge to dry to the des

final solids concentration. (This concentration can vary from 18 to 60 pen

depending on several factors, including type of sludge, processing rate nee

and degree of dryness required for lifting. Nominal drying times vary fror

to l5 d under favorable conditions, to 30 to 60 d under barely acceptable

ditions.)

4. Remove the dewatered sludge either mechanically or manually.

5. Repeat the cycle.

ludge landfill can be defined as the planned burial of wastewater solids, including

rocessed sludge, screenings, grit, and ash, at a designated site. The solids are placed

ito a prepared site or excavated trench and covered with a layer of soil. The soil

over must be deeper than the depth of the plow zone (about 0.20 to 0.25 tn). For

ie most part, landlilling of screenings, grit, and ash is accomplished with methods

milar to those used for sludge landfilling. I

Medicated Land Disposal (DLD)

>edicated land disposal means the application of heavy sludge loadings to some

nite land area that has limited public access and has been set aside or dedicated for

l time to the disposal of wastewater sludge. Dedicated land disposal does not mean

i-place utilization. No crops may be grown. Dedicated sites typically receive liquid

udges. While application of dewatered sludges is possible, it is not common. In

ldition, disposal ofdewatered sludge in landfills is generally more cost-effective.

'tilization

’astewater solids may sometimes be used beneficially in ways other than as a soil

itiient. Of the several methods worthy of note, composting and co-tiring with mu-

cipal solid waste are two which have received increasing amounts of interest in

e last few years. The recovery of lime and the use of the sludge to form activated

urbon have also been in practice to a lesser extent.

ludge Disposal Regulations

n Febmary l9, 1993, the EPA promulgated risk-based regulations that govern the

e or disposal of sewage sludge. These regulations are codilied as 40 CFR Part 503

td have become known as the “SO3 Regulations." The regulations apply to sewage

idge generated from the treatment of domestic sewage that is land-applied, placed

_ a surface disposal site, or incinerated in an incinerator that accepts only sewage

idge. The regulations do not apply to sludge generated from treatment of industrial

ocess wastes at an industrial facility, hazardous sewage sludge, sewage sludge with

lychlorinated biphenyls (PCB) concentrations of 50 mg/L or greater, or drinking

iter sludge.

Figure 543 summarizes the sludge quality requirements for use or disposal.

ie regulation establishes two levels of sewage sludge quality with respect to heavy-

:tal concentrations: ceiling concentration limits and pollution concentration limits.

i be land-applied, bulk sewage sludge must meet the pollutant ceiling concentration

nits and cumulative pollutant loading rates (CPLR) or the pollutant concentration

nits (Table 5»l7). Bulk sewage sludge applied to lawns and home gardens must

:et the pollutant concentration limits. Sewage sludge sold or given away in bags

Sand drying beds are the oldest, most commonly used type of drying b@d_

Many design variations are possible, including the layout of drainage piping, thi¢k_

ness and type of gravel and sand layers, and construction materials. Sand drying

beds for wastewater sludge are constructed in the same manner as water treatment

plant sludge-drying beds. Current U.S. practice was discussed and illustrated in

Section 3-9.

Sand drying beds can be built with or without provision for mechanical sludge

removal, and with or without a roof. When the cost of labor is high, newly constructed

beds are designed for mechanical sludge removal.

Vacuum filtration. A vacuum tilter consists of a cylindrical drum covered with

a tiltering material or fabric, which rotates partially submerged in a vat of condi~

tioned sludge (Figure 542). A vacuum is applied inside the drum to extract Water,

leaving the solids, or hlter cake, on the filter medium. As the drum completes its

rotational cycle, a blade scrapes the filter cal-ce from the filter and the cycle begins

again. ln some systems, the hlter fabric passes off the drum over small rollers to

dislodge the cake. There is a wide variety of filter fabrics, ranging from Dacron to

stainless-steel coils, each with its own advantages. The vacuum filter can be applied

to digested sludge to produce a sludge cake dry enough (15 to 30 percent solids)

to handle and dispose of by burial in a landiill or by application to the land as a

relatively dry fertilizer. lf the sludge is to be incinerated, it is not stabilized. ln thisCase, the vacuum tilter is applied to the raw sludge to dewater it. The sludge cak

then fed to the furnace to be incinerated.

Continuous belt filter presses (CBFP). The CBFP equipment used in treat

wastewater sludges is the same as that used for water treatment plant sludges. l

is described and illustrated in Section 3-10.

The CBFP is successful with many normal mixed sludges. Typical dewa

ing results for digested mixed sludges with initial feed solids of 5 percent gi\

dewatered cake of 19 percent solids at a rate of 32.8 kg/h - ml. ln general, mos

the results with these units closely parallel those achieved with rotary vacuum

ters. An advantage of CBFPS is that they do not have the sludge pickup problem

sometimes occurs with rotary vacuum filters. Additionally, they have a lower ent

consumption.

Reduction

Incineration. lf sludge use as a soil conditioner is not practical, or if it site is

available for landtill using dewatered sludge, cities may turn to the alternativ

sludge reduction. Incineration completely evaporates the moisture in the sludge

combusts the organic solids to a sterile ash. To minimize the amount of fuel used

sludge must be dewatered as completely as possible before incineratioii. The exh

gas from an incinerator must be treated carefully to avoid air pollution.

5-12 SLUDGE DISPOSAL

Ultimate Disposal `

The WWTP process residuals (leftover sludges, either treated or untreated) are

bane of design and operating personnel. Of the five possible disposal sites for ri

uals, two are feasible and only one is practical. Conceivably, one could ultimz

dispose of residues in the following places: in the air, in the ocean, in “outer spz

on the land, or in the marketplace. Disposal in the air by buming is in reality

ultimate disposal but only temporary storage until the residue falls to the grour

you use air pollution control devices, then the residue from these devices mu;

disposed of. Disposal of sewage sludge at sea by barging is riow prohibited it

United States. “Outer space” is not a suitable disposal site. Thus, we are left

land disposal and utilization of the sludge to produce a product.

For ease of discussion, we have divided land disposal into three categc

land spreading, landnlling, and dedicated land disposal. We have grouped all o

utilization ideas under one category,

Land Spreading

The practice of applying WWTP residuals for the purposes of recovering nutri

water. or reclaiming despoiled land such as strip mine spoils is called land sprea

ln contrast to the other land disposal techniques, land spreading is land-use inter

must meet the pollutant concentration limits or the annual sewage sludge product

application rates that are based on the annual pollutant loading rates.

Two levels of quality for pathogen densities (class A and class B) are defined

in the regulation. All class A pathogen reduction alternatives require that either fe-

cal coliform density be less than 1,000 most probable number (MPN) per gram of

total solids, or Salmonella bacteria be less than 3 MPN per 4 grams of total solids.

The class A treatment alternatives include treating the sludge for a specified time

and temperature combination, heat-enhanced alkaline stabilization, treatment in a

process to further reduce pathogens (PFRP), and use of processes that are proven

to reduce virus plaque-forming units and helminth ova to less than 1 per 4 grams of

sludge. PlfRPs include composting, heat drying, heat treatment, thermophilie aerobic

digestion, beta~ and gamma-ray irradiation, and pasteurization. The class B pathogen

standard is less than 2 million fecal coliforms per gram of sludge or treatment in a

process to signincantly reduce pathogens (PSRP). The PSRPS include aerobic di-

gestion, air drying, anaerobic digestion, composting, and lime stabilization. Sludges

meeting the class A pathogen densities may be land-disposed immediately. Time

restrictions are placed on harvesting crops, grazing of animals, and public access to

sites on which class B sludge is applied.

Vectors are insects (or other animals) that transmit disease. The organic na»

tune of sludge often attracts vectors after the sludge is land-applied. The 503 regu~

lations provide ll alternatives to reduce vector attraction, Some of the alternatives

are; volatile solids reduction of 38 percent of more, achieving a standard oxygen

uptake rate of less than 1.5 mg O; per hour per gram of dry solids at 20°C, aerobic

lrteatniunl at greater than 40°C with an average temperature greater than 45°C for

|11 days, alkaline stabilization, sludge drying, surface incorporation, and soil cover.

'l`lit~ 503 regulations are “self-impleinenting” in that permits are not required

to n~i|niit' tronformance.

For each type of decomposition (aerobic, anoxic, and anaerobic). list the electroi

acceptor, important end products, and relative advantages and disadvantages a

a waste treatment process.

List the growth requirements of bacteria and explain why the bacterium need

them. `

Sketch and label the bacterial growth curve for a pure culture. Dehne or explai

each phase labeled on the curve.

List a BOD value for strong, medium, and weak municipal waste.

List and describe five on-site alternatives for treating and/or disposing of dt

mestic sewage.

Choose the correct on-site treatmentldisposal system based on population_ lar

use, and soil conditions. ,

Explain the difference between pretreatment. primary treatment, seconda=

treatment, and tertiary treatment, and show how they are related.

Sketch a graph showing the average variation of daily tlow at a municip

wastewater treatment plant (WWTP).

Define and explain the purpose of equalization,

Sketch, label, and explain the function of the parts of an activated sludge pla

and a trickling filter plant.

Define HC, SRT, and sludge age, and explain their use in regulating the activat

sludge process.

Explain the purpose of the F/M ratio and dehne F and l\/l in terms of BGD5 a

mixed liquor volatile suspended solids.

Explain the relationship between F/M and 66.

Explain how cell production is regulated using F/M and/or 9,

Compare two systems operating at two different F/l\/I ratios. f

Define SVI and explain its use in the design and operation of an activated slut

plant.

Explain the difference between bulking sludge and rising sludge and what t

cumstances cause each to occur.

List and explain the relationship of the five types of oxidation ponds to oxyg

Explain what an RBC is and how it works.

Compare the positive and negative effects of disinfection of wastewater et*

ents.

List the four common advanced wastewater treatment (A\\/T) processes and

pollutants they remove.

Explain why removal of residual suspended solids effectively removes resic

BOD;

Describe refractory organics and the method used to remove them.

List three chemicals used to remove phosphorus from wastewaters.

Explain biological nitrihcation and denitrification either in words or with an

equation.

Explain ammonia stripping either in words or with an equation.

Describe the three basic approaches to land treatment of Wastewater.

State the two major purposes of sludge stabilization.

Explain the purpose of each of the sludge treatment steps and describe the major

processes used.

Describe the locations for ultimate disposal of sludges and the treatment steps

needed prior to ultimate disposal.

ll the aid Qfllzis text, you should be able to do I/zefollowing:

Calculate the bacterial population at a time, I, given the initial population and

the number of generations. »

Determine the volume of a septic tank and the area ofa tile field to treat waste-

water from a family or institution, given the proper data.

Determine whether or not a grit particle of given diameter and density will be

captured in a given velocity~controlled grit chamber, or detemiine the minimum

diameter that will be captured under a given set of conditions.

Detemiine the required volume of an equalization basin to dampen a given pe-

riodic How.

Determine the effect of equalization on mass loading of a pollutant.

Evaluate or size primary and secondary sedimentation tanks with respect to de-

tention time, overtiow rate, solids loading, and weir loading.

Use the appropriate trickling hlter equation to determine one or more of the

following, given the appropriate data: treatment efficiency, filter volume, filter

depth, hydraulic loading rate.

Estimate the soluble BOD5 in the efliuent from a completely mixed or plug-flow

activated sludge plant; detemiine the mean cell residence time or the hydraulic

detention time to achieve a desired degree of treatment; determine the “wasting”

fiow rate to achieve a desired mean cell residence time or F/M ratio.

Calculate the F/M ratio given an influent BOD5, flow, and detention time, or

:alculate the volume of the aeration basin given F/M, BOD5, and flow.

Calculate SVI and utilize it to determine retum sludge concentration and/or fiow

'ate_

Ialculate the required mass of sludge to be wasted from an activated sludge

)rocess given the appropriate data.

falculate the theoretical mass ofoxygen required and the amount of air required

o supply it given the appropriate data.

’erform a sludge mass balance. given the separation efficiencies and appropriate

nass flow rate.

Design a septic tank and tile held system for a highway rest area. Use the i’ol~

lowing assumptions:

a. Average daily trafhc = 6,000 vehicles/d

b. % turn in = 10 percent

c. Use rate = 20.0 liters/turn in

maximum use rate I 2.5 >< average

d. Terrain = Flat

e. GWT = Average 4.2 m below grade

f. Soil percolation rate: 5 min/cm

Ginger Snap is planning to expand her Kookie Iar restaurant to a full-size

restaurant to he called the Pretzel Bowl. The existing septic tank has a vol-

ume of 4_0 rn; and the existing tile held has a trench area of 100.0 m2. If the

anticipated wastewater production from the Pretzel Bowl is 4000 L/d, will l\/ls.

Snap have to expand either the septic tank or the tile held or both? Assume the

soil is a sandy loam.

If a particle having a 0.0170 em radius and density of |.95 g/em; is allowed to

fall into quiescent water having a temperature ot`4°C, what will he the terminal

settling velocity? Assume the density of water 1 1,000 kg/m3.

A/zsiifer: 3.82 >< l0"l m/S

If the terminal settling velocity of a particle falling in quiescent water having

a temperature of 15°C is 0.0950 cm/s, what is its diameter? Assume a particle

-lcnsity of 2.05 g/cm" and density of water equal to 1,000 kg/in ‘_

Determine the surface area of a primary settling tank sized to handle a maxis

mum hourly flow of0.570 m3/s at an overflow rate of 60.0 m/d. If the effective

tank depth is 3,0 m, what is the effective theoretical detention time?

Answers: Surface area = 820.80 or 821 mf; to = 1.2 h

If an equalization basin is installed ahead of the primary tank in Problem 5_

14, the average flow to the tank is reduced to 0.400 m3/s. What is the new

overflow rate and detention time?

Envirotech Systems markets a synthetic media for use in the construction of

triclding Hlters. Envirotech uses the following formula to determine BOD rg,

moval efficiency:

Le l k6Dl

-4 j exn ~- ~~~' ~

L, l Q" l

where Le BOD; of effluent, mg/L

Li BOD5 of influerit, mg/L

. _ _V ` (m/(1)05

k treatability factor, ff-

0 2 temperature correction factor

(l.035)7 ‘Zo

T wastewater teniperature. ° C

D media depth, in

Q hydraulic loading rate, m/d

n 0.5

Using the following data for domestic wastewater, determine the treatability

factor, lc.

Wastewater temperature = 13°C

Hydraulic loading rate 1 41.1 m/d

% BOD remaining Media depth, m

100.0 0.00

80.3 1.00

64,5 2.00

41.6 4.00

l7,3 8.00

where r = recirculation ratio and all other terms are as described in Prob-

lem 5-l6_ Use this equation to determine the efficiency of a L8 m deep

synthetic media filter loaded at a hydraulic loading rate of 5.00 rn/d with

a recirculation ratio of 2.00. The wastewater temperature is l6°C and the

_ _ _ 0 `

treatability factor is l_79 @§f; at 20°C

Y

Determine the concentration ofthe effluent BOD5 for the two-stage trickling

filter described belowr The wastewater temperature is l7°C. Assume the NRC

equations apply.

Design flow 1 0.0509 mi/s 5

lnlluent BOD; (after primary treatment) 1 260 mg/L

Diameter of each filter 1 24.0 m

Depth ofeach filter 1 L83 m

Recirculation flow rate for each hlter 1 0.0594 mi’/s

Determine the diameter of a single-stage, rock media filter to reduce an

applied BOD5 of l25 mg/L to 25 mg/L. Use a hydraulic loading rate of

l4 in"/ni? - d, a recirculation ratio of 12.0, and a filter depth of l.83 m. As-

sume the NRC equations apply,

Using the assumptions given in Example 5-7, the rule of thumb values for

growth constants, and the further assumption that the infiuent BOD; was re-

duced by 32.0 percent in the primary tank, estimate the liquid volume of an

aeration tank required to treat the wastewater in Problem 5-7. Assume an

Ml_,\/SS ot’ 2,000 mg/L_ '

Answer: Volume = 4,032 or 4,000 m3

Repeat Problem 5-20 using the wastewater in Problem 5-8.

Using a spreadsheet program you have written, rework Example 5-7 using

the following MLVSS concentrations instead of the 2,000 mg/L used in the

example: l,000 mg/L; 1,500 mg/L; 2,500 mg/L; and 3,000 mg/L.

Using a spreadsheet program you have written_ determine the effect of

MLVSS concentration on the effluent soluble BOD5 (S) using the data in

Example 5-7. Assume the volume of the aeration tank remains constant at

970 m3_ Use the same MLVSS values used in Problem 5-22.

lf the E/M of a 0.4380 m3/s activated sludge plant is 0.200 d", the inliuent

BOD; after primary settling is I5() mg/L, and the MLVSS is 2,200 mg/L, what

is the volume ofthe aeration tank?

.~'\r1.s`w<'1'_' Volume fl L29 >< l()4 Ill; `

What sludge volume would you expect to find after settling the mixed liquor

described in Example 5-10 for 30 minutes in a one-liter graduated cylinder

(magna cum laude).

Answer." Volume 2 500 ml,

What MLVSS and SVI must be achieved to reduce the return sludge flow rate

of Example 5-10 from 0.150 m3/s to 0.0375 m3/s? (Note that there are several

combinations that will be satisfactory.)

Two activated sludge aeration tanks at Turkey Run, lndiana, are operated in

series. Each tank has the following dimensions: 7.0 in wide by 30.0 m long by

4.3 m effective liquid depth. The plant operating parameters are as follows:

Flow = 0.0796 m3/s

Soluble BOD5 after primary settling 1 l30 mgfla

MLVSS = 1,500 mg/L

MLSS = 1.40 (MLVSS)

Settled sludge volume after 30 min I 230.0 mL/L

Aeration tank liquid temperature = 15°C

Determine the following: aeration period, F/M ratio, SVI, solids concentration

in the return sludge, and return sludge rate,

Answers: aeration period 1 6.3 h; F/M I 033; SVI 1 110 mL/g;

X; = 9,130 mg/L; Q, I 0.0238 m3/s

The 500-bed Lotta Hart Hospital has a small activated sludge plant to treat its

wastewater. The average daily hospital discharge is 1,200 liters per day per

bed, and the average soluble BOD5 after primary settling is 500 mg/'L_ The

aeration tank has effective liquid dimensions of 10.0 m wide by 10.0 m long

by 4.5 m deep. The plant operating parameters are as follows:

MLVSS = 2,000 mg/L

MLSS = 1.20 (MLVSS)

Settled sludge volume after 30 min I 200 mL/L

Determine the following: aeration period, F/M ratio, SVl, solids concentration

in return sludge, and return sludge rate.

Using the following assumptions, determine the sludge age and cell wastage-

flow rate for the Turkey Run WWTP (Problem 5-27).

Assume: SS in the effluent are negligible

Wastage is from the aeration tank

Yield coefficient = 0.40

-1

Bacterial decay rate = 0.040 d

Effluent BOD5 = 5.0 mg/1, (soluble)

Answers: 6, = ll.5

Using the following assumptions, determine the solids retention time and the

cell wastage flow rate for the Lotta Hart Hospital WXVTP (Problem 5-28).

Assume: SS in effluent I 30,0 mg/L

Wastage is from the return sludge line

Yield coefficient 1 0.60

Bacterial decay rate 1 0.060 d"

lnert fraction of SS 1 66.67%

Allowable BOD in effluent 2 30.0 mg/L

The two secondary settling tanks at Turkey Run (Problem 5-27) are 16.0 min

diameter and 4.0 m deep at the side wall. The effluent weir is a single launder

set on the tank wall. Evaluate the overflow rate, depth, solids loading, and

weir length of this tank for conformance to standard practice.

Answers: U0 2 l'/,l m/d < 33 rn/d. OK.

SWT) > 3_7 m recommended depth. OK.

Sl- 2 46.65 kg/ml ~ d <1 253 kg/m3 ~ tl. OK

WL 1 68.4 m3/d - m, which is acceptable,

The single secondary settling tank at the Lotta Hart Hospital WWTP (Prob-

lem 5-28) is 10.0 m in diameter and 3_4 m deep at the side wall. The eflluent

weir is a single launder set on the tank wall. Evaluate the overflow rate, depth,

solids loading, and weir length for conformance to standard practice.

An oxidation pondhaving a surface area of 90,000 ml is loaded with a waste

flow of 500 m3/d cdntaining l80 kg of BOD, The operating depth is from 0.8

to l.6 m. Using the Michigan miles of thumb, determine whether or not this

design is acceptable,

Answers: Loading rate 2 20.0 kg/ha- d

Detention time = l80 d

This design is acceptable.

Determine the required surface area and the loading rate for a facultative ox-

idation pond to treat a waste flow of 3,800 m3/d with a BOD5 of 100.0 mg/L.

Rework Example 5-15 using alum {Al;(SO4)3 ~ ISHZO] to remove the phos-

phorus.

Answer: 86_l mg/L of alum

Rework Example 5-15 using lime (CaO) to remove the phosphorus.

Prepare a monthly water balance and estimate of the storage volume required

(gin m3) for a spray irrigation system being designed for Wheatvillei iowa,

The design population is l,000 and the design wastewater generation rate ig

280.0 Lpcd. Based on a nitrogen balance, the allowable application rate is

27.74 mm/mo. The area available is 40.0 ha. The percolation rate during the

spray season is l50 mm/mo. Assume that the runoff is contained and reap~

plied. Assume “spray season" is when temperature is above 0°C. In fact,

spraying can continue to about 44°C but once spraying has stopped, it may

not recommence until temperatures exceed +4°C. The following clirnatolog-

ical data (from Kansas City) may be used. This is a direct application ofthe

hydrologic balance equation, Equation 2-2 may be rewritten as:

(

Q =P+WW~ETYG=R

dz

where % 1 change in storage, mm/mo

P 1 precipitation, mm/mo

WW 1 wastewater application rate, mm/mo

ET = evapotranspiration, mru/mo

G I groundwater infiltration, mm/mo

R 1 runoff,n1ni/mo

Clirnatological data from Kansas City, Missouri

Average Evapotranspiration Precipitation

Month temperature (°C) (mm) (mm)

JAN -O. 2 23

FEB 2. l 28

MAR 6.3 43

APR 13.2 79

MAY l 8. 7 l l 2

JUN 24.4 155

JUL 27.5 203

AUG 26.6 l98

SEP 21.8 l52

OCT l5.7 l 14

NOV 7_0 64

DEC 2.1 25

Prepare a monthly water balance and estimate the storage volume (in ml) for

Flushing Meadows. The design population for Flushing Meadows is 8,880

The average wastewater generation rate is 485.0 Lpcd. The area available is

l25.0 ha. The percolation rate is 200 mm/mo during the spray season. As-

sume that runoff is to be contained and reapplied. Assume also that the fol-

lowing climatological data apply. Assume “spray season" is when temperature

is above O°C. In fact, spraying can continue to about -4"C but once spraying

has stopped it may not recommence until temperatures exceed +4°C.

Prepare a monthly water balance and estimate the storage volume (in ml) for

Flushing Meadows. The design population for Flushing Meadows is 8,880

The average wastewater generation rate is 485.0 Lpcd. The area available is

l25.0 ha. The percolation rate is 200 mm/mo during the spray season. As-

sume that runoff is to be contained and reapplied. Assume also that the fol-

lowing climatological data apply. Assume “spray season" is when temperature

is above O°C. In fact, spraying can continue to about -4"C but once spraying

has stopped it may not recommence until temperatures exceed +4°C.

A 185.6240 Mg/d

1,115 (),9()(); 1;j I 0250; UN = 0.00;1]p = 0.150

UH 0.190

A ll.S`H/(3f_S`.` B

= 21.112 or 21.1 Mg/<11

E I 190.011 or 190. Mg/d

J = 47.503 or 4715 Mg/d

K I 142.509 or 143. Mg/d

L = 144.147 or144_1\/Ig/d

Rework Problem 5-41 assuming that the digestion solids are not dewatered

prior to ultimate disposal, that is, K 1 L.

The Flowsheet for the Doubtfu1 WWTP is shown in Figure P-543. Assuming

that the appropriate values of 77 given in Figure 5-33 may be used when needed

and that A I 7.250 Mg/d, X = 1.288 Mg/d, and N = 0.000 Mg/d, what is

the mass How (in kg/d) of sludge to be sent to u1timate disposal?

"" 1 Q».~fm.1-mm5-44. Using the following mass How data from the Doubtful WWTP (Problem 5-43)

determine 175, 170, 11/V, 171, and ryx.

Mass Flows for Doubtful WWTP in M g/d:

A = 7.280 J I 4.755

B 1 7.798 K = 6.422

D = 0.390 N 1 9.428

E = 8.910 X = 0.468

F I 6.940

5-45. Determine the surface area required for the gravity thickeners (assume that

no thickener is greater than 30.0 in in diameter) to thicken the waste activated

sludge (WAS) at Grand Rapids, Michigan, from 10,600 mg/L to 2.50 percent

solids. The waste activated sludge tlow is 3,255 ml/ii. .fxssurne that the

settling curves of Figure 5-36 apply.

Answer: A, 1 2,379.5 or 2,380 m~ depending on graph reading.

choose four thickeners at 27.5 m diameter.

7

5 46. Determine the surface area required for the gravity thickeners of Problerr

if7l0 m3/d of primary sludge is mixed with the WAS to form a sludge h

2.00 percent solids. The final sludge is to have a solids concentration oi

percent. The batch settling curve for mixed WAS and PS in Figure 5

assumed to apply.

3 47. The Pomdeterra wastewater treatment plant produces thickened sludg

has a suspended solids concentration of 3_8 percent. They are investi;

a filter press that will yield a solids concentration of 24 percent. lf the;

produce 33 m3/d of sludge, what annual volume savings will they achi

they install the press?

3 48. Ottawa’s anaerobic digester produces 13 rn;/d of sludge with a suspt

solids concentration of7.8 percent. What volume otsludge must they di

of each year if their sand drying beds yield a solids concentration of 3

cent?

15 DISCUSSION QUESTIONS

You are touring the research labs of the environmental engineers at you

versity. Two biological reactors are in a controlled temperature room that

temperature of 35°C Reactor A has a strong odor. Reactor B has virtually nt

What electron acceptors are being used in each reactor?

If the state regulatory agency requires tertiary treatment of a municipal \

water, what, if any. processes would you expect to find preceding the tt

process?

What is the purpose of recirculation and how does it differ from return slut

In which of the following cases is the cost of sludge disposal higher?

a. 06 = 3 days

b. 05 = 10 days

Would an industrial wastewater containing only NH; at a pH ot 7.00 be dent

if pure oxygen was bubbled into it? Explain your reasoning.

-16 ADDITIONAL READING

ooks

D. Benetield and C. \V. Randall. Ui<>[1tg1'c'u/ I’/I1c'4*.~'t I);-'xigri /‘ur ¥\*li\fi'1\;1Iu1' I`r»'t:m1<'r1I. llppct

River, NJ: Prentice Hall. V980

W. Clark. \\'. Viessnuui. Jr.. and M. J. llztminerp llkizw .Snpyilt mul lbllirrzwz (.`<»/if/uf/_ Net

llurpcr & Row. 1077.

l.. l)t'oSle_ 17/lr`(Vl'\' will l’r'z1r'Iir*e ff llflm'/' run/ Vl'l1\‘It'n'i1I€r Trvtiinzwll. New York \\.'ilt:§.‘. l‘l*>i