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    5-7 UNIT PROCESSES OFSECONDARY TREATMENTOverviewTrickling FiltersActivated Sludge

    Oxidation PondsRotating Biological Contactors (RBCs) 5-8 DISINFECTION 5-

    9 ADVANCED WASTEWATER TREATMENTFiltrationCarbon AdsorptionPhosphorus RemovalNitrogen Control 5-10 LAND TREATMENT

    Slow Rate

    Overland FlowRapid Infiltration 5-11 SLUDGE TREATMENTSources and Characteristics of Various SludgesSolids Computations

    ThickeningStabilizationSludge ConditioningSludge DewateringReduction 5-12 SLUDGE DISPOSALUltimate DisposalLand SpreadingLandfillingDedicated Land Disposal (DLD)Utilization 5-13 CHAPTER REVIEW 5-14



    Role of Microorganisms

    The stabilization of organic matter is accomplished biologically using a variety ofmicroorganisms. The microorganisms are used to convert the colloidal and dissolvedcarbonaceous organic matter into various gases and into cell tissue. Because cell tissue has aspecific gravity slightly greater than that of water, the resulting tissue can be removedfrom the treated liquid by gravity settling.

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    It is important to note that

    unless the cell tissue producedfrom the organic matter isremoved from the solution,complete treatment will not beaccomplished because the celltissue, which itself is organic,will be measured as BOD inthe effluent. If the cell tissue isnot removed, the onlytreatment that will be achievedis that associated with the

    bacterial conversion of aportion of the organic matteroriginally present to variousgaseous end products.1

    Classification of


    By kingdoms. Microorganismsare organized into three broadgroups based on their structuraland functional differences. Thegroups are called kingdoms.The three kingdoms are

    animals, plants, and protista.Representative examples andcharacteristics of differentiation are shown inFigure 5-1.

    By energy and carbon source.The relationship between thesource of carbon and the sourceof energy for themicroorganism is important.Carbon is the basic building

    block for cell synthesis. Energy

    must be obtained from outsideof the cell to enable synthesisto proceed. Our goal inwastewater treatment is toconvert both the carbon and theenergy in the wastewater intomicroorganisms that we canremove from the water bysettling. Therefore, we wish toencourage the growth oforganisms that use organicmaterial for both their carbonand energy source.

    If the microorganism usesorganic material as a supply ofcarbon, it is calledheterotrophic. Autotrophsrequire only CO2 to supply

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    thatrelyonlyon thesunforenergyarecalled


    sextractenergyfromorganic orinorganicoxidation/reduction reac-


    s useorganicmaterials,whilelithotrophsoxidize


    Bytheirrelationshiptooxygen.Bacteria alsoareclassified bytheir

    ability or inability to utilizeoxygen as a terminal electronacceptor3 inoxidation/reduction reactions.Obligate aerobes aremicroorganisms that must have

    oxygen as the

    'Metcalf &Eddy, Inc., andG.Tchobanoglous, Wastewater

    Engineering: Treatment,Disposal, Reuse, New York:McGraw-Hill, p. 395, 1979.Reprinted by permission.2J. E. Bailey and D. E Ollis,

    Biochemical EngineeringFundamentals, New York:McGraw-Hill, p. 222, 1977.3An organic substrate is not

    directly oxidized to carbondioxide and water in a singlechemical step because there isno energy-conservingmechanism that could trap somuch energy. Thus, biologicaloxidation occurs in small steps.Oxidation requires the transferof an electron from thesubstance being oxidized tosome acceptor molecule thatwill subsequently be reduced.In most biological systems,each step in the oxidation

    process involves the removal oftwo electrons and thesimultaneous loss of two

    protons (H+). The combinationof the two losses is equivalentto the molecule having lost twohydrogen atoms. The reactionis often referred to asdehydrogenation. The electronsand protons are not releasedinto the cell, but are transferredto an acceptor molecule. Theacceptor molecule will notaccept the protons until it hasaccepted the electrons and thusit is referred to as an electronacceptor. Since the net result ofaccepting an electron and

    proton is the same as acceptinga hydrogen atom, suchacceptors are also calledhydrogen acceptors. (C. P. L.Grady and H. C. Lim,

    Biological WastewaterTreatment, Theory and

    Applications, New York:Marcel Decker, 1980.)

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    AnimalsMulticellularExhibit Tissue DifferentiationMotile

    o ersrus aceans

    PlantsMulticellularExhibit Tissue Differentiation







    hibit Tissue


    ac er a

    ue- reenae


    ll lar

    libit Tissue



    FIGURE 5-1

    Classification of microorganisms by kingdom.

    terminal electron acceptor. When wastewater contains oxygen and can

    support obligate aerobes, it is called aerobic.Obligate anaerobes are microorganisms that cannot survive in the

    presence of oxygen. They cannot use oxygen as a terminal electronacceptor. Wastewater that is devoid of oxygen is called anaerobic.

    Facultative anaerobes can use oxygen as the terminal electron acceptorand, under certain conditions, they can grow in the absence of oxygen.

    Under anoxic conditions, a group of facultative anaerobes calleddenitrifiers utilizes nitrites (NO^~) and nitrates (NO^~) as the terminalelectron acceptor. Nitrate nitrogen is converted to nitrogen gas in theabsence of oxygen. This process is called anoxic denitrification.

    By their preferred temperature regime. Each species of bacteriareproduces best within a limited range of temperatures. Fourtemperature ranges are used to classify bacteria. Those that grow best attemperatures below 20C are called psy-chrophiles. Mesophiles grow

    best at temperatures between 25 and 40C. Between 45 and 60C, thethermophiles grow best. Above 60C, stenothermophiles grow best.The growth range of facultative thermophiles extends from thethermophilic range into the mesophilic range. These ranges arequalitative and somewhat sub-

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    jective. You will note the gaps

    between 20 and 25C andbetween 40 and 45C. Don'tmake the mistake of saying thatan organism that grows well at20.5C is a mesophile. Therules just aren't that hard andfast. Bacteria will grow over arange of temperatures and willsurvive at a very large range oftemperatures. For example,

    Escherichia coli, classified asmesophiles, will grow attemperatures between 20 and50C and will reproduce, albeitvery slowly, at temperaturesdown to 0C. If frozen rapidly,they and many other microorganisms can be storedfor years without a significantdeath rate.




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    highestpopulation ofmicroorganisms inawastewatertreatment


    willbelongto the



    usesolublefood.Conditions inthetreatment

    plantareadjusted so



    minate. No particular species isselected as "the best."

    Fungi. For our purpose, wedefine fungi as multicellular,non-photosynthetic,

    heterotrophic protists. Fungiare obligate aerobes thatreproduce by a variety ofmethods including fission,

    budding, and spore formation.They form normal cell materialwith one-half the nitrogenrequired by bacteria. In anitrogen-deficient wastewater,they are found to predominateover the bacteria.4

    Algae. This group of microorganisms are

    photoautotrophs and may beeither unicellular or multicellular. Because of thechlorophyll contained in mostspecies, they produce oxygenthrough photosynthesis. In the

    presence of sunlight, thephotosynthetic production ofoxygen is greater than theamount used in respiration. Atnight they use up oxygen in

    respiration. If the daylighthours exceed the night hoursby a reasonable amount, thereis a net production of oxygen.

    Protozoa. For our purpose, wemay say that protozoa aresingle-celled animals thatreproduce by binary fission(dividing in two). Most areaerobic chemoheterotrophs,and they often consume

    bacteria. They are desirable in

    wastewater effluents becausethey act as polishers inconsuming the bacteria.

    Rotifers and crustaceans.Both rotifers and crustaceansare aerobic, multicellularchemoheterotrophs. The rotiferderives its name from theapparent rotating motion of twosets of cilia on its head. Thecilia provide mobility and amechanism for catching food.Rotifers consume bacteria andsmall particles of organicmatter.

    Crustaceans arecharacterized by their shell

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    structure.Theyare asourceof

    foodforfishandare notfoundinwastewatertreatmentsystems toanyextent







    y forSanita








    Hill, p.



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    u s ra e

    awa er

    \ / /








    End Products 1'New

    FIGURE 5-2


    except in underloaded lagoons. Their presence is indicative of a highlevel of dissolved oxygen and a very low level of organic matter.

    Bacterial Biochemistry

    Metabolism. The general term that describes all of the chemicalactivities performed by a cell is metabolism. This in turn is divided intotwo parts: catabolism and anabolism. Catabolism includes all the

    biochemical processes by which a substrate is degraded to end productswith the release of energy.5 In wastewater treatment, the substrate isoxidized. The oxidation process releases energy that is transferred to anenergy carrier which stores it for future use by the bacterium (Figure 5-2).

    Anabolism includes all the biochemical processes by which thebacterium synthesizes new cells. 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 thetype of decomposition (that is, aerobic, anoxic, or anaerobic) used by amixed culture of microorganisms. Each type of decomposition has

    peculiar characteristics which affect its use in waste treatment.

    Aerobic decomposition. From our discussion of bacterial metabolismyou will recall that molecular oxygen (O2) must be present as theterminal electron acceptor for decomposition to proceed by aerobicoxidation. 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 primarilycarbon dioxide, water, and new cell material (Table 5-1). Odiferousgaseous end products are kept to a minimum. In healthy natural watersystems, aerobic decomposition is the principal means of self-


    'Substrate is food. For our application, "food" is the organic material

    from the human digestive tract.

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    TABLE 5-1

    Waste decomposition end


    Representative end


    S A A A



























    es C02 +

    C02 + H20C02 + H20C02 + H20COz + H20

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    Source: After

    Pelczar and Reid,Microbiology,

    New York:



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    A wider spectrum of organic material can be oxidized aerobicallythan by any other type of decomposition. This fact, coupled with thefact that the final end products are oxidized to a very low energy level,results in a more stable end product (that is, one that can be disposed ofwithout damage to the environment and without creating a nuisancecondition) than can be achieved by the other oxidation systems.

    Because of the large amount of energy released in aerobicoxidation, most aerobic organisms are capable of high growth rates.Consequently, there is a relatively large production of new cells incomparison with the other oxidation systems. This means that more

    biological sludge is generated in aerobic oxidation than in the otheroxidation systems.

    Aerobic decomposition is the method of choice for large quantitiesof dilute wastewater (BOD5 less than 500 mg/L) becausedecomposition is rapid, efficient, and has a low odor potential. For highstrength wastewater (BOD5 is greater than 1,000 mg/L), it is notsuitable because of the difficulty in supplying enough oxygen and

    because of the large amount of biological sludge that is produced. Insmall communities and in special industrial applications where aeratedlagoons (see Section 5-7) are used, wastewaters with BOD5S up to3,000 mg/L may be treated satisfactorily by aerobic decomposition.

    Anoxic Decomposition. Some microorganisms will use nitrate (NO^~)as the terminal electron acceptor in the absence of molecular oxygen.Oxidation by this route is called denitrification.

    The end products from denitrification are nitrogen gas, carbondioxide, water, and new cell material. The amount of energy madeavailable to the cell during denitrification is about the same as thatmade available during aerobic decomposition. As a consequence, the

    production of cells, although not as high as in aerobic decomposition, isrelatively high.

    Denitrification is of importance in wastewater treatment wherenitrogen must be removed to protect the receiving body. In this case, aspecial treatment step is added to the conventional process for removalof carbonaceous material. Denitrification will be discussed in detaillater.

    One other important aspect of denitrification is in relation to final

    clarification of the treated wastewater. If the environment of the finalclarifier becomes anoxic, the formation of nitrogen gas will cause largeglobs of sludge to float to the surface and escape from the treatment

    plant into the receiving water. Thus, it is necessary to ensure that anoxicconditions do not develop in the final clarifier.

    Anaerobic decomposition. In order to achieve anaerobicdecomposition, molecular oxygen and nitrate must not be present asterminal electron acceptors. Sulfate (SO4-), carbon dioxide, and organiccompounds that can be reduced serve as terminal electron acceptors.The reduction of sulfate results in the production of hydrogen sulfide(H2S) and a group of equally odiferous organic sulfur compoundscalled mercaptans.


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    The anaerobic decomposition (fermentation) of organic mattergenerally is considered to be a two-step process. In the first step,complex organic compounds are fermented to low molecular weightfatty acids (volatile acids). In the second step, the organic acids areconverted to methane. Carbon dioxide serves as the electron acceptor.

    Anaerobic decomposition yields carbon dioxide, methane, andwater as the major end products. Additional end products includeammonia, hydrogen sulfide, and mercaptans. As a consequence of theselast three compounds, anaerobic decomposition is characterized by anunbelievably horrid stench!

    Because only small amounts of energy are released duringanaerobic oxidation, the amount of cell production is low. Thus, sludge

    production is low. We make use of this fact in wastewater treatment byusing anaerobic decomposition to stabilize sludges produced duringaerobic and anoxic decomposition.

    Direct anaerobic decomposition of wastewater generally is not

    feasible for dilute waste.6

    The optimum growth temperature for theanaerobic bacteria is at the upper end of the mesophilic range. Thus, toget reasonable biodegradation, we must elevate the temperature of theculture. For dilute wastewater, this is not practical. For concentratedwastes (BOD5 greater than 1,000 mg/L), anaerobic digestion is quiteappropriate.

    Population Dynamics

    Bacterial growth requirements. In the discussion of the behavior ofbacterial cultures which follows, there is the inherent assumption thatall the requirements for growth are initially present. Since theserequirements are fairly extensive and stringent, it is worth taking a

    moment to recapitulate them. The following list summarizes the majorrequirements that must be satisfied:

    1. A terminal electron acceptor

    2. Macronurrientsa. Carbon to build cells

    b. Nitrogen to build cellsc. Phosphorus for ATP (energy carrier) and DNA

    3. Micronutrientsa. Trace metals

    b. Vitamins are required by some bacteria

    4. Appropriate environment

    a. Moistureb. Temperaturec. pH

    6Some researchers are exploring the use of anaerobic systems fortreatment of dilute wastes, especially groundwater contaminated withhazardous waste. 1

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    Growth in pure cultures. As an illustration, let us examine ahypothetical situation in which 1,400 bacteria of a single species areintroduced into a synthetic liquid medium. Initially nothing appears tohappen. The bacteria must adjust to their new environment and begin tosynthesize new protoplasm. On a plot of bacterial growth versus time(Figure 5-3), this phase of growth is called the lag phase.

    At the end of the lag phase the bacteria begin to divide. Since allof the organisms do not divide at the same time, there is a gradualincrease in population. This phase is labeled accelerated growth on thegrowth plot.

    At the end of the accelerated growth phase, the population oforganisms is large enough and the differences in generation time aresmall enough that the cells appear to divide at a regular rate. Sincereproduction is by binary fission (each cell divides producing two newcells), the increase in population follows in geometric progression: 1

    -* 2 -* 4 -> 8 -* 16 -* 32, and so forth. The population of bacteria(P) after the nth generation is given by the following expression:

    p = p0(2y (5-1)

    where P0 is the initial population at the end of the accelerated growthphase. If we take the log of both sides of Equation 5-1, we obtain thefollowing:

    logP= logP0 + n log2 (5-2)












    : --- - - -- ---





    L G r h

    / A l dy ^


    5 10 15 20 25 30 35 40 45 50 55 60 Time (h) FIGURE 5-3Bacterial growth in a pure culture: The "Log-Growth Curve."

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    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 andintercept Po at to equal to the end of the accelerated growth phase.Thus, this phase of growth is called the log growth or exponential

    growthphase.The log growth phase tapers off as the substrate becomes

    exhausted or as toxic byproducts build up. Because of this, at somepoint in time, the population becomes constant either as a result ofcessation of fission or a balance in death and reproduction rates. This isdepicted by thestationary phase on the growth curve.

    Following the stationary phase, the bacteria begin to die fasterthan they reproduce. This death phase is due to a variety of causes thatare basically an extension of those which lead to the stationary phase.

    Growth in mixed cultures. In wastewater treatment, as in nature, purecultures of microorganisms do not exist. Rather, a mixture of species

    competes and survives within the limits set by the environment.Population dynamics is the term used to describe the time-varyingsuccess of the various species in competition. It is expressedquantitatively in terms of relative mass of microorganisms.7

    The prime factor governing the dynamics of the various microbialpopulations is the competition for food. The second most importantfactor is the predator-prey relationship.

    The relative success of a pair of species competing for the samesubstrate is a function of the ability of the species to metabolize thesubstrate. The more successful species will be the one that metabolizesthe substrate more completely. In so doing, it will obtain more energyfor synthesis and consequently will achieve a greater mass.

    Because of their relatively smaller size and, thus, larger surface

    area per unit mass, which allows a more rapid uptake of substrate,bacteria will predominate over fungi. For the same reason, the fungipredominate over the protozoa.

    When the supply of soluble organic substrate becomes exhausted,the bacterial population is less successful in reproduction and the

    predator populations increase. In a closed system with an initialinoculum of mixed microorganisms and substrate, the populations willcycle as the bacteria give way to higher level organisms which in turndie for lack of food and are then decomposed by a different set of

    bacteria (Figure 5-4). In an open system, such as a wastewatertreatment plant or a river, 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 dynamic equilibrium. It is ahighly sensitive state, and changes in influent characteristics must beregulated closely to maintain the proper balance of the various


    7If each individual organism of Species A has, on the average, twicethe mass at maturity as each individual organism of Species B, and bothcompete equally, we would expect that both would have the same mass

    but that there would be twice as many of Species B as there would beof A.

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    0 50 > 1

    30 0 020 a: 4




    Time (h)FIGURE 5-4Population dynamics in a closed system. (Source: Curds, "ATheoretical Study of Factors Influencing the Microbial PopulationDynamics of the Activated Sludge Process-I." Water Resources, vol. 7,

    p. 1269, 1973.)



    "6b 30 -


    S 20


    0 -I

    - 8

    *' \ / ' -



    u s rae n

    ac e ac er a-

    onsum n

    ewa eac er a n'/^_ Free-


    5 6

    S 5

    " -

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    Time(h) FIGURE 5-5Population dynamics in an open system. (Source: Curds, "A TheoreticalStudy of Factors Influencing the Microbial Population Dynamics of theActivated Sludge Process-I." Water Resources, vol. 7, p. 1269, 1973.)

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    The Monod Equation. For the large numbers and mixed cultures ofmicroorganisms found in waste treatment systems, it is convenient tomeasure biomass rather than numbers of organisms.8 In the log-growth

    phase, the rate expression for biomass increase isdX

    It= (JLX (5-3)


    dX_= growth rate of the biomass, mg/L t

    fi = growth rate constant, t !

    X= concentration of biomass, mg/L

    Because of the difficulty of direct measurement of k in mixed

    cultures, Monod9 developed a model equation that assumes that the rateof food utilization, and therefore the rate of biomass production, islimited by the rate of enzyme reactions involving the food compoundthat is in shortest supply relative to its need. The Monod equation is




    fim = maximum growth rate constant, t_1

    S= concentration of limiting food in solution,

    mg/LKs = half saturation constant, mg/L

    = concentration of limiting food when ju, = 0.5fim

    The growth rate of biomass follows a hyperbolic function asshown in Figure 5-6.

    Two limiting cases are of interest in the application of Equation 5-4 to wastewater treatment systems. In those cases where there is anexcess of the limiting food, then S Ksand the growth rate constant,fi,is approximately equal to /tm. Equation 5-3 then becomes first-order in

    biomass. At the other extreme, when S

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    Limiting food concentration S, mg/L

    FIGURE 5-6

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

    Equation 5-4 assumes only growth of microorganisms and doesnot take into account natural die-off. It is generally assumed that thedeath or decay of the microbial mass is a first-order expression in

    biomass and hence Equations 5-3 and 5-4 are expanded to

    dX dt

    Ks+SkdX (5-5)

    where kj = endogeneous decay rate constant, t_1 .

    If all of the food in the system was converted to biomass, the rateof food utilization (dS/dt) would equal the rate of biomass production.Because of the inefficiency of the conversion process, the rate of foodutilization will be greater than the rate of biomass utilization, so

    dt 1 dX Y~dt(5-6)


    Y= decimal fraction of food mass converted to

    biomass = yield coefficient, jflffXd

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

    _dS _ 1/xmSX

    It ~ Y Ks +S(5-7)

    Equations 5-5 and 5-7 are a fundamental part of the development ofthe design equations for wastewater treatment processes.

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    Physical Characteristics

    Fresh, aerobic, domestic wastewater has been said to have the odor ofkerosene or freshly turned earth. Aged, septic sewage is considerablymore offensive to the olfactory nerves. The characteristic rotten eggodor of hydrogen sulfide and the mercaptans is indicative of septicsewage. Fresh sewage is typically gray in color. Septic sewage is black.

    Wastewater temperatures normally range between 10 and 20C. Ingeneral, the temperature of the wastewater will be higher than that ofthe water supply. This is because of the addition of warm water fromhouseholds and heating within the plumbing system of the structure.

    One cubic meter of wastewater weighs approximately 1,000,000grams. It will contain about 500 grams of solids. One-half of the solidswill be dissolved solids such as calcium, sodium, and soluble organic

    compounds. The remaining 250 grams will be insoluble. The insolublefraction consists of about 125 grams of material that will settle out ofthe liquid fraction in 30 minutes under quiescent conditions. Theremaining 125 grams will remain in suspension for a very long time.The result is that wastewater is highly turbid.

    Chemical Characteristics

    Because the number of chemical compounds found in wastewater isalmost limitless, we normally restrict our consideration to a few generalclasses of compounds. These classes often are better known by thename of the test used to measure them than by what is included in theclass. The biochemical oxygen demand (BOD5) test, which wediscussed in Chapter 4, is a case in point. Another closely related test isthe chemical oxygen demand(COD) test.

    The COD test is used to determine the oxygen equivalent of theorganic matter that can be oxidized by a strong chemical oxidizingagent (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 biologically, and becauseBOD5 does not equal ultimate BOD.

    The COD test can be conducted in about three hours. If it can becorrelated with BOD5, it can be used to aid in the operation and controlof the wastewater treatment plant (WWTP).

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

    and ammonia nitrogen in the wastewater.10

    TKN gives a measure of theavailability of nitrogen for building cells, as well as the potentialnitrogenous oxygen demand that will have to be satisfied.

    10Pronounced "kell dall" after J. Kjeldahl, who developed the test in


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    TABLE 5-2

    Typical composition of untreated domestic wastewater

    Weak Mediu Stron

    Constituent (all mg/L exceptAlkalinity (as 50 100 200


    Sus ended solids 100 200 350

    Total dissolved 200 500 1 000

    Total or anic carbon 75 150 300Total phosphorus (as 5 10 20

    a To be added to amount in domestic water supply.Chloride is exclusive of contribution from water

    softener backwash.

    Phosphorus may appear in many forms in wastewater. Among theforms found are the orthophosphates, polyphosphates, and organicphosphate. For ourpurpose, we will lump all of these together underthe heading 'Total Phosphorus (as P)."

    Three typical compositions of untreated domestic wastewater aresummarized in Table 5-2. The pH for all of these wastes will be in therange of 6.5 to 8.5, with a majority being slightly on the alkaline sideof 7.0. Industrial wastewater composition may be significantlydifferent from domestic wastewater.


    Without Water Carriage

    The pit privy. Although most modern environmental engineeringtexts would skip this subject, the mere existence of10,000 of these ortheir modern equivalent in the United States is just too much for us toignore. Furthermore, the facts of the matter are that junior engineersare the most likely candidates for designing, erecting, operating,dismantling, and closing the beasts.

    Figure 5-7 provides most of the information you will ever wantto know about the construction of an outhouse. The slab is usually

    poured over fiat ground on top of roofing paper. The riser hole is

    formed using 12-gauge galvanized iron. Once the slab has set, it islifted into place over the pit. The concrete is a 1:2:3 mix, that is, one

    part Portland cement, two parts sand, and three parts gravel less than25 mm in diameter.

    The principle of operation of the pit privy is that the liquidmaterials percolate into the soil through the cribbing and the solids "dryout." A pit of the dimensions shown in Figure 5-7 should last a familyof four about ten years. Rainwater is to be prevented from entering the

    pit. A cup of kerosene at weekly intervals


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    ----25"-- -------'! HI

    l ,




    " -

    ---J 1 i



    i! 1 1 1

    :.|.: 'Side Elevation

    FIGURE 5-7onstruct on eta s o t e p t pr vy: a cross sect on; p an o

    concrete s a ; an c eta s o r ser orm. Source: E ers an Stee ,

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    will discourage mosquito breeding, and odors are reduced by the use ofa cup of hydrated lime. Unfortunately, this will also slow thedecomposition of paper. Therefore, the use of lime is not encouraged.Disinfectants should never be used.

    The vault toilet. This is the modern version of the pit privy. Itsconstruction is the same as that of the pit privy with the exception thatthe pit is formed as a watertight vault. A special truck (fondly called a"honey wagon") is used to pump out the vault at regular intervals.Because of the liquefying action of the bacteria and incipient anaerobicdecomposition, vault toilets are much more odiferous than the old pit

    privies. Many masking agents (perfumes) and disinfectants areavailable to mitigate the stench. Unfortunately, most of them haveunpleasant odors themselves. If electricity is at hand, an ozonegenerator, set to vent into the gas space above the waste, will perform

    near-miracles in odor reduction.

    The chemical toilet. The airplane toilet, the coach-bus toilet, and theself-contained toilets of recreation vehicles are all versions of thechemical toilet. The essence of the system is a strong disinfectantchemical used to carry the waste to a holding tank and render itinoffensive until it can be pumped from the holding tank. While thesevehicular systems are quite effective, the chemical must be selectedwith an eye toward its impact on the treatment system which ultimatelymust receive it. The chemical toilet has not found wide acceptance in

    permanent installations. This is due to the cost of the chemical and tothe impracticality of maintenance.

    With Water Carriage

    Septic tanks and tile fields. A typical septic tank and tile fieldarrangement for a residential dwelling is illustrated in Figure 5-8. Theseptic tank and tile field are a unit. Neither part will function asintended without the other.

    The main function of the septic tank is to remove large particlesand grease which would otherwise clog the tile field. Heavy solidssettle to the bottom where they undergo anaerobic decomposition.Grease floats to the surface and is trapped. It is only slightlydecomposed.

    Since the septic tank is not heated, little reduction in BOD5occurs. Rather, the solid organic material which settles out is liquified.It then passes to the tile field. Since not all of the solid material can beliquified, the tank must be pumped at periodic intervals. The timeinterval between pumping depends on the amount of use and theobjects which find their way to the tank. Toilet paper is easilydegraded; however, plastic-lined disposable diapers cannot be degradedwithin a reasonable time. A family of four with young children canexpect to have their septic tank pumped every two years. A householdof two may not have to have its septic tank pumped in five or ten yearsof use. Grease accumulation is often the major factor in determining thefrequency of cleaning.

    In the past, the volume of the septic tank has been a function ofthe number of bedrooms in the dwelling. Current practice suggests that

    a 24-hour hydraulic

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