Laboratory Scale Activated Sludge Unit'

ANTHONY F. GAUDY, JR., RICHARD S. ENGELBRECHT, AND RALPH D. DE MOSS2

Departm-ents of Civil Engineering and Mlliciobiology, University of Illinois, Urbana, Illinois

Received for publication Jantuary 7, 1960

Recently, many investigators have turned attentionito devices for continuous culture of microorganisms.In the basic sciences, the externally controlled "chemo-stat" of Novick and Szilard (19'50) and AMonod's "bac-togen" (1950) have found considerable usefulness instudies where fairly large amounts of cells were requiredand where it was desirable to maintaini the populationat a specific growth rate. Continuous culturing devicesalso find useful application in the saniitary enigineeringfield. Garrett and Sawyer (1952), Schulze (1956),Garrett (1958), and Busch (1959) have made use ofcontinuous flow bench scale activated sludge units.The basic advantages and re(luirements for these

units for use in sanitary engineering studies have beendiscussed by Coe (1952), and Hatfield and Stirong(1954) and Busch (1959). The advantages lie primarilyin similarity of operation to the prototype as comparedwith the dissimilarity of batch units. The requiremenitsas set forth by Busch embody positive control of airflow, aeration solids, and solids wasting.

Regardless of the degree of control, extrapolation ofbench scale data for the prediction of prototype opera-tion is precarious. A more important need for suchcontrol is to insure that the sludge used for experimen-tation retains some degree of constancy throughout theinvestigation. Ideally, the most useful experimentationrequires that the system remain constant as one vari-able is studied. This is one of the valid arguments forworking with pure cultures. Howrever, in applying bio-chemistry and microbiology to the treatment of organicwastes, a heterogeneous population should be used,since under natural conditions, such systems wouldevolve subject to the intrinsic selectivity embodied inthe enrichment culture principle. To control the con-stancy of a heterogeneous population is a far moredifficult problem than that presented by a pure culturesince selection in a pure culture depends oni randommutation which under normal conditions is a compara-tively rare occurrence. Therefore, before confidentlyapplying the chemostat principle to a heterogeneouspopulationi, it is necessary to obtain some assurance

1 This investigation was stupported by a fellowship (GF-7973)from the Division of Research Grants of the National Insti-tuites of Health, U. S. Public Health Service.

2 Assistant professor and professor of Sanitary Eingineer-ing and professor of Microbiology, Departnments of CivilEngineering and Microbiology, respectively, University ofIllinois.

that system constancy can be at least roughly maini-tained or controlled.A bench scale unit which is to be used for research

on the activated sludge process must, in addition tomaintaining sludge constancy, have a high degree offlexibility since the many modifications now in userender it no longer possible to deal with the activatedsludge process per se. Although the biochemical prini-ciples of all successful biological treatment systems aresimilar, the various environmental conditions imposedby each modification make each unique in operatinigcharacteristics. It is not felt that continuous flow unitsshould replace batch systems for sanitary engineeringresearch, but it seems ideal that they be used to pro-vide a source of biological solids for experimentation.Such experimentation may be carried out in batch orin continuous flow depending on the particular require-ments of the research.The present interest in continuous culturing de-

vices arose during an investigation into the biochemi-cal effects of a rapid change in the chemical structureof a waste during waste treatment by the activatedsludge process. The term "qualitative shock load" hasbeen adopted to distinguish this type of shock loadfrom phenomena usually implied by the term "shockload." To study these effects it was necessary to gainsome assurance that, throughout the investigation,the population used for study would remain fairlyconstant in metabolic characteristics. It was proposed,therefore, to design and operate a continuous culturingdevice from which cells could be harvested for subse-quent subjugation, in batch study, to a qualitativeshock load.The purposes of the present paper are to review

briefly the principles upon which externally controlledcontinuous flow units are based, to describe the unitdesigned, and to present operational data and observa-tions which may be useful to others interested in theuse of such units for experimental purposes.

FUNDAMENTAL CONCEPTS

Kinetic treatment of continuous flow units has beendeveloped in essentially three fields. Denbigh (1947),in considering the kinetics of steady state polymeriza-tion, points out that the mathematical simplicity ofcompletely mixed continuous flow apparatuses makesthe study of steady state systems more attractive than

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discontinuous or semidiscontinuous systems. Also inthe field of physical chemistry, Bransom et al. (1949)used a similar kinetic development for continuouscrystallization operations and verified the results ofsteady state systems using batch studies to controland correlate operational variables.

In the biological field, Novick and Szilard (1950)and Monod (1950) used identical kinetic developmentsto describe the steady state kinetics of continuous cul-turing devices.

In the sanitary engineering field, Garrett and Sawyer(1952) and Schulze (1956) showed identical kineticsfor control of laboratory scale activated sludge plants.Later, Garrett (1958) recommended use of these kineticprinciples as a means of "hydraulic control" for theoperation of a full scale activated sludge plant.

In relation to Garrett's recommendation, it shoul(dbe noted that the kinetic development of continuousflow systems requires complete mixing in the aerator.Although most systems tend toward this condition, itdoes not rigidly hold for long rectangular aeration tanks.Also, the new steady state which may be set up inresponse to external changes such as feed concentra-tion, rate of flow, and temperature may not always bethe most desired one. In addition, the chemical compo-sition of the waste stream may change, which mayaffect the predominance of bacterial species and therate of sludge growth. Therefore, the principle may notbe applicable to activated sludge plants. However, itmay be applied to industrial fermentations where thesevariables may be subject to close control.

Concerning the activated sludge process, the principleis of value in providing the experimenter with a theo-retically sound laboratory apparatus which can besubject to controls not easily attained with the fullscale plant. The development of system kinetics is thesame for both the chemostat and the laboratory scaleactivated sludge plant.However, there is a difference in the apparatus, that

is, the incorporation of a sludge return line for recyclingthickened sludge to the aerator, which is a salient fea-ture of the activated sludge process. This does not af-fect the theoretical operation but does have an effecton the over-all rate of sludge growth. If the rate ofsludge return is constant, the effect on the growthrate is constant and does not directly enter into theconsiderations.Novick and Szilard (1954), Spicer (1955), and Moser

(1958) have dealt with the development of the kinetictheory of continuous flow units in considerable detail.The brief presentation below may serve to bring intofocus the essential identity of the kinetic fundamentalsemployed in the three fields previously cited.

In an undiluted (batch) biological system, the generalkinetic expression for bacterial growth is:

dN/dt = KN (1)

that is, growth follows first order kinetics wherein thechange in population is proportional to the amountpresent at any time. In a diluted (continuous flow)system this equation does not hold since the over-allchange in solids production is affected by the continu-ous dilution provided by the incoming flow of substrate.In a completely mixed system, biological solids arecarried out of the tank at a rate equal to the inflow rate,Q, thus diluting the solids concentration by the factorQ/V, that is, the ratio of the inflow rate to the fixedvolume of the tank. Under these conditions, systemkinetics may be expressed as:

dN/dt = KN - (Q/V)N (2)

In the steady state dN/dt is zero. Therefore, trans-posing and cancelling like terms, it is seen that theproportionality factor (K, the growth rate constant) iswholly a function of the hydraulic rate of flow Q, thatis'

K = Q/V (3)

which states that K is equal to the reciprocal of thedetention time, T (T = V/Q; mean residence time).

Internally controlled devices usually employ somemethod of measuring cell density in the aerator. Aphotoelectric cell is commonly used; it responds tochanges in cell density and can be made to control therate of feed flow, thus controlling K. However, it is theexternally controlled unit which applies more directlyto the activated sludge model. More precisely, it is thefact that a continuous culturing device can be ex-ternally controlled which makes its application to theactivated sludge process possible.Under conditions of external control Q/V is held at

any desired value below Kmax. This is essential since ifQ/V were greater than K, the solids would be dilutedout and if it were equal to K, the system could notrespond to an increase in organic feed concentration.This aspect is particularly important concerning ef-fluent control in the activated sludge system. If the ef-fluent biochemical oxygen demand (BOD) is to besubject to hydraulic control, the system must be poisedto respond successfully to any possible increase in in-fluent BOD concentration. One such response is anincrease in the number of organisms metabolizing thewaste. This can be accomplished artificially by return-ing more sludge to the aerator or naturally, accordingto equation (2), by increasing the growth rate. If thegrowth rate is already at its maximum, that is, Kmax -Q/V, the natural response is impossible.K can be externally controlled through the design

of the substrate. All essential nutrients except one can

be provided at nonlimiting concentrations. The limit-ing or controlling constituent, depending on the systemstudied, may be an amino acid, a nitrogen source, phos-phorus, sulfur or, as in the case most applicable to the

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study of the activated sludge process, the energy source,which may be expressed as the pollutional strength orBOD of the waste.Under these conditions K becomes a function of CG

the concentration of the conitrolling nutrient and equa-tion (3) is modified to:

K(CG) = Q/V (4)

The rate of sludge growth is thus controlled by therate of flow, the physical boundaries of the aerator, andthe substrate composition. With a constant sludge re-cycling rate, the cell density, N, is accordingly fixedl atany specific steady state condition. Under defined ex-perimental conditions, the factors affecting such asteady state condition cani be varied and studied in acontrolled manner. Conversely, if all controlling vari-ables are held constant, it would seem possible that thesludge produced should maintain some degree of re-producibility or constancy in its metabolic activity.

Although there may be valid objections to the applieduse of this principle as a field control device for fullscale activated sludge processes, it should find extensive

and valid use in controlled laboratory experimentationiaimed at delineating fundamental phenomena and ef-fects of changes in operational variables, both of whichwill increase understanding of the biological treatmentprocesses.

EXPERIMENTAL METHODS

Description. The experimental unit is shown in figure1. Organic and inorganic constituents are fed separatelyfrom 4-L and 11-L reservoirs. Both tanks are petcock-controlled. A constant hydrostatic head is maintainedthrough the use of the air inlet tube shown in the dia-gram. Successful operation requires an air-tight seal.To insure that air bubbles are not drawn into the siphon,the air inlet tube should be at least /,4 in. above thesiphon inlet. This device has been recommended byHoover (1951) for maintaining a constant feed rate atminute flows. It was found to be wholly satisfactory atthe low flow rate used during the present experimenta-tion.The aerator is made from a 4-L Erlenmeyer flask

cut as shown and fitted with )3j in. diameter outflow

AERATOR LUE

Figure 1. Continuous feed activated sludge unit

COMPRESSED AIRINORGANIC FEED

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pipe set to provide a 2-L volume of aerated liquor.Aeration is provided by compressed air introducedthrough two carborundum diffusers.The sludge return-settling tank and accompanying

sludge return air lift were included to incorporate thecharacteristics of the activated sludge process. If theunit is to operate as a true chemostat, these componentsare not needed. However, since the return of thickenedsludge will exercise a pronounced effect on activatedsludge systems, it is ideally included in a laboratoryunit designed for such research.The sludge return-settling tank consists of a 4-L

Erlenmeyer flask, cut as shown and fitted with outletand inlet devices. The operating capacity of the tankis 2 L. The tank bottom is rubber-stoppered and fittedwith an air inlet tube and sludge draw-off tube. Thisunit serves to partially thicken the sludge and providesa means of daily wasting a fixed volume of thickenedunderflow. It also provides a reservoir of thickenedmixed liquor for return to the aerator.The sludge return air lift serves to reaerate the

thickened mixed liquor and to return a portion of it tothe aerator. The sludge reaeration line was incorpo-rated into the design to insure proper control of thereturn flow and the hydraulic head in the sludge dis-tribution hopper. It also provides reaeration and re-

circulation of the flocculated cells. All compressed airenters the aerator and sludge return line through theliquid and dry traps shown.

Standard waste. The basic unit was designed to pro-vide an external and known control on system kinetics.The operational techniques and selection of the stand-ard waste were designed to enhance this control, toprovide an experimental population which would re-

main fairly constant through the investigation, and tosimulate the heterogeneous population in an activatedsludge aerator.The continuous flow unit was seeded with 200 ml of

settled domestic sewage and diluted to 2 L with organicand inorganic feeds. Microscopic examination revealeda heterogeneous population consisting of many morpho-logically different bacteria and a moderately large andvaried protozoan population. Filamentous forms were

present but not abundant. No algae or diatoms were

observed. The system was batch operated for 1 weekwhile the solids built up and thereafter as a continuousflow unit.

In designing the organic feed, consideration was

given to the carbon source which consisted solely ofglucose, fed at a concentration of 1,000 mg per L. Thisconcentration was chosen since it falls well within the

range of concentrations treatable with the activated

sludge process and because it is low enough to providean external control for the unit. Glucose was chosen in

order to develop a population which might be expectedin activated sludge and since it was desirable to use a

compound readily used by all aerobic organotrophicbacteria.

It should be recognized that any waste treatmentsystem will function as an enrichment culture for theorganism(s) best suited to the particular environmentcreated by the physical conditions of the process andthe chemical composition of the waste (Gaudy, 1955).An aerobic system fed glucose, an inorganic nitrogenand phosphorous source, and inorganic constituents inproper concentration, will foster the predominance ofaerobic organotrophs, in particular the pseudomonads.The inorganic constituents and their concentrations,shown in table 1, vwere based on cultural requirementsfound to be optimum in the bacteriological literatureand were in general present in excess concentration.This was necessary to insure that the external controlwould be the carbon source only. The BOD/N andBOD/P ratios found in the sanitary literature werenot maintained because these are minimal requirementsempirically determined on the basis of economical BODremoval. Although they can be defended on a theo-retical basis, it cannot be unequivocally stated thatthey will not limit a biological system.

Loading parameters. The activity of an aerobic cul-ture may be measured on the basis of oxygen used perunit cell mass per unit of time. This is often expressedas

Qo2 = mliters 02 uptake/mg dry weight of cells/hr

This same parameter is also used to express theloading of a biological treatment process

Loading = lb 5 day BOD/1000 lb suspended solids/hr

Although different interpretations may be placedon these two expressions, they are nevertheless funda-mentally similar. Use was made of the latter expres-sion in checking the efficiency of the model activatedsludge system. The former expression was used tocheck the activity of the sludge in the unit.Loading chart for various solids concentrations. The

system was operated at a constant volumetric loadingof 2 L per day. However, due to periodic withdrawals

TABLE 1Standard experimental waste

Constituent Concentration

per L

Glucose ..................................... 1000mgNH4C1 ..................................... 300 mgMgSO4-7H20................................. 250 mg

FeSO4-7H20 ................................. 10 mgMnSO4-H20.................................. 10 mg

CaCI2 -2H20 .................................. 10 mgTrace elements (tap water) ................... 100 ml1 M Phosphate buffer, pH 7.0 ................. 10 ml

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for various experiments, it was expected that the solidsconcentration would vary from time to time.

It was desirable therefore to construct a loadingchart for the plant so that its efficiency could be readilymeasured at any time a sample was taken.The basis for calculations is as follows:

Theoretical BOD = (1000) (192/180) (0.7) = 750 mg/L

where 0.7 = fraction of total BOD exerted in 5 daysat 20 C (k taken as = 0.1).

BOD loading.

2 L at 750 mg/L = 1.5 g 5-day BOD/day1.5 X 1/453.6 = 0.00332 lb BOD/day

Design volume.

(a) Aerator volume = 2 L(b) Settling-return tank volume3 = 2 L

4 L total

A table of loadings at various values of aerationsolids and a sample calculation are shown in table 2.The information is shown graphically in figure 2. Systemefficiency can be easily computed from BOD or chemicaloxygen demand (COD) data and compared withgeneralized or average findings readily available in theliterature for municipal activated sludge plants (Imhoffand Fair, 1956). It should be noted that the efficiencyof substrate removal was measured using COD resultsand that the effluent tested was "membrane filtereffluent" obtained in conjunction with biological solidsmeasurement using the membrane filter technique(Engelbrecht and McKinney, 1956). Therefore, thesystem was tested as to its biochemical efficiency ratherthan over-all efficiency which includes settleability ofthe aeration solids. This was desirable because of thenature of subsequent experiments for which the sludge

I Although the aeration efficiency was undoubtedly muchlower in the settling return tank, it is felt that in calculatingthe loading the entire volume of 4 L should be considered sincethe airlift method has been used as a means of aeration forculturing bacteria (Lundgren and Russell, 1956). This wouldlead to a more conservative estimate of system efficiency.

TABLE 2System loading at various solids concentrations

Aeration Calculation Loading*Solids

mg./L

250 lb BOD 62.8500 day 31.4750 g lb hr 1 lb 20.91000 - X1X-X x 15.71250 L g day 1000 lb 12.61500 10.51750 0.00332 X 1000 9.02000 0.25 X 4 X 0.0022 X 24 7.9

* Poundssolids/hr.

was developed (not herein reported). However, inapplying the principles involved in hydraulically con-trolled continuous flow units, the more standardmethods of measurement may be employed with equalfacility.

RESULTSSystem efficiency. Operational results for 3 months

are shown in table 3. The observed efficiencies are some-

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40

20

00 500 1000 1500 2000

AERATION SOLIDS, mg/

Figure 2. System loading at various solids concentrations

TABLE 3Operational results

Biological COD Theoretical System ExpectedtDate Solids Remaining Re ing ading Efficiency Efficiency

mng/L mg/L mg/L % %3/18 820 135 95 19 87.3 913/22 990 175 123 15.5 85.6 913/25 1020 180 126 15 85.2 913/28 960 205 144 14.5 80.8 923/31 1050 135 95 16.2 87.4 914/7 890 185 130 17.4 82.7 914/22 1080 195 137 14.2 81.7 924/23 960 160 112 16.2 85.1 914/25 1010 205 144 15.1 80.8 914/28 1110 195 137 14.0 81.7 925/2 1000 125 88 15.5 88.3 915/8 985 215 151 15.7 79.6 915/13 990 185 130 15.6 82.7 915/17 1030 210 147 15.0 80.4 916/6 1100 205 144 14.0 80.6 926/13 880 190 133 17.5 82.3 916/16 1060 165 116 14.5 84.5 926/20 1170 155 109 13.1 85.4 92

* May be obtained from figure 2; pounds 5 day BOD/1000lb suspended aeration solids/hr.

t Approximate efficiency to be expected at the stated load-ing (Imhoff and Fair, 1956).

L .L. L I

-

5 day BOD/1000 lb of suspended aeration

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what lower than those predicted for domestic treat-ment plants. Had the calculations been based on a 2-Lvolume under air (the aerator volume), these valueswould have coincided more closely. The point to beemphasized is that the organic removal remained fairlyconstant throughout (80 to 87 per cent removal) whichin some degree substantiates the conclusions of Gar-rett concerning hydraulic control of BOD removal.

Sludge activity. The measurement of system efficiencyis in some respects a measure of sludge activity. How-ever, it is a measure of sludge activity in a limited sys-

tem controlled by the hydraulic rate of a feed used insuch concentration that the maximal growth rate isnot attained. The Qo2 measurement is made in an

undiluted batch system under respiring conditions at a

sufficiently low cell concentration or high substrateconcentration to insure that substrate is not limiting.With sufficient agitation, the manometric measurementof Qo2 should yield that uptake attributable to theintrinsic nature of the cells rather than to the systemin which they are grown. The absence of an extracellu-lar nitrogen source other than that of the atmospherealso tends to remove the complicating factor of cellreplication during the test.The Qo2 values obtained are shown in figure 3. The

values are fairly closely grouped but are by no means

identical. The most probable reason for this is discussedbelow.

Operational observations. While the constant-headtank and petcock arrangement provided an excellentfeed control, it was found that the organic feed orificerequired close surveillance since the aerosol producedin the aerator provided ample seed for growth at theorifice thereby altering its hydraulic characteristicsand thus its flow. Flow rates were checked for bothfeed lines three to five times daily during the firstmonth of operation and daily thereafter. The effluentvolume was also measured as a check. Early in thework, losses varied from 20 to 40 ml per day. Duringthe more humid periods prevalent in the latter part ofthe investigation, an evaporative loss was not detect-able.

U)

a

0U)

0

co

E

N.

0

:40

a

20

0

MARCH APRIL MAY

TIME, MONTHSJUNE JULY

Figure S. Variability in metabolic activity of sludge pro-duced in activated sludge unit.

During the first few weeks of operation, it was foundthat if the pH were allowed to stay below 6 for 24 hrthe flocculating characteristics of the system were de-creased and the color of the aerator culture turned fromgreenish-yellow characteristic of the pseudomonads toa more gray appearance, indicating a change in pre-

dominant species. Microscopic examination did notreveal any readily discernable morphological differ-ences in the species present, but there was considerablyless aggregation of the cells. After pH adjustment, 48hr were required to restore the system to its usualappearance. It should be noted, however, that duringa period when pH was purposely held low there was no

apparent difference in the substrate removal efficiencyof the system. It would seem entirely possible that a

slight change in the pH could cause a large change inspecies predominance but might not affect the bio-chemical efficiency of the system. In substantiation ofthis view, Sawyer et al. (1955) have reported very littledifference in BOD removal efficiency at widely differ-ing pH values. However, since a varying pH might beexpected to effect some changes in biochemical response

in subsequent experiments with the sludge, the pH was

adjusted to 7 whenever it approached 6.7.One aspect of operation that militated against suc-

cessful application of external control principles was

the proliferous amount of growth found adhering tothe aerator walls. True hydraulic control requires thatthe culture be wholly suspended. Some investigatorshave employed a "windshield wiper" arrangement toprevent accumulation of side growth (Novick, 1955).Having observed the operational characteristics of theunit for some time, it is felt that one of the prime causes

for side growth in this unit involves the relatively smallamount of agitation near the wall of the tank. Vigorousaeration was maintained at all times, but the shape ofthe aerator militated against vigorous agitation at thebottom and lower portion of the walls of the tank.Although this was ideal in assisting to simulate theflocculated growth prevalent in the activated sludgeprocess, there was not sufficient agitation to slough theside growth. The effects of side growth, although veryimportant in the control of continuous flow bench scaleunits, are probably insignificant in a full scale plantsince the ratio of tank surface to liquid volume is de-creased. However, in applying the principle of hy-draulic control to the activated sludge process, thesettling tank must be considered and a specific amountof sludge wasted. Therefore, the development of sludgepockets in the settling tank, which is essentially identi-cal in effect with side growths, must be avoided.The above comment illustrates what is believed to

be a rather important point concerning the reconcilia-tion of phenomena observed in small scale laboratoryunits as compared to large scale pilot plants and fullscale plants. Although some of the physical peculiarities

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of each may be different, they may have formallyanalogous counterparts which if recognized can assistin analyzing data obtained using either approach tosanitary engineering research.

Settling characteristics of the sludge were extremelyvariable, ranging from 74 to 90 per cent compaction in30 min. Settling tests were not run regularly on theunit since this data was of no immediate concern to theinvestigation. The effluent was always observed to beturbid. It was noted, however, that this turbiditycould be sharply decreased by gentle stirring of the ef-fluent followed by 1 hr quiescent settling. Microscopicobservations of the culture were made approximatelytwice weekly. It is rather surprising that although theoriginal sewage seed contained a moderate amount offilamentous forms, these forms were never observedexcept in trace amounts in the experimental system.The system appeared to consist wholly of flocculatedand individual bacteria with varying numbers of freeswimming and attached protozoa. No attempt wasmade to control temperature; however, it was meas-ured on the average of 2 times daily and found to varyfrom 19 to 23 C.

SUMMARY

It is concluded that the unit described provides afairly constant biological system based on organic re-moval efficiency and sludge activity. Analysis of theresults of the response studies for which these cellswere used also substantiated this conclusion (Gaudy,1959). The unit is not identical with the "chemostat"or "bactogen" after which it was modeled because thereturn of sludge, which was necessary to simulate anactivated sludge treatment process, provided a partialinternal control of growth rate. The return of sludgeallows a higher population to exist than the energysource would permit without recirculationi. This situa-tion is characteristic of most activated sludge systems.However, with a constant amount of wasted sludge andeffluent, the over-all characteristics and the mathe-matical validity of the concept are identical for theunits described in the biological literature, the oneused here, and that described by Garrett (1958) in thesanitary literature. The unit recently described byBusch (1959) also functions in the same way.

If the unit, or a modification of it, is used for thevarious studies for which it may usefully serve, asmuch automation as possible should be incorporatedinto its design. The incorporation of a wall scrapingdevice would considerably increase the cost of theunit; however, the elimination of side growths wouldassure a more constant sludge activity since it preventsretention of older cells. The continuous flow modelplant, operating on the chemostat principle, is con-

sidered to be sufficiently validated for use in sanitaryengineering research to warrant more elaborate con-struction and inclusion of such automation as positiveand continuous removal of side growth and continuouspH and temperature control.

REFERENCESBRANSOM, S. H., DUNNING, W. J., AND MILLARD, B. 1949 Ki-

netics of crystallization in solution. Disc. Faraday Soc.,5, 83-95.

BUSCH, A. W. 1959 Laboratory units for bench scale studies.Water & Sewage Works, 106, 254-256.

COE, R. H. 1952 Bench scale biological oxidation of re-fining wastes with activated sludge. Sewage and Ind.Wastes, 24, 731-749.

DENBIGH, K. C. 1947 Continuous reactions. Part II. Thekinetics of steady state polymerisation. Trans. FaradaySoc., 43, 648-660.

ENGELBRECHT, R. S. AND MCKINNEY, R. E. 1956 Membranefilter method applied to activated sludge suspended solidsdetermination. Sewage and Ind. Wastes, 28, 1321-1325.

GARRETT, M. T., JR. 1958 Hydraulic control of activatedsludge growth rate. Sewage and Ind. Wastes, 30, 253-261.

GARRETT, M. T., JR., AND SAWYER, C. N. 1952 Kinetics ofremoval of soluble BOD by activated sludge. PurdueUniv. Eng. Bull. No. 36, 51-77.

GAUDY, A. F., JR. 1955 Mode of bacterial predominancein aerobic waste disposal systems. Master's Thesis,Massachusetts Institute of Technology, Cambridge,Massachusetts.

GAUDY, A. F., JR. 1959 Biochemical aspects of qualitativeshock loading of aerobic waste treatment systems. Doc-toral Thesis, University of Illinois, Urbana, Illinois.

HATFIELD, R. AND STRONG, E. R. 1954 Small scale labora-tory units for continuously fed biological treatment experi-ments. I. Aeration units for activated sludge. Sewageand Ind. Wastes, 20, 1255-1258.

HOOVER, C. P. 1951 Water supply and treatment. NationalLime Association (Washington, D. C.), Bulletin No. 211.

IMHOFF, K. AND FAIR, G. M. 1956 Sewage treatment. JohnWiley and Sons, Inc., New York, New York.

LUNDGREN, D. C. AND RUSSELL, R. T. 1956 An air liftlaboratory fermentor. Appl. Microbiol., 4, 31-33.

MONOD, J. 1950 La technique de culture continue. Theorieet applications. Ann. inst. Pasteur, 79, 390-410.

MOSER, H. 1958 The dynamics of bacterial populationsmaintained in the chemostat. Carnegie Inst. Wash.Publ. No. 614.

NOVICK, A. 1955 Growth-of bacteria. Ann. Rev. Microbiol.9, 97-110.

NOVICK, A. AND SZILARD, L. 1950 Description of the chemo-stat. Science, 112, 715-716.

NOVICK, A. AND SZILARD, L. 1954 Dynamics of growth proc-ess. Princeton University Press, Princeton, New Jersey.

SAWYER, C. N., FRAME, J. D., AND WOLD, J. P. 1955 Revisedconcepts on biological treatment. Sewage and Ind.Wastes 27, 929-938.

SCHULZE, K. L. 1956 The effect of phosphorus supply on therate of growth and fat formation in yeasts. Appl. Mi-crobiol., 4, 207-210.

SPICER, C. 1955 The theory of bacterial constant growthapparatus. Biometrics, 11, 225-230.

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