Microbiological Process Report

Activity of Microorganisms in Organic Waste Disposal

II. Aerobic Processes'

Ross E. McKINNEY

Massachusetts Institute of Technology, Cambridge, Massachusetts

Received for publication November 27, 1956

The aerobic biological treatment systems are ex-tensively used for the stabilization of dilute organicwastes. Activated sludge, trickling filters, and oxidationponds are the major basic types of aerobic systems.Each of these systems operates on the same fundamen-tal biochemical principles and differs one from an-other primarily by the method of oxygen transfer.Activated sludge utilizes compressed air for its oxygensource and for mixing. The microorganisms in tricklingfilters are attached to stones and obtain their oxygen bydiffusion from the air. Oxidation ponds represent thecombined efforts of algae and bacteria with the algaeproducing part of the oxygen for the bacteria and thebacteria producing the carbon dioxide desired by thealgae.

MICROORGANISMS

The microorganisms which inhabit the aerobic bio-logical treatment systems include bacteria, fungi, algae,protozoa, rotifers, and other higher animals. The growthof any or all types of microorganisms in a given in-dustrial waste disposal system will depend upon thechemical characteristics of the industrial waste, theenvironmental limitations of the particular wastesystem and the biochemical characteristics of themicroorganisms. All of the microorganisms which growin a given industrial waste disposal system contributeto its over-all characteristics, both good and bad. It isimportant to recognize the contributions made by eachtype of organism to the over-all stabilization of theorganic wastes if the waste treatment system is to beproperly designed and operated for maximum efficiency.

Bacteria. The bacteria are the basic biological unitsin aerobic waste treatment systems. The diverse bio-chemical nature of bacteria makes it possible for themto metabolize most, if not all, organic compounds foundin industrial wastes. Obligate aerobes. and facultativebacteria are found in all aerobic waste treatmentsystems. Growth of any particular species is dependent

1 Presented at the 13th General Meeting of the Society forIndustrial Microbiology, Storrs, Connecticut, August 26-30,1956.

upon its competitive ability to obtain a share of theavailable organic material in the system.

Bacterial predomination will normally divide itselfinto two major groups, (1) the bacteria utilizing theorganic compounds in the waste and (2) the bacteriautilizing the lysed products of the first group of bacteria(Gaudy, 1954). The bacteria utilizing the organic com-pounds in the waste are the most important group andwill determine the characteristics of the treatmentsystem. The species with the fastest growing rate andthe ability to utilize the majority of the organic matterwill predominate. The extent of secondary predomina-tion will depend upon the length of starvation. Deple-tion of the organic substrate is followed-by death andlysis of the predominate bacteria. Release of the cellularcomponents of the bacteria permits other bacteria togrow up. Since all biological treatment systems arenormally overdesigned as a safety factor, secondarypredomination will occur.

Aside from the metabolic characteristics of thebacteria, the most imnportant characteristic is theirability to flocculate. All of the aerobic biological wastetreatment systems depend upon the flocculation of themicroorganisms and their separation from the liquidphase for complete stabilization. It was first thoughtthat flocculation was caused by a single bacterial species,Zoogloea ramigeria (Butterfield, 1935; Heukelekian andLittman, 1939; Wattie, 1942), but recent studies(McKinney and Horwood, 1952; McKinney and Weich-lein, 1953) have shown that there are many differentbacteria which have the ability to flocculate. It has beenpostulated (McKinney, 1955) that all bacteria have theability to flocculate under certain environmental condi-tions. The prime factors affecting flocculation are thesurface charges of the bacteria and their energy level.The electrical surface charge on bacteria grown indilute organic waste systems has been shown to bebelow the critical charge for autoagglutination, 0.020volts (McKinney, 1953). This means that Brownianmovement provides sufficient energy to overcome therepelling electrical forces when two bacteria approacheach other and to permit the Van der Waal forces of

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attraction to predominate and hold the two bacteriatogether. Autoagglutination does not take place if theenergy level of the system is sufficiently high to permitthe bacteria to multiply and to be rapidly motile.Autoagglutination, or flocculation, occurs only after thebacteria lack the energy of motility to overcome theVan der Waal forces. Once floc has started to form, someof the bacteria die and lyse. An insoluble fraction of thebacterial cell is left which is primarily polysaccharide.The older the floc becomes, the more polysaccharidebuilds up and the less active bacteria are entrained in it.

Fungi. Fungi play an important role in the stabiliza-tion of organic wastes. Like the bacteria, the fungi canmetabolize almost every type of organic compoundfound in industrial wastes. The fungi have the potentialability to predominate over the bacteria but they donot except under unusual environmental conditions.The filamentous nature of most of the fungi found inindustrial wastes makes them undesirable since they donot form a tight compact floc and settle easily. For thislatter reason, considerable efforts are expended to makethe environmental conditions more favorable for bac-teria predomination than for filamentous fungi pre-domination.The filamentous fungi predominate over the bacteria

at low oxygen tensions, at low pH, and at low nitrogen.Low oxygen tension results from a low oxygen supply or

from a high organic load causing the demand to exceedthe supply. Under reduced oxygen levels, metabolismdoes not proceed to carbon dioxide and water but stopswith the formation of organic alcohols, aldehydes, andacids. If the system lacks sufficient buffer, the organicacids depress the pH to the more favorable range forfungi. Thus, it can be seen that low oxygen tension andpH can be interrelated. Many of the fungi grow well atpH 4 to 5 while few bacteria are able to grow wellenough to compete. Fungi require less nitrogen thanbacteria per unit mass of protoplasm. In nitrogendeficient wastes, the fungi are able to synthesize more

active masses of protoplasm from the wastes than are

the bacteria and predominate. Bacteria average approx-

imately 10 to 12 per cent nitrogen (Porter, 1946) whilefungi range from 5 to 6 per cent nitrogen.Under normal environmental conditions fungi will be

present and will aid in the stabilization of the organicmatter. But the fungi are of secondary importance andwill not predominate.

Algae. The algae are the third form of biologicalplants which play a part in the over-all stabilization oforganic wastes. Since the algae obtain their energy forsynthesis from sunlight, they do not have to metabolizethe organic compounds like the bacteria and the fungi.To form protoplasm the algae primarily utilize theinorganic components of the wastes, for example, am-

monia, carbon dioxide, phosphate, magnesium, potas-sium, iron, calcium, sulfate, sodium and other ions. It

is possible to have algae and the bacteria predominatetogether since they do not utilize the same waste com-ponents. The bacteria metabolize the organic com-ponents of the waste and release some of the inorganiccomponents utilized by the algae. During protoplasmsynthesis the algae release oxygen which is taken by thebacteria to bring about complete aerobic stabilizationof the organic matter.

In the absence of sunlight the algae must obtain theenergy required to stay alive from the metabolism oforganic matter in the same manner as bacteria andfungi. This organic matter normally comes from storedfood within the cell but in some algal species it can comefrom the organic material in the wastes.

Protozoa. The protozoa are the simplest animalsfound in waste disposal systems. The role that theprotozoa play in stabilizing organic wastes has onlyrecently been clarified by combining a study of pureculture protozoa (A. Gram, 1953, unpublished observa-tions) with the natural observations in various biologi-cal treatment systems. This study showed that ratherthan being the primary mechanism of purification(Pillai, 1941; Pillai and Subrohmanyan, 1942, 1943,1944; Pillai et al., 1947) the protozoa were responsiblefor reducing the number of free-swimming bacteria,thus aiding in producing a clarified effluent.The succession of protozoa had long been observed

in biological waste disposal systems (Buswell and Long,1923; Agersborg and Hatfield, 1929; Barritt, 1940)but no explanation of the reasons for this successionwere put forth. The succession of protozoa is affected bythe same factors which affect the predomination of anybiological species. The type of food and the competitionfor food are the major factors which determine thepredomination of the protozoa. The Sarcodina are onlybriefly found in aerobic waste treatment systems sincethey do not find sufficient food to compete with the bac-teria and other biological forms. The Phyto-Mastigoph-ora survive a little longer than the Sarcodina as they takein soluble organics for their food but they are unable tocompete against the bacteria and are soon displaced.The Zoo-Mastigophora predominate over the Phyto-Mastigophora in that they are able to utilize the bacteriafor food rather than compete with the bacteria for food.But the Zoo-Mastigophora give way to the free-swim-ming Ciliata which have a better mechanism for ob-taining the bacteria and other food components. As thesystem becomes more stable, there are less and lessfree-swimming Ciliata. The low-energy-requiring stalkedCiliata displace the high-energy-requiring free-swim-ming Ciliata. But soon the system becomes so stablethat the stalked Ciliata cannot obtain enough energyand die out of the system.The succession of protozoa offers a good index of

stability of the biological waste treatment system.Efforts have been made to relate the numbers of pro-

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tozoa to the degree of stabilization (Lackey, 1949;Jenkins, 1942) but they have not been successful sincethe same numerical population exists at two separateand distinct levels of purification. Low numbers of free-swimming Ciliata occur at both a low degree of purifica-tion, 20 to 40 per cent, and at high purification, 75 to 95per cent. The relative types of protozoa and relativenumbers can be used for any particular system to esti-mate the rough efficiency, i 10 per cent, of any bio-logical treatment system.The protozoa have more complex metabolic systems

than do bacteria or fungi which makes the protozoamore sensitive to toxic organic compounds. In systemscontaining toxic organic compounds, regular observa-tions of the protozoa can be used as an indicator of thetoxic conicentration and to warn of potential toxicity tothe bacteria which are responsible for stabilization ofthe wastes. The protozoa can also be used to indicatedeficiencies of certain essential elements such as nitrogenor phosphorus. Nutrient deficiencies will reduce bothnumber of species and number of any particular species.

Rotifers and higher animals. Rotifers, as well as higheranimals, occur in aerobic biological waste disposalsystems. Because of their complex metabolic systemsthe higher animals occur only in highly stabilizedsystems where an excess of oxygen occurs at all times.The rotifers can be used as indicators of a very highdegree of purification, 95 to 100 per cent. They canmetabolize solid floc particles which the protozoa cannotmetabolize and predominate after all of the protozoahave died off for lack of food.Daphnia and other crustaceans grow well in stabilized

aerobic systems, utilizing bacteria, fungi, and algae astheir major source of food. Nematodes are also foundunder certain conditions in high solids concentrations.

BIocHEMISTRY

The basic biochemical reaction in aerobic biologicalwaste disposal systems is the same for all microor-ganisms.

Food + Microorganisms + Oxygen -* IncreasedA1Iicroorganisms + Carbon dioxide + Water

+ Waste products (+ Energy)

This reaction can be divided into two components:synthesis and endogenous respiration. The synthesisreaction determines the maximum quantity of micro-organisms produced and the minimum quantity ofoxygen required per uniit mass of organic waste stabi-lized. The endogenous reaction determines the break-down of protoplasm to yield energy for the micro-organisms to remain alive. The synthesis reaction canbe established by a complete chemical balance of thebasic biochemical reaction when the microorganismsare growing unrestricted in an excess of organic sub-strate. The endogenous respiration reaction can be

established after the organic substrate has been ex-hausted.The stabilization of organic matter by micro-

organisms follows a definite pattern of growth which canbe divided into three sections: log growth, declininggrowth, and endogenous metabolism. Log growth is theunrestricted growth which occurs in the presence ofunlimited food. Declining growth results when foodbecomes limiting and the rate of growth becomesproportional to the amount of food remaining. Assynthesis begins to fall off, endogenous respirationbecomes significant and increases until a maximum isreached when synthesis ceases. The rate of endogenousmetabolism is directly proportional to the mass ofactive protoplasm.As indicated by the basic biochemical reaction, the

growth of the microorganisms produces definite changesin the removal of food from the system and in theoxygen utilized. Figure 1 shows the general relationshipsexisting between mass of microorganisms, substrate andoxygen. This diagram is the key to understanding thedesign and operation of the aerobic biological treatmentsystems.

Considerable effort has been made by the sanitaryengineer to operate in the log growth phase since themaximum conversion of organic matter in the wastes toprotoplasm and the minimum quantity of oxygen perunit of protoplasm formed occurs in this phase. Whilethis sounds good from a theoretical standpoint, it doesnot work out as well from a practical standpoint. Bac-teria will not flocculate and settle as long as they aregrowing at a log rate which means that the micro-organisms will be completely dispersed and will go outin the effluent to the receiving body of water. A maxi-mum of 50 per cent of the organic matter will bestabilized before the bacteria pass from the log growthphase into the declining growth phase. The remainder

LOG \ DECLINING | ENDOGENOUSGROWTH GROWTH

UX I, 0I l0 T l. 01

0.

0° m j SolubleI -o

- orgonics0040U)n ( I

TIME

FIG. 1. Theoretical metabolism of organic matter by micro-organisms.

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of the load will be passed oIn to the receiving water.Lastly, the rate of oxygen uptake is a maximum at thechange between log growth and declining growth. Ifaerobic conditions are to be maintained, it will beimperative to have equipment capable of transferringthe oxygen demanded. At this time, the rate of oxygentransfer obtainable from current equipment is thelimiting factor in the design and operation of bio-logical treatment systems.Most biological treatment systems operate ba3tween

the declining growth and endogenous phase with adefinite flowing-through period which simulates thetime axis of figure 1. The high degree of purificationobtained by the aerobic biological treatment systems,85 to 99 per cent, is brought about by growth in theendogenous phase. Endogenous metabolism has theadvantage of complete oxidation without increasedsynthesis, a low organic level remaining in the wastesand excellent flocculating characteristics. But it hasits disadvantages also. Oxygen must be supplied for theentire oxidation of the organic matter since none isconverted to synthesis. The rate of oxygen uptake isvery low so that a long aeration period must be usedto stabilize the organic matter.

Industrial wastes often contain unusual organiccompounds not readily found in large concentrationsin nature. Many of these compounds are toxic to micro-organisms in very low concentrations. It is possible totreat wastes containing toxic organic compounds byslowly adapting the bacteria to the toxic compound.This is usually done by growing a large quantity ofmicroorganisms on nontoxic organic compounds andthen slowly removing the nontoxic compound and sub-stituting the toxic compound. The acclimated systemcan stabilize the new organic material as rapidly as itcould the nontoxic compounds. Cyanogenic, phenolic,formaldehyde, acrolein and many other toxic wasteswhich were once thought to be biologically untreatableare now being stabilized in biological systems. Bio-logical stabilization is not a factor of a particular chemi-cal compound but rather a factor of the chemicalstructure of the compound (McKinney, 1956). Attackon the molecule is directed at certain chemical groups.The carboxy group is most readily attacked, followedby the aldehydic carbonyl, the hydroxyl, the ketocarbonyl and the methyl groups. Amino groups areattacked as readily as hydroxyl groups. Sulfonategroups tend to act as carboxyl groups but block thecarboxyl enzyme reaction and prevent metabolism.Continual growth in sulfonic acid groups permits thebacteria to adapt to them and to metabolize thesulfonated molecule completely (McKinney et al.,1955). Very little is known of the effect of other sub-stituted groups except that adaptive metabolism isrequired.One of the major problems in the aerobic stabilization

for the stabilization of organic matter is through theacid form. Unless the system has sufficient buffer toneutralize the acid that is formed, the pH will quicklydrop below the level for optimum efficiency. Bacterialsystems prefer a pH between 6.5 and 9.5. The bufferproblem is most critical with metabolism of neutralcompounds such as sugars, alcohols, aldehydes, andketones. Organic acids are usually neutralized at thestart and form their own buffer with metabolism. Con-tinued operation below pH 6.5 will usually cause fungito predominate in the biological treatment system.While the fungi will stabilize the organic matter asreadily as bacteria, the fungi do not have the same

flocculating characteristics as the bacteria. The fungitend to filamentous forms which do not compact to areadily settleable mass. But there are some wastesystems which will cause the nonfilamentous fungi topredominate and which will have excellent settlingcharacteristics. Above pH 9.5 the hydroxide ion con-centration begins to have a toxic effect on the bacteria.It is standard procedure to neutralize high pH wastesdown to pH 9.5 with acid before treating biologically.The carbon dioxide produced by the microorganismswhile stabilizing the organic wastes can be used toneutralize the hydroxide alkalinity rather than acids,provided the pH of the mixed wastes and micro-organisms does not exceed 9.5 for too long a period.The synthesis of bacterial protoplasm requires

carbon, hydrogen, oxygen, nitrogen, phosphate, sulfate,potassium, sodium, magnesium, calcium, iron, andmolybdenum. The wastes must supply these items ifmaximum efficiency is to be obtained. The carbon andhydrogen are supplied by the organic wastes. Oxygen issupplied from water or from air as dissolved oxygen.

Nitrogen can come from the organic waste or frominorganic forms such as ammonia, nitrite, and nitrate.It is even possible to use nitrogen gas from the air bynitrogen fixation to supply the necessary nitrogen forprotoplasm (McKinney et al., 1955). Phosphate usuallycomes from the wastes. The present use of syntheticdetergents results in large quantities of phosphateentering the wastes from cleaning processes alone. Thecarriage water for many of the industrial wastes is tapwater in the area. In hard water areas there are more

than enough of the trace elements. In soft water areas,

or in waste streams with demineralized water, theabsence of sufficient sulfate or trace metal could very

definitely limit the process and must be considered as a

vital part of the design.Oxygen is supplied to aerobic biological treatment

systems primarily from the air. Activated sludge uses

diffused air at low pressure, 6 to 8 psi, while tricklingfilters and oxidation ponds depend on air diffusion. Therate of stabilization is limited by the rate of oxygen

transfer. Standard diffused air equipment in activated

of organic wastes is that of pH. The metabolic pathway

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L of waste per hr. Special equipment is available fortransfer rates as high as 100 to 300 mg per L per hr. Thehigher rates of oxygen transfer are obtained as a resultof combining compressed aeration with high-speedmixing. The high-speed mixing tends to tear the flocapart and prevents complete flocculation and clarifica-tion of the effluent. The operating costs also go up quitemarkedly with high-speed mixing.

If the oxygen demand is greater than the oxygensupplied to the system, the bacteria turn to chemicalsources of oxygen as the hydrogen acceptors. Nitritesand nitrates are the most readily utilized hydrogenacceptors. Nitrites and nitrates are reduced to gaseousnitrogen rather than to ammonia. The most commonoccurrence of denitrification is in the settling compart-ment of activated sludge systems where the nitrogengas released catches under the floc and causes it to riseto the surface of the settling tank. Release of the gas atthe surface causes the floc to break up and to be par-tially lost in the effluent. Sulfates become the hydrogenacceptor after nitrates. The reduction of sulfates resultsin the production of hydrogen sulfide. Although the onlygroup of sulfate reducing bacteria are the obligateanaerobes, Desulfovibrio, the rapid hydrogen sulfideproduction in high sulfate systems with the subsequentsulfate reduction points to the existence of a largenumber of Desulfovibrio in the aerobic system or toother sulfate reducing bacteria. Since organic matter isthe hydrogen donor for the Desulfovibro, they could bepresent in large quantities and could contribute to theover-all stabilization of the organic wastes.Temperature plays a very important role in biological

stabilization. The rate of biochemical reactions increasesrapidly as the temperature increases, doubling withevery 10 C increase. Trickling filters act as coolingtowers to a certain extent and rapidly lose the heatcontent of the wastes and that produced by biologicalmetabolism to the surrounding air. This makes tricklingfilters less desirable in colder climes since the filtersmust be enclosed to prevent freezing and the metabolicefficiency is considerably reduced. Activated sludge withcompressed aeration loses some of its heat but retains agood portion of it. This is important in the winter whenthe heat content of the wastes is used to hold theactivity of the microorganisms high. The maximumsustained temperature of the aerobic biological systemsshould be limited to 40 C. It is possible to operate at55 C with the thermophilic aerobes but heat losses areso high that it is hard to maintain the true thermophilicrange and efficiency.

BIOLOGICAL TREATMENT SYSTEMS

The three major aerobic biological treatment systemsare oxidation ponds, trickling filters, and activatedsludge. All three systems have definite places in thefield of biological stabilization of industrial wastes. Noone system can be singled out as the best system for

industrial wastes. Failure to consider all of the factorsin a particular waste problem is poor engineering.The final selection of a particular treatment systemcan be made only after careful consideration of all fac-tors.

Oxidation ponds. Oxidation ponds are the simplestaerobic treatment systems to construct since they arenormally made by making a 2- to 3-foot depression inthe ground with a bulldozer, setting the influent and theeffluent lines and starting the wastes. The bacteria bringabout stabilization of the wastes and derive their oxygenfrom surface reaeration caused by wind over the largesurface area. As an excess of nitrogen and phosphorusbecomes available, algae take up these elements alongwith carbon dioxide released by the bacteria to producenew cells. The production of new algae releases oxygenfrom the carbon dioxide back into solution for theaerobic bacteria to utilize. The single chamber oxidationpond utilized in this country is normally loaded at amaximum of 65 pounds of BOD per acre per day. Itproduces an effluent having 10 to 25 ppm BOD. Theeffluent also contains a large quantity of algae andbacteria as these are not removed in the large singlepond. The factors affecting the design and operation ofthe single tank oxidation ponds have been set forth byothers (Oswald and Gotaas, 1955) and will not becovered here for lack of space.A Swedish oxidation pond system for sewage (Wenn-

strom, 1955) has set the pattern for the future design ofoxidation ponds. This system has four ponds in serieswith a definite cross-flow pattern. The influent devicesspread the wastes over one side of the almost squareponds and collect uniformly over the opposite side. Theflow through each pond is recirculated through thatpond 6 to 7 times before being discharged to the nextpond. The final pond is equal in size to the first threeponds and is used to cultivate crustaceans whichutilize the algae for food. In the summer months theeffluent from the system is free of algae and high incrustaceans. The crustaceans are a good fish food andaid in making the receiving body of water a goodfishing stream. The over-all pond loading is 90 poundsBOD per acre per day, almost 50 per cent higher thanconventional ponds in this country.

In areas where the terrain is relatively flat and landis inexpensive and easily available, oxidation ponds willbe more economical than any other type of biologicaltreatment. The operating expenses for oxidation pondsare very low with the recirculation pumps being theonly mechanical item. Care must be taken so that theorganic load is not so high that the demand for oxygenexceeds that available through surface reaeration. Algaewill supply oxygen in proportion to the protoplasmproduced during sunlight hours but will have an oxygendemand at night. Since the production of new algae hasnot stabilized the organic matter but has actuallyresulted in the creation of organic matter from inorganic

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end products, stabilization of the waste has not beencompleted. Failure to keep the system aerobic willresult in odors and nuisance complaints. The maximumorganic load on the oxidation ponds will be limited bythe rate of surface reaeration and will form the basis ofdesign.

Trickling filters. Trickling filters obtain their oxygen

by diffusion from the air as the liquid wastes splashand flow through a layer of stones, 6 to 10 feet deep.The aerobic microorganisms stabilize the organic matterwith the production of new cells, which cling to thesurface of the stones. As the system operates, additionalgrowth builds up on the stones, thereby cutting down on

the void space. The hydraulic velocity of flow increasesuntil the hydraulic velocity is great enough to shear thegrowth from the surface of the stones and to wash it outof the filter into the sedimentation unit.As the depth of growth on the stone increases, the

diffusion of nutrients and oxygen into the lower reachesnext to the stone surface becomes limited. The micro-organisms at the stone surface undergo endogenousmetabolism for the lack of nutrients and anaerobicmetabolism for the lack of oxygen. Anaerobic degrada-tion of the cellular components produces organic acidsas end products. The organic acids diffuse back throughthe layer of microorganisms and actually increase theorganic loading at the surface. The efficiency of sta-bilization is reduced in proportion to the extent ofanaerobic degradation taking place in the filter. As a

result, the efficiency of stabilization undergoes a cyclicvariation rather than producing a constant efficiency.The anaerobic degradation of the microorganisms at thestone surface reduces the holding surface and aids inshearing the growth from the stone.High rate trickling filters are used almost exclusively

in industrial waste treatment systems today. Thesefilters are loaded at hydraulic loadings of 20 to 40million gallons per acre per day and at organic loadingsup to 90 pounds BOD per 1000 cu ft filter volume. Theefficiency of stabilization is approximately 85 per cent.With strong wastes the hydraulic loading is maintainedby a high rate of recirculation of effluent. The majorproblem with very strong soluble wastes is rapid con-

version of the organic matter into protoplasm and theclogging of the void space with growth. Ponding of thefilter results and anaerobic conditions set in on theentire filter.

Sedimentation is an integral part of trickling filters.All of the organic matter removed by a filter is notoxidized. A part of the organic matter removed from theliquid phase is adsorbed to the filter slimes and is stillattached to those slimes when they are washed into thesedimentation tank. The slimes are removed from thesedimentation tank and stabilized by anaerobic diges-tion. Approximately 20 per cent of the stabilization ofthe waste is brought about through sedimentation of thefilter slimes. Because of their adsorptive characteristics

the trickling filters are best suited for colloidal wastesrather than soluble wastes. The contact time throughthe filter is so short that soluble wastes can be removedonly by several passes through the filter.As in the case of oxidation ponds, the rate of oxygen

transfer puts an upper limit on the quantity of organicmatter which can be stabilized by trickling filters.Efforts have been made to increase the oxygen transferby blowing air up through the filter. Physical chemicalrelationships show the fallacy of this idea. The concen-tration of oxygen in the air surrounding the filter stonesis very high compared to the oxygen in solution. Thetransfer of oxygen from the air into the liquid is de-pendent upon the partial pressure of oxygen in the airand the oxygen deficit in the liquid. Increased oxygentransfer can be brought about by increasing the partialpressure of oxygen. Blowing air through the filter willnot increase the partial pressure of oxygen and will notincrease the oxygen transfer. The natural flow of air in afilter will keep the partial pressure of oxygen in theair in the filter at normal atmospheric conditions. Thus,it appears that the limit of operation of trickling filtersis being reached.

Activated sludge. Activated sludge is rapidly becomingthe most popular method for stabilizing liquid organicindustrial wastes. The basic process depends upondiffused air at 6 to 8 psi to supply oxygen to the micro-organisms and to supply the necessary force to keep themicroorganisms, waste, and oxygen well mixed at alltimes. The microorganisms after stabilizing the organicwastes flocculate and settle, leaving a clear supernatantcontaining 10 to 15 ppm BOD. The settled micro-organisms are partially returned to the head end of theaeration tank as seed to maintain a high microorganismpopulation in the aeration tank and the remainder arewasted to anaerobic digestion for further stabilization.The primary problems with activated sludge systems

have been their inability to absorb shock organic ortoxic loads, their complex operation with higher operat-ing costs than trickling filters, the limit of oxygentransfer, and the low efficiency of oxygen utilization.Normal design of activated sludge systems are long,narrow aeration tanks in which the wastes and returnedsludge are introduced at one end and flow toward thefar end with a definite retention period. The diffused airis introduced along one side of the tank to produce aspiral flow pattern and to insure good mixing. A suddenshock organic load produces an immediate increase ingrowth response by increasing the food:microorganismratio over that for normal operation. The period of flowthrough the tank is not sufficient for the microorganismsto come back to stability and enter the sedimentationtank only partially stabilized. Poor flocculation andsedimentation occur with a loss of the solids out theeffluent and reduced seed for return. Toxic materialsare mixed with the entire mass of the microorganismsand produce an immediate killing of all microorgan-

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isms. On a trickling filter only the surface layer of micro-organisms in contact with the wastes are killed. Theremaining microorganisms are able to respond to theorganic waste after the toxic shock has passed in thefilter but an entirely new sludge mass must be producedin activated sludge.

Activated sludge with its compressed air system, itsbalance of return sludge, its variation in sludge syn-thesis, and its reaction to variations in flow is at best acomplex system requiring considerable mechanical andbiochemical knowledge and skill. Only those industrieswhich will employ a skilled operator should venture intoactivated sludge. The large number of mechanicaloperations makes activated sludge more expensive tooperate than trickling filter by a factor of three.Oxygen transfer again limits the total stabilization of

organic material. Normal diffused air equipment limitsthe rate of oxygen transfer in the field to between 30and 50 mg per L per hr. But the high demand for oxygenoccurs only at the head end of the aeration tank wherethe starved microorganisms are mixed with the food. Asthe sludge progresses through the tank the demand foroxygen decreases until more is supplied than is needed.Poor efficiency of oxygen utilization results.Many modifications of activated sludge systems have

been used to overcome the problems of oxygen transferand utilization. For the most part, these systems fallinto two classes, the complete treatment and the partialtreatment systems. Conventional activated sludgesystems, step aeration, Kraus process, and taperedaeration are loaded at 20 to 50 lb of BOD per 1000 cu ftaeration volume and give 90 to 95 per cent efficiency.The partial treatment systems consist of high rateactivated sludge and dispersed aeration. High rateactivated sludge is loaded at 90 pounds BOD per 1000cu ft aeration volume and gives 65 to 75 per centefficiency. Dispersed aeration is loaded at several hun-dred pounds BOD per 1000 cu ft aeration tank volumebut gives only 30 to 50 per cent stabilization. Activatedsludge systems are best suited for completely solubleorganic wastes but can be used for colloidal wastes. Theactivated sludge produced in the aeration system hasexcellent adsorptive characteristics. New York Citytakes advantage of these adsorptive characteristics tostabilize sewage which contains a high percentage ofcolloidal solids (Chasick, 1954). Biosorption is anothermodification of activated sludge which depends uponthe adsorption of colloidal organics for its operation. Itproduces an excellent effluent in completely colloidalwastes but falls short in soluble wastes since the contactperiod between wastes and sludge is only 30 minutes.

FUTURE DESIGNS

The future design of biological waste treatmentsystems will depend upon utilization of the fundamentalbiochemistry of the microorganisms which make up thesystem and its translation into practical design systems.

The basic considerations for the most efficient econom-ical biological treatment system for industrial wastesare as follows:

1. Uniform waste feeding.2. Proper pH adjustment.3. Proper nutrient adjustment.4. Uniform air utilization.5. Instantaneous waste mixing.6. Uniform organic level in the aeration system.7. A minimum of mechanic equipment.8. Simplicity of operation.9. Ability to absorb shock organic and hydraulic

loads.Needless to say, it is impossible to design a system thatcompletely solves all these problems. One effort toeliminate these problems and to design a new type ofbiological treatment system is given here.The basic unit is the conventional activated sludge

aeration tank. The tank is modified by running a longi-tudinal wooden baffle along the length of the tank todivide it into an aeration section and a sedimentationsection. Diffused air is supplied along the aeration sideof the tank wall and the wastes are fed uniformly alongthe long wall. The flow goes across the tank instead ofdown the tank. This permits instantaneous mixing of thewastes with the aeration section contents, uniform airdemand throughout the entire aeration tank and auniform organic content in the aeration section. Thesludge leaves the aeration tank by flowing under thebaffle and up into the sedimentation section. The massof the sludge prevents its rise to the surface and onlythe clear effluent is drawn off. The sludge mass increasesuntil its weight forces it back into the aeration sectionwhere the mixing forces disperse it with the aerationtank contents. In this way the sludge is returned to theaeration section without the benefit of mechanicalequipment. The system can be operated in the endoge-nous phase with the organic matter being completelyoxidized, or in the declining growth phase with somesludge being wasted. The choice of operation is prima-rily one of economics. Operation with sludge wastingrequires anaerobic digestion for stabilization of theexcess sludge but is more economical for large plantsthan complete aerobic oxidation.

If the system is operated with endogenous metab-olism, the nutrients added to the system must match thesmall quantity lost in the effluent. This is especiallyvaluable with nutritionally deficient wastes as only asmall quantity will be lost and required to be made upto hold the solids at equilibrium. If synthesis is used andexcess sludge produced, the nutrients must make up forthose used in synthesis. The exact quantity of nutrientscan be calculated from the structure of protoplasm.Since the systems are to be operated at endogenousphase or just adjacent to it in the final phase of thedeclining growth phase, the rate of oxygen utilized perunit mass of protoplasm is very small, 1 to 2 mg per L

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per hr per 1000 ppm suspended solids. To give maxi-mum stabilization 20,000 to 30,000 ppm suspendedsolids must be carried in the unit. The upper limitwill be determined by the physical characteristics ofthe floc and by the quantity of oxygen transferred. Itappears that this system will be limited more by thequantity of solids carried than by oxygen transfer.The heavy sludge mass carried in the system will tend

to buffer the wastes somewhat. The strong acid wasteswill have to be partially neutralized before entering theunit but strongly alkaline wastes will not have to bepreneutralized if there is sufficient organic matter in thewastes. The complete mixing of the wastes distributesthe hydroxide alkalinity uniformly over the entireaeration tank instantly where the carbon dioxide beinguniformly produced can neutralize it. The rate of carbondioxide produced actually limits how high the pH ofthe raw wastes can be. A recent pilot plant investigationwith this system showed that it was possible to have araw waste with pH 11.5 and 1000 mg per L glucose asorganics and to have complete stabilization withoutpreneutralization. Pilot plant studies have also shownthat the system can absorb radical fluctuations inorganic strength and hydraulic flow without upsettingthe system.

This biological treatment system has been namedHi-Lo activated sludge since it carries high solids andproduces a low BOD effluent regardless of the influentconcentration. Hi-Lo activated sludge has been pilotplanted for two industrial waste systems, with onesystem partially constructed and in operation. The costof this system is only one-third to one-fifth of the cost ofconventional systems and can be set up to operate itselfcompletely automatically. Hi-Lo activated sludge is notthe ultimate answer but merely the first step in aneffort to tie together the fundamental microbiology andbiochemistry with the economics of construction andoperation. Better aerobic biological waste treatmentsystems will be designed and built only when properconsideration is given to both the microorganisms andthe physical structures.

ACKNOWLEDGMENT

This study was supported in part by research grantRG-4395, National Institutes of Health, Public HealthService.

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