Aerobic biological treatment. Water treatment processes


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Aerobic biological treatment Thejnal part of a six-part series on water lrealment processes A -

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The traditiod goals of aerobic biological treatment are the oxidation of organic material (collectively measured as biochemical oxygen demand BOD]) and the oxidation of ammonium @VI&+). These reactions are brought about with oxygen as the fmal electron acceptor. Organic material is mineralized to H,O. CO,, “a+, and other constituenk, and &e is oxidized to nitrate ma-). Ea& reaction also results in the synthesis of new cell mass. One feature that makes biological treatment practical is the retention of cells, because this fosters accumulation of a large biomass to effect rapid and complete oxidation withim a relatively short liquid detention time. The use of aerobic biological treatment can be traced back to the late nineteenth century (1, 2), and by the

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Bruce E. Rittmann University of Illinois at Urbana-champaign Urbana, Ill. 61801

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128 Envimn. Sci. Technol.. Vol. 21, No. 2,1987

1930s it was a standard methcd of wastewater treatment. The most common and longest-standing methods of aerobic treatment are the activatedsludge and the tricklmg fiter processes. Although each performs the same oxidation reactions and accumulates similar microorganisms, they differ in the manner in which cells are retained.

In the activated-sludge process, microorganisms must accumulate into relatively large aggregates, called flocs. Because they are much larger than single bacterial cells, the flocs can settle out in a quiescent settler after they exit the aeration tank. After settling, most of the settled cell mass is returned to the aeration tank to allow build-up of activated sludge. In trickliig-fdter systems, the cell mass is retained directly in the filter; it is attached to fixed, solid surfaces. (Originally, rocks were used.) This attached cell mass is called a biofiim. Organic contaminant and removal, oxygen use, new cell mass growth, and b i o f h retention all occur in the trickliig fiter. The wastewater moves from the trickling filter to a settler to improve the quality of the effluents, but

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0013936W87/[email protected] 1987AmerkanChemical society

the settler is not needed for retention and return of cell mass.

Fundamental design Aerobic biological processes can be designed and operated so that oxidation of organic contaminants occurs alone or with N&+ oxidation. These operating options are possible because the bacteria responsible for the two varieties of oxidation are physiologically different. Table 1 lists some typical parameters of the microorganisms that carry out aerobic oxidation and the first (and usually rate-limiting) step of nitrification. The first and most critical difference is that the oxidizers are heterotrophs, whereas the nitrifiers are autotrophs. Heterotrophs use organic molecules as a source of cellular carbon to gain energy and electrons for synthesis of new cell mass. The prime characteristic of an autotroph is that it must reduce inorganic carbon from CO?, for example, to obtain carbon for new cell growth: nitrifying autotrophs also oxidize N&+ or NO2- to gain energy and electrons. The reduction of C 0 2 to the oxidation state of cellular carbon requires a large expenditure of energy and electrons by the autotroph. Thus, the yield ( Q of new cell material per unit of oxidized electron donor substrate (organic

contaminants and NH,+) is much lower for an autotroph than it is for a heterotroph. Because of this lower yield, autotrophs also have a much lower maximum specific growth rate, pmax. which means that under conditions favorable to both types of microorganisms, the heterotrophs will grow and accumulate much faster than the autotrophs can. Table 1 shows that the ratio of maximum specific growth rates is roughly 1.5/0.11 = 13.6at2OoC. Listed at the bottom of Table 1 are three process characteristics useful for practical design and operation of biological processes. The first is the limiting value of the minimum sludge age, [(8cmi")lim].This limiting value defines the sludge age, ll,, at which the biomass will wash out even under the most favorable conditions. It therefore represents a limitation on the loading rate to the microorganisms. For practical design, ll,, which is defined as the mean residence time of cell mass in the process, must be several times greater than [(8,m'")lim] (4). For an activated-sludge process that does not receive a significant amount of active cells, 8, is defined as

8, = ( X V ) W P + PQw) (1) in which X = concentration of biomass in the process, V = volume of the proc-

rABLE 1

Characteristics and typical kinetics for aerobic heterotrophs and Nitrosomonae bmmMtsr

Physiology rrophic type Electron donor Electron acceptor Carbon source Nitrogen source Kinetic paramete+ Maximum specific substrate utilization rate, k True yield, Y Maximum specific growth .r rate. ,~ Halt maximum rate concentration, K, Specific maintenance rate coewicient. b' Molecular dinusion coefficient.D Process parameters Limiting value of the minimum sludge age [(sCm'")lim] Minimum concentration to support steady-state biomass, S,,, Minimum substrate flux to support adeep biofilm.J,,

Oxldlzsr

Nllmaomonas

Heterotroph Organic material

Autotroph

0 2

02

Carbon in organic material NH,*

COP NH.'

20 mg BOD,lmg VSS,day 0.45 mg VSSlmg BODl

2.3 mg Nlmg VSS.day 0.33mg VSSlmg N

91day

0.761day

10 mg BODJL

1.0 mg NIL

0.2lday

0.ltlday

0.6 cm2/day

1.5 cm2/day

0.1 1 day

1.5 day

0.23mg BODl/L

0.17 mg NIL

0.1 mg

BOD,lcm2.day

NH,*

0.1 mg Nlcm2,day

.Temperature = 20 O C . OReferences 3-5. BOO, = Ullimate BOD. or maximum degradable organic material. as measured by oxygen

demand.

VSS = Volume of Suspended solids

ess, X' = concentration of biomass in the effluent, XW = concentration of biomass in the wasted sludge, p = effluent flow rate, and Q" = flow rate of wasted sludge. The design engineer has a measure of control over all parameters that define 8, and must ensure that 8, > > [(ll,"'")lim]. The second process parameter is the minimum concentration of organic material needed to support steady-state biomass, Smin,This is defined in terms of the basic kinetic parameters as K,b'/(Yk - b') (2) in which Ks = half-maximum rate concentration, Y = true yield, k = maximum specific rate of substrate use, and b' = overall, first-order biomass loss coefficient. S,,, defines a lower limit on process loading, because at substrate concentrations 0.3 + 0.22(DO) ( 5 ) substrate concentrations and growth in which U = specific COD loading, rates. During this period, the slowkg COD/kg VSS'day (VSS = volume growing filaments are unable to comof suspended solids), and DO = dis- pete because of their low maximum solved oxygen concentration (mg/L) in specific growth rate. A subsequent pethe aeration basin. Bulking by these riod of treatment provides very low fast-growing forms can be suppressed specific growth rates, which are inimiby U values lower than those produced cal to the fast-growing filaments because they decay rapidly. by Equation 5 .

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132 Environ. Sci. Technol.. Vol. 21. NO.2. 1987

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The strategy of intermittent treatment periods was applied successfully to control and reverse bulking in sequencing batch reactors (16). In this case, the fill phase was unaerated to allow buildup of a high concentration of soluble substrate. In continuous-flow units, intermittent treatment periods were used, and the influent and return sludge were sent to a small selector tank that contained high concentrations of substrate (21, 22). The criteria for selector operation are not well established, but high COD concentration ( 2 0 4 0 mg CODiL 1211) and high specific loading (15-25 kg COD/kg VSS.day) have been suggested as necessary (22). Alternative separation approaches. Another possible means to improve solids separation for all biological processes is to replace the settler with another device. Microstrainers or screens with slit sizes of no more than 30 pm could be useful substitutes for gravity clarification (23). Another method of interest is membrane separation, for example, by hollow-fiber membrane fiters. The economic aspects of the alternative techniques for solids separation have not been evaluated adequately. Biofilm kinetics. Over the past decade or so, major improvements have been made in describing the kinetics of biofilm processes. ?he first important finding was that mass transport mechanisms exert significant control over the overall rate of substrate use (24). Within the biofilm, substrate use and mass transport by molecular diffusion occur simultaneously to create a substrate concentration gradient in the biofilm and to expose the microorganisms in the film to lower substrate concentrations than those at the surface. In addition, external mass transport resistance to the solid is imwrtant in most

situations because it lowers the surface concentration from the concentration in the bulk liquid. A second improvement in biofilm kinetics was the definition of the mechanisms for controlling biofilm accumulation. For most reactors, the main accumulation mechanism is growth through substrate use (6). Biofilm is lost mainly through maintenance decay (decay of cells to provide energy) and through the removal of microorganisms from the solid to the liquid (6, 25). If the flux of substrate into the biofilm is just great enough to balance biofilm growth and loss, there is a steady-state biofilm (6).Otherwise, the biofilm will exhibit net growth or loss (7, 26). A third advancement for biofilm kinetics was the development of simple solutions to the differential equations that describe substrate use in the biofilm, mass transport inside and outside of the biofilm, and biomass accumulation. A simple solution gives the flux of substrate into the biofilm directly as a function of the bulk-liquid substrate concentration and some kinetic parameters: No further numerical solution of the differential equations is needed. Simple solutions are available for steady-state biofilms (6) and nonsteady-state biofilms (7); they can be used for manual calculation or they can be incorporated as rate terms in computer models. Although having simple

sources and sink kinetics of SMPs may lead to strategies that will minimize their presence in treatment effluents. The first advance involves defining source mechanisms. For some time it has been postulated that the rates of formation for different fractions of SMPs are related to different aspects of the cells’ metabolic rates (30, 31). Studies using various combinations of “‘C-labeled substrate and biomass demonstrate that some SMPs-the utilizationassociated products-are produced directly by original-substrate m e t a b lism (24). The rest-the biomass-associated products-derive from the decay and metabolism of existing biomass. The formation rate of utilization-associated products is proportional to the rate of substrate use, but the rate of formation of biomass-associated products is proportional to the amount of accumulated active biomass (29). A second advancement in understanding SMPs involves the fact that they are biodegradable. Gaudy and Blachly showed that most newly formed SMPs are biodegradable after culture acclimation (32). Schultz and Keinath report that some of the SMP that is left over after activated-sludge treatment is adsorbed to powdered activated carbon and can be desorbed and solutions for substrate flux makes for mineralized to C 0 2 (33). Although practical use of biofilm modeling, kimuch of the residual SMP in an effluent netic analysis is not as straightforward may be relatively refractory and exert for biofilm reactors as it is for sus- litlle BOD, the originally formed SMP pended-growth reactors. The use of seems to be largely biodegradable. substrate flux as a loading criterion (8) Work has been done to combine the is a good first step toward translating kinetics of formation of utilization-asthe results of sophisticated mathematisociated and biomass-associated prodcal modeling into routine practice. ucts with the kinetics of biodegradation More progress is needed, however, to for these materials (34).The key to the make design and analysis of biofilm model is the description of the SMP processes simple but still based directly biodegradation kinetics according to a on all the important principles. model that represents SMPs as multiMicrobial products. A final ad- ple-component material that has a range vance in the understanding of the per- of biodegradabilities (35).The overall formance of biological processes has model of SMP formation and biodegracome through investigations of the for- dation (34) offers the only successful mation of soluble microbial products explanation of the experimental obser(SMPs) by microorganisms. For a vation that a minimum in net SMP proproperly performing aerobic process, duction seems to occur for moderate effluent soluble organic carbon or COD sludge ages in activated-sludge treatusually can be attributed to the products ment. Although more experimental inof microbial metabolism, and the origi- formation and evaluation are needed, nal feed substrate can be quite low in modeling of SMP formation and degraconcentration. dation may offer a rational approach for Optimizing removal of the soluble improvement in the performance of bifraction of effluent organic material in- ological reactors for soluble materials volves minimizing the net formation of and may complement improvements reSMPs. The SMPs formed from microlated to solids separation. bial metabolism are made up of a wide Innovations range of molecule sizes and types, most of which are polymers of > loo0 moAreas of innovation in aerobic biolecular weight (27-29). Apparently, the logical treatment have included ideas SMPs are mostly cell synthesis compo- and methods that are too new to be in nents that escape the cells by diffusion common use. Four areas are of particuor during lysis. lar importance: fluidized-bed procImportant advances in defining the esses, anoxic systems, biodegradation Envifon. Sci. Tmhnol.. Vol. 21. NO.2. 1987 133

of hazardous chemicals, and treatment of drinking water. Fluidized-bed processes. The use of fluidized beds is a relatively recent development in biofilm technology. In this case, the reactor is composed of a bed of small solid particles with an attached biofilm. The particles are fluidized by the upward movement of the liquid. Although currently associated mainly with methanogenesis (36) and denitrification (37),this process is applicable to and offers advantages for aerobic treatment. The most obvious advantage of a fluidized-bed biofilm reactor is that it can have a very high specific surface area that is not prone to clogging because the small particles are fluidized. The high specific surface area allows accumulation of a high-volume density of biofilm, which usually has low resistance to external mass transport. This makes it possible to build compact reactors. Detention times measured in minutes are possible, making it possible to process loads many times greater than those treated by conventional aerobic processes. For nitrification, fluidized beds may offer even greater advantage kcause of their ability to retain the slow-growing nitrifiers. A less obvious advantage is that very low effluent concentrations can be achieved if good biofilm mixing occurs and surface loadings are not too high (8, 38). Thus, fluidized-bed treatment processes can be especially valuable for small reactors or for low effluent concentrations. Before the potential of fluidized-bed biofilm reactors can be realized for aerobic systems, at least two issues must be resolved. First, supplying oxygen at rates sufficient to meet the demands of very high volumetric loadings will require oxygen transfer rates in excess of those currently possible. One approach is to use an oxygenation device in a recycling line. A second approach is to apply diffused aeration within the column, creating a three-phase reactor. Three-phase reactors may offer additional advantages if the direct transfer of oxygen from bubbles to biofilm (as appears to occur in some fixed-bed processes [39, 40]), is greater than that in nonbiofilm reactors. The second issue that requires anention involves controlling the accumulation and distribution of biomass within the fluidized column. When the surface loading is high, biofilm hold-up can kcome too great. This makes bed expansion difficult to control, and particles can be lost to the effluent or to the recycling line. For low-loading reactors, optimal distribution of biomass is essential for achieving low effluent concentrations (38). Although valuable 134 Enviran. Sci. Technol., Vol. 21. NO.2. 1987

Variations in aerobic, suspended-growth biological processes The commonly employed suspended-growth processes described here were developed to overcome difficulties of conventional activated-sludge processes and to provide improved treatment in special cases. Conventional activated sludge. Aeration occurs in long, narrow tanks and results in quasi-plug flow. Compressed air is used in the aeration system. Sludge moves to the Settler after processing in the aeration tank. Although most of the settled sludge is then returned to t h e aeration tank, some is removed from the system to control the sludge age. Tapered aeration. This system is similar to conventional activated-sludge processes, except that t h e coc!pressed-air supply is greatest near the inlet of the aeration tank and lowest at the outlet. The goal is to counteract oxygen deficiency near the inlet and to eliminate oxygen excess near the outlet. Step aeration. This system is similar to conventional activated-sludge treatment. except that the wastewater is fed to the aeration tank at several locations, or steps, along the flow path. ihis technique evens out BOD and counteracts oxygen deficiency near the inlet. It also provides extra sludge accumulation in the aeration tank. which can increase process capacity. Completely mixed. The aeration tank has a square or circular plan view; aerators mix the contents throughout the tank. The main advantages have to do with equalized loading and oxygen demands. This is the simplest system to model. Contact process. The wastewater flows into a Small contact tank (30-90-min detention), where colloidal organic contaminants are captured in flocs and soluble contaminzits are oxidized. The senled sludge is sent to a re-aeration tank (3-6-h detention of sludge) before it is returned to the cr, :!act tank. Reaeration provides oxidation of colloidal material and endogenous decay of biomass. It also reduces the tankvolume needed for treatment, because both reactions are possible when biological solids are highly concentrated. Oxidation ditch. The aeration tank contains continuous channels in which the mixed liquor is aerated and circulated by rotating brushes. Usually, oxidation ditches are operated in the extended-aeration mode and frequently are not preceded by primary sedimentation. The large volume, long sludge age, and complete mixing normally associated with extended aeration offer process stability and simplicity. Pure oxygen. The aeration tanks are coverF-(,and pure oxygen is provided instead of air. Increased O2transfer rates allr iigher dissolved oxygen concentrations, higher concentrations of mixeL .duor suspended solids, and smaller reactor volumes. This method requires an O2generation system. Sequenced batch reactors. One tank is used for filling with influent, aeration, settling, decanting effluent, and retaining sludge from cycle to cycle. The advantages are that a separate settler is not required and reaction conditions can be controlled for each phase. Extended aeration. Almost all of the above processes can be operated in the extended-aeration mode by lengthening the sludge age to more than 15 days. Sludge production is minimized, and nitrification usually occurs. Aerated or stabilization lagoons. Because this method does not usually require sludge recycling or settlers, liquid detention times are equal to the sludge age. Surface aerators or natural oxygen exchange provide 02. Lagoons are simple to operate, but require large amounts of land. work on biofilm accumulation and distribution has already been done (4143), considerable research is needed to define the accumulation and loss rates of biofilm and the mechanisms that control media mixing and distribution. Anoxic and anaerobic zones. The second innovation for aerobic treatment is the inclusion of anoxic and anaerobic zones within a basically aerobic system. In a properly designed system, the anoxic zones allow denitrification to occur. Establishment of a preliminary anaerobic zone has been used to enhance phosphorus removal. Denitrification ' ; a commonly used method for removing nitrate from wastewater; the nitrate is strivped to the

atmosphere as N2 gas. Traditional wastewater treatment techniques use denitrification as an additional process that follows aerobic treatment for oxidation of organic material and NH4+ (2). Because heterotrophic bacteria perform denitrification using NO3- or NO2- instead of O2 as the final electron acceptor, an exogenous source of BOD must be added to the reaction when traditional techniques are used. One innovation that offers important economic advantages is carrying out denitrification and aerobic reactions in the same system. This is called a onesludge system, because the microorganisms for all reactions are contained in one culture. The first advantage of a

Variations in aerobic biofilm processes Biofilm processes have evolved to take advantage of new construction materials and to provide improved treatment efficiency (2,9, 70). Conventional trickling filter. Wastewater is distributed over a bed of rocks. where it comes into contact with the biofilm. O2is supplied by air that flows through the medium, and effluent is sent to a settler for clarification. The effluent often is recycled to increase the hydraulic loading, which improves the distribution of biofiim and helps overcome oxygen limitation. Biological tower. The biological tower is similar to the trickling filter, except that plastic media can be stacked to heights of 12m. The use of lightweight plastic media allows construction of tall towers (thus conserving land) with high specific surface areas (allowing higher volumetric loading than possible in conventional trickling filters). Rotating biological contactor. Lightweight plastic media that contain the biofilm rotate into and out of the liquid to provide aeration. The effluent is sent to a settler for clarification. Increased volumetric loadings can reduce the total tank volumes from those required by conventional trickling filters. Activated biofilter. A biofilm first stage is followed by an activated-sludge second stage and a settler. Sludge is recycled to the biofilm stage and to the activated-sludge tank. This variation combines biofilm and suspended-growth chaiacteristics. Biological aerated flRer. Wastewater is filtered downward through a fully submerged bed of small rocks. which help to form the biofilm. and air is forced into the bed. No settler is used.. but oeriodic backwashina is reauired. This is a . compact treatment system. Fluidized-bad filter. A bed of small Darticles is fluidized bv the uDward flow of water. Very high specific surface areas can be achieved wittiout iniroducing the problem of clogging. Fluidized beds are sometimes called expanded beds. I

one-sludge system is that the influent organic material can be used as the electron acceptor for denitrification, thus, no exogenous source is required. Second, the removal of the influent's BOD by denitrification reduces the oxygen requirement, because NO3- and NO2- are used instead of 02.The economic advantage of reducing . oxygen requirement can make deni.:ification an attractive option, even when nitrogen removal is not mandated, as long as nitrification is already a treatment requirement. The most widely used means of providing denitrification along with aerobic reactions in a one-sludge system is to maintain separate anoxic and aerobic compartments. First, the wastewater and activated sludge rex1 in an anoxic compartment in which denitrification occurs; the organic material is oxidized in proportion to the NO3- reduced to N2. but N&+ is not removed. Then, the liqiiid passes to an aerobic zone in which the NH4+ is nitrified to NO3-, and the remaining organic contaminants are oxidized aerobically. Because nitrification must occur, sludge age usually is controlled by the nitrifiers. Such compartmentalized treatment has been demonstrated with multiple tanks in series (44), with biofilm reactors in series (4.9, with multiple-channe1 oxidation ditches (46). and with sequencing batch reactors (SBRs) (47). One-sludge systems work because NO3- is recycled from the aerobic stage to the anoxic stage. With multiple tanks or biofilm reacfa

tors in series, a flow of mixed liquor or effluent is recycled continuously from the aerobic reactor to the anoxic reactor to return NO;. Multiple-channel oxidation ditches use the normal recirculation of mixed liquor through continuous channels to transfer NO3- from aerobic to anoxic sections. With an SBR, recycling occurs by alternating aeration and nonaeration during the filling and reaction phases and by retaining a significant fraction of wastewater volume in the tank between cycles. Batchelor modeled one-sludge systems with alternating anoxic and aerobic compartments and discussed efficient operation strategies (48, 49). Recent work in single-channel oxidation ditches demonstrates that having separate compartments for aerobic and anoxic reactions is not 1.-quired (50). If the dissolved-oxygen concentration is suitably poised and if sludge flocs are well formed, anoxic reactions occur in the interior and aerobic reactions occur near the exterior of the flocs. Thus, both reactions occur in microzones within the floc, even though the the liquid contents of the reactor are aerobic everywhere. For the wastewaters and conditions tested, dissolved-oxygen concentrations below 0.5 mglL are are enough to allow denitrification with nitrification in the single-channel oxidation ditches (50). Modeling work suggests that the same phenomenon-nitrification in the outer portion and denitrification in deeper portions-is possible in biofilm reactors (51).

The inclusion of an anaerobic first stage of treatment before one-sludge denitrification is important to enhanced phosphorus removal by activated sludge. Some strains of bacteria incorporate twice the normal amount of phosphorus if the sludge is exposed to strictly anaerobic conditions and high concentrations of organic material (52). For continuous-flow systems, the influent wastewater and the return sludge are mixed in a first stage, from which 01,NO;, and NO2- are excluded (53). Then, the wastewater and sludge flow to an anoxic tank, which receives recycled NO3- from an aerobic tank downstream. The same phenomena are possible in an SBR when the fill stage omits O2 and when NO3- is consumed rapidly while the organic material is still plentiful (52). Hazardous-chemical removal. A third area of innovation is in the use of aerobic biological processes to detoxify hazardous organic compounds. In one sense, this is an old subject, because biological processes have been used for many decades to treat industrial wastewaters. On the other hand, several new methods are being added to the tools available for aerobic biological detoxification. Those of most interest are the use of activated carbon in biological reactors and the secondary utilization of organic contaminants. Because activated carbon can adsorb large amounts of hydrophobic organic compounds, it can enhance the performance of biological treatment processes in two situations. First, adsorption of an organic compound is useful when that compound inhibits the growth of microorganisms. As the adsorbed molecules are held within the micropores of the carbon, they are sequestered from the microorganisms, which are too large to penetrate the micropores. Biodegradation can proceed for other materials and for the inhibitor if the microbial culture acclimates to it. Second, when a molecule is adsorbed, it is retained in the biological system. In some instances-for example, when there is irregular loading of anthropogenic compounds-retention allows time for culture acclimation and ultimate biodegradation. Activated carbon is applied mainly in two ways. The first, which is suitable for suspended-growth reactors, is to add powdered activated carbon (PAC) to the influent of an activated-sludge system (33, 54). The PAC, which builds up in the activated sludge in a manner similar to buildup of the activated sludge itself, sequesters and retains organic molecules, as described above. The addition of PAC also seems to improve sludge settling. The second method uses granular acEnviron. Sci. Technol., Vol. 21. NO. 2.1987 135

tivated carbon (GAC) as the attachment medium in a fluidized-bed biofilm process. Although previously found most often in anaerobic fluidized beds (59, aerobic processes also benefit from GAC as an attachment medium because of its molecule retention and sequestering properties, its light weight for easy fluidization ( 2 9 , and its superior surface characteristics for biofilm accumulation (56). Another useful way to remove organic contaminants is called secondary utilization. It is particularly important when biodegradation of compounds in low concentrations is desirable. Secondary utilization is the biodegradation of a compound, called a secondary substrate, by microorganisms that gain their energy for growth and maintenance from another energy source, the primary substrate (57). The primary substrate can be a single compound or many used simultaneously. Namkung et al. demonstrated the best way to model a biological process for secondary utilization (58). They point out that the amount of accumulated biomass is determined by primary-substrate use; secondary-substrate use is determined by the amount of accumulated biomass and the intrinsic biodegradation kinetics of each secondary substrate. One design and analysis technique for aerobic systems involving trace amounts of contaminants indicates that high removal efficiencies for individual secondary substrates are possible, even when a compound's intrinsic biodegradation kinetics are relatively slow (59). Drinking-water treatment. The last area of innovation is in the use of aerobic biological processes to produce potable water. Although biological treatment is often used in Europe (3),in the United States it must be considered a major innovation in drinking-watertreatment practice. Aerobic biofilm processes can remove NH4+, Fe'+, Mn2+, and organic compounds. All of these materials are found routinely, and their presence results in biological instability, which in turn causes troublesome bacterial growth during physicochemical treatment processes and in distribution systems (3). In addition, aerobic biological treatment should be able to remove certain organic pollutants and precursors to the formation of trihalomethanes. These descriptions of important improvements and innovations illustrate the potential offered by aerobic biological treatment and highlight the value of advancements in our understanding of the mechanisms that control the treatment abilities and kinetics of biological processes. Research that expands our understanding of the fundamental 136 Environ. Sci. Technol., Vol. 21. NO. 2. 1987

mechanisms and applies that understanding to practice will go a long way toward improving traditional processes and developing new systems.

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