Aerobic microbial transformations of resuspended sediments in combined sewers — A conceptual model

~

Pergamon

Wat. Sci. Tech. Vol. 37, No. I, pp. 69-76,1998. iC) 1998 IAWQ.Published

PIT: S0273-1223(97)OO757-9

by ElsevierScienceUd Printed in Great Britain.

0273-1223198 $19.00 + 0.00

AEROBIC MICROBIAL TRANSFORMATIONS OF RESUSPENDED SEDIMENTS IN COMBINED SEWERS - A CONCEPTUAL MODEL Jes Vollertsen and Thorkild Hvitved-Jacobsen Environmental Engineering Laboratory, Aalborg University, Sohngaardsholmsvey 57, DK·9000 Aalborg, Denmark

ABSTRACT A methodology for characterisation and a concept for modelling of aerobic microbial transformations of resuspended sewer sediments based on long term measurements of oxygen utilisation rates (OUR's) are presented. The OUR measurements were evaluated applying a conceptual model for aerobic microbial transformations of sewer solids based on methodologies originating from description of activated sludge processes. Validation showed that yield coefficient and maximum growth rate could be considered constant when readily biodegradable substrate was added to the resuspended sewer solids during the OUR experiment. Maintenance energy requirement of the biomass was argued to be a better concept for modelling microbial transformations compared with either considering decay of biomass into hydrolysible substrate or including endogenous decay of biomass. OUR measurements are recommended as a valuable methodology for characterisation of suspended sewer sediments in terms of COnfractions and related biotransformations. @ 1998 IAWQ. Published by Elsevier Science Ltd

KEYWORDS Oxygen utilisation rate (OUR), combined sewer sediments, resuspension, biodegradability, hydrolysis, decay, maintenance energy requirement, endogenous respiration, modelling. INTRODUCTION The objective of this study was to establish a methodology for characterisation of the biodegradability of organic matter in resuspended sewer sediments based on experimental procedures and the use of a conceptual model. Aerobic biodegradability of sewer solids is of interest in relation to effects on receiving waters during combined sewer overflows (CSO's) as well as to their impact on wastewater treatment processes. Particulate organic matter in sewer sediments can not be utilised directly by micro-organisms but must first undergo hydrolysis into readily biodegradable substrate (Ss). Hydrolysis is conducted by exoenzymes produced by active cells. Hydrolysis products are utilised by heterotrophic biomass under consumption of an electron acceptor, I.e, dissolved oxygen (DO) under aerobe conditions. By measurement of the oxygen utilisation rate (OUR), and calibration of a model including biomass growth and substrate formation. it is possible to estimate the amount and quality of the hydrolysible substrate (Bjerre et al., 1997). Model constants were determined by addition of biodegradable substrate to the suspended sewer sediments. MATERIALS AND METHODS The equipment used for continuous OUR measurements consists of two identical systems operated at 20°C, Figure I. Sewer sediments were suspended in tap water and aerated to about oxygen saturation. The amount of sediments in suspension was chosen to keep the OUR between 2 and 20 g 02 m') h", For determination of 69

70

J. VOLLERT5EN and THORKILD HVITVED·JACOB5EN

O UR , the su spension wa s pumped fro m the vesse l into a closed reac to r where the DO co nce ntra tio n was measured. When the DO con centration dropped below 5.4 g O 2 m' a new. sa tu rated sa mple was pu mped into a reactor and the old sam ple returned to the vess el. INGOLD oxyge n se nso rs wit h 12 mm di am ete r Teflon membranes were used for DO measure ment s. T he OUR was ca lculated by co nsi de ring line ar reg ression o f the measured DO concentration s. The sediments were taken from a 1000 mm diameter concrete pip e in a combined sewer system of the city of Aalborg. During dry weather per iods the flow is ve ry low and the pipe is normally filled with 10-1 5 ern of sedime nts . COD, TS, and VS wer e determined acco rding to St and ard Method s ( 1995) . OXYJ:IOU

.' t lrrer

O X Y~ f' n

S t rr rvr

m e t vr

-~

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)

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:.

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~~

I

-J

:0:

:i:

J ~~ ~ ~ ~ ~ ~ = =~ ~ = = = = = ~ = = = = = ~ = = = = = = = ~

i I

: J

Figure I: Diagram o f the two parall el sys tems for co ntinuo us O UR mea su rem ent s. THE MODELLING APPROA CH T he model desc ribing the aerob ic mic robia l transformati on s of the organic mat ter in sewer so lids origi na tes in principle from wastewater treatment as pre sented in the Activa ted Sludge Mod el No . I (He nze et aI., 1987). The main differences - as discussed later in this paper - are that typically 2-3 fractions of hydrolysible substrate were introduced (Bjerre et aI., 1997) and that the concept o f maintenance ene rgy requirement, i.e. the respiratory energy necessary to maintain the biomass in an ac tive state, wer e intro duced instead of decay of biom ass. T able I. This con cept is si mi lar 10 what is reco mmended for microbial was tewa ter tran sformation s in gr av ity se wers, (Hv itved- Jacobse n et al., 1998). Ta ble I: Proc e ss kinetics and stoichiometry for biotransform at ion of su spended sewe r sul ids .

OC:::::::::::~mponent Ss Process ' ::-:----. Aerobic growth ------I

X.I"I

XS1

Proc ess rare

X S .1

Ss

JJH - - - X H K , + S,

Y" Maintenance energy requirement Hydrolysis. fast Hydrolysis. med ium Hydrolysis . slow

-I

-I

Cf IllX

R

,I

-I ·1

• if 5, is insufficient. the remaining CO D for the maintenance energy requ irement

t:

taken from the biumavs.

Biomass Characteristics, Maximum Growth Rate and Yield Coefficient Maximum growth rate, J.1H, and yie ld coefficient, Y H, arc ma in biomass character istics to he co nsid e red to evaluate the ass umption of constant composition o f the biomass durin g the O UR ex pe riment. Th e evaluation was done by add itio n of Ss to the reactors. Comparis on betw een heel' and ace tate showed that J.111 was

71

Aerobic microbial transformations of resuspended sediments

approximately the same for the two substrates while YH was higher using beer when all the organic matter was considered being readily biodegradable. The initial respiration rate was higher when using acetate. Because beer consists of a range of readily biodegradable and readily hydrolysible organic species, beer was chosen as substrate for these experiments. When calculating the YH it was assumed that all COD was readily biodegradable.

0.40 .---

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-

-

-

-

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0.35

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1

Experiment 1. 3.1.96 0.00 l-~~~-....!:;::=======1 12 18 24 30 36 42 o 6 Tine. h

1- -

1

Experiment 2. 3.1.96 0.00 l-~_~-...!:;::======:::::!..l 12 18 24 30 36 42 o 6 Time. h

Figure 2: Addition of readily biodegradable substrate (beer) in the two parallel OUR experiments, Figure I, showing a corresponding biomass growth. The area between the curve and the dashed line is equivalent with the amount of readily biodegradable substrate removed by respiration parallel with the growth process. This area is referred to as au. Growth rates were calculated from experiments under substrate unlimited growth conditions as shown in Figure 2 using Equation I. This equation was obtained from the process kinetics and stoichiometry presented in Table I as shown by Kappeler and Gujer (1992). (I)

Table 2: Variation in IlH and YH for sewer sediments related biomass based on OUR measurements. Experiment number I

I 2 I 2 I

Start date

03.01.1996

09.01.1996 17.01.1996

2 I

2

24.01.1996

TimeofS s addition, h

3.05 24.00 24.00 2.98 23.80 22.81 46.17 22.64 46.99

iBiomass characteristics I I I I I I I I I I

!!Hd·

2.0 2.1 2.3 4.5 5.9 4.4 4.3 7.4 6.0

1

YH

0.73 0.71 0.64 0.77 0.67 0.74 0.72 0.65 0.63

Sewer sediment characteristics TS,% VS,% COD, mg COD/g TS

76.1

1.0

32.1

45.1

3.86

106

15.5

15.9

285

7.8

32.7

586

YH is defined as the increase in biomass COD per unit COD of totally consumed Ss. Assuming that all Ss added is utilised by the biomass for growth and that biomass is the only product, the produced biomass is the amount of Ss added minus the amount of oxygen removed by respiration (Ol,'), Equation 2, Figure 2:

(2) The results of these experiments and the characteristics of the investigated sediments are shown in Table 2. The results indicate that IlH is constant during an OUR measurement and that YH can be considered constant although it seems to be slightly decreasing. The assumption of an unchanged composition - and activity - of the sewer solids related biomass during several days of growth is therefore acceptable.

J. VOLLERTSEN and THORKILD HVITVED-JACOBSEN

72

Biomass Decay versus Maintenance Energy Requirement In the Activated Sludge Model No. 1 (Henze et al., 1987), decay is assumed to result in the transformation of biomass into hydrolysible substrate. Kappeler and Gujer (1992) assumed decay through endogenous respiration. Both concepts involve a decay rate proportional to the biomass. For heterotrophic biomass under aerobic conditions and with endogenous respiration, a decay rate constant of 5% of JlH was suggested by Kappeler and Gujer (1992). In the Activated Sludge Models 1 and 2 (Henze et al., 1987; Henze et al., 1995), a decay rate constant of 7.5-10% of JlH were suggested. Bjerre et a1. (1995) found an endogenous decay rate constant in wastewater to be 15% of JlH. As shown later in this paper decay rate constants and endogenous decay rate constants in sewer sediments using the above mentioned concepts were found to be 60% and 40% of JlH, respectively. Such decay rates for bacteria are far above what is realistic (Kurland and Mikkola, 1993). As discussed by Tempest and Neijssel (1984) it should be more correct to assume a fixed substrate requirement for growth and a constant maintenance energy requirement in form of substrate utilisation without biomass growth for the micro-organisms. This will be done in this study. Tempest and Neijssel (1984) found that for a glycerol limited chemostat culture of K. Aerogenes growing at low K+ concentrations, the maintenance energy requirement rate, qm, was 2.1 mmol 02 h' (g dry weight cells)" that approximately equals 1.1 g 02 d' (g COD biomass)" . Furthermore, they observed, that qm could be significantly higher if there were other substrate limitations (ammonia, sulphate, phosphate, K+) and that qm not necessarily was constant over the growth range. In a review upon energetics of bacterial growth by Russel and Cook (1995), it was made clear that maintenance energy requirement is not constant over the growth range. Nevertheless, qrn will be considered constant in this approach . Maintenance Energy Requirement, Decay and Endogenous decay Decay rates and maintenance energy requirement rates can be calculated based on a model similar to what is shown in Table I when the biomass concentration can be determined at two different periodes. When the growth is not substrate limited, the biomass concentration can be determined as follows, Table 1: OUR(t}

= I~YII /lIlXs(t)+q..xs(t)

(3)

H

In this model JlH is found from Equation I. In the models presented by Henze et al. (1987) and by Kappeler and Gujer (1992), JlH in Equation I is replaced by JlH - bH (Kappeler and Gujer, 1992). From Table I it is seen that the DO removed by respiration in the time interval tl to t2 is:

1- Y !!.So=-H

YII

1" /lH s, "

K s +ss

()d Xstt+

1" q",Xstt ( )d "

(4)

and that biomass growth is determined by

(5)

Solving Equations 4 and 5 with respect to the maintenance energy term gives

1"'. q",X s(t)dt =ss; - 1-Y YH (X s(t

2) -

X s(t l ) )

(6)

H

The change in biomass from tl to t2 is assumed linear in time. Model computations show that this is a good assumption when no readily biodegradable substrate is present:

Aerobic microbial transfonnations of resuspended sediments

73

(7) Substituting Equat ion 7 into Equation 6, Equation 6 can easily be solved with respect to qm. Using the parallel experiments presented in Table 2, the biomass concentrations can be calculated using Equation 3; XO(tl) from experiment 1 and X O(t2) from experiment 2. Solving Equation 3 with Equation 6 and 7 leads by iteration to maintenance energy rate constants which seems to be a constant fraction of ~H. This fraction was 118% of ~H with a 95% confidence interval of 21%. When the same type of calculations were carried out with the model presented by Henze et al. (1987), the decay rate constants was 60% of ~H with a 95% confidence interval of 3%. When calculations were carried out based on the endogenous rate concept of Kappeler and Gujer (1992), a corresponding level of rate constant was 43% of ~H with a 95% confidence interval of 2%. The finding that the decay rate was a constant multiplied with ~H is also reported by Kurland and Mikkola (1993) who find that death rates of natural isolates correlates with the growth rates. If the 3 presented models were used with the corresponding calculated rate constants to simulate the OUR of the same experiment. unreasonable simulations were obtained. This leads to the conclusion that none of the model approaches are capable of simulating the biomass concentration satisfactorily. This could partly be due to growth related product formation other than cell biomass e.g. as it occurs in pure culture as well as in sewer biofilms (Nielsen et al.,1997). Another explanation might be that the maintenance energy requirement is not constant over the growth range as e.g. discussed by Tempest and Neijssel (1984) and Russel and Cook (1995) . Decay of biomass, endogenous respiration and maintenance energy requirement are all processes expected to take place during microbial growth . As requirement of maintenance energy is believed to be the dominating process (Russel and Cook, 1995). the other processes are neglected in this study. The maintenance energy requirement per unit of biomass is assumed constant. The experimental procedures used in this study does not allow a determination of the variability of the maintenance energy requirement. Growth related product formation is omitted because it would increase the model complexity significantly and the rate of product formation can not easily be determined. Hydrolysis Different kinetics have been used to describe hydrolysis. Sollfrank (1988) and Kappeler and Gujer (1992). used a first order rate expression in the concentration of hydrolysible substrate. Xs. When the growth is substrate limited, this implies that an increase in the concentration of Xs results in an increase in the production of readily biodegradable matter and hereby in an increase in growth and growth related oxygen consumption. This hypothesis was tested by adding different concentrations of X s in form of protein (Bovine Serum Albumin (BSA» and cellubiose to suspended sewer solids . The initial OUR was seen not to depend on the concentrations added, Figure 3 and Figure 4. Neither did this rate expression lead to a satisfactory simulation result of the measurements. If there is an unlimited amount of Xs• the parameter governing hydrolysis must be the enzymatic activity which can be substituted by X B as a first approximation. If there is an unlimited enzymatic activity (i.e. a high concentration of Xo) the governing parameter must be the number of sites available for enzymes to react with; i.e. hydrolysis is proportional with Xs. Therefore, a rate expression is needed which for Xs approaching infinity at constant X o equals klXB and for X B approaching infinity at constant Xs equals k2XS' As this is achieved with the rate expression used in the Activated Sludge Model (Henze et al., 1987), this expression will be used in this approach, see Table 1. More than one fraction of hydrolysible substrate must be introduced in modelling because a single fraction gives unsatisfactory simulation results of the OUR measurements. This has also been observed for Wastewater by Bjerre et al. (1997) and for activated sludge by Solifrank (1988). The more fractions that are

74

J. VOLL ERTSEN and T HORKILD HVITV ED -JACOBSEN

introduced the better the simulation result inev itabl y will be , independent of the valid ity of the model (La rsen, 1996). For this rea son the lowest number o f fracti on s giv ing a s atisfac tory simul ati on result is c hos e n. This is fo r sus pended se we r sedi me nts normall y se e n to be 3 fractions w ith different hydrol ys is rate s wh en simulating experiments over 60- 200 hou rs.

0.20 0. 16

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5

0.04 0.00

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-

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aocuon

12

:16

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0.00 • 12

24

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48

60

72

o

Figure 3: Add ition of 7 .5 mg ce llubiose/g TS to sewer sed ime nt at time 8 h. M odel simulation is sho wn with and without subst rate addition.

24

48

60

72

T,",o. h

Tim • h

Figure 4 : Addition of 15.2 mg cc llubiosc/ g TS to sewer sediment at time 8 hand 9.2 mglTS celluhiosc at 47 h. Simulat ion is wi th and with out subs trate add ition .

In order to evaluate the process rat e des cription of hydrolysis, cellubiosc and protein in form o f BSA we re added to sediments in known q uantities, Figure 3 and Figure 4 . The mod el used (Ta ble I ) was able to s im ulate e xperimental results if the s im ulated amount of hydrolysible s ubstrate was only a s ma ll fract ion of what was actually added , Figure 3 and Figure 4 . The yield coe ffici e nt was ca lculated accordi ng to Eq ua tio n 2 but with X, inste ad of Ss. In the experiments shown in Figure 3 and Figure 4 , the o bserved yield co e fficie nts were all determined to be 0.97 . and generally the observed yield co effic ie nts we re in the range of 0.78-0 .97. Two experiments were carried out during 270 hours were the O UR was continuously measured and BSA was added 4 times. The same sediment was used in both experiments but in the firs t study the sediment was aerated for 3 day s before the first addition of BSA took pla ce. In the othe r study the sediment was kept cool and anaerobic for the same period . For the preaerated sedi me nts . yie ld coefficie nts we re !I.88. D.X:; . !I.XX and 0.90 and for the sto red sediment the yie ld coefficie nts were cor res po nd ing ly 0 .1)2. !I.XI. O.XI) and lU X. During the se long term experiments. a build up of floes in the reactors we re o bserved vis ually . S imilar o bse rvatio ns we re made using cellubiose . Thi s ind ica tes that some o f the mat e rial added was not avai lable s ubstrate for the biomass or the s ubs trate wa s tran sformed into non or ver y s lo w ly bio de gr adable prod ucts.

OxidatIon of reduced, lnorganlc substances Sulfide and iro n have heen measured in preliminar y experiments. It was found that oxidation of sulfide and Fct ll) can only account for a small fraction of the measured OUR . However. an initial decrease in O UR often seen during the first hour or so can he modelled as a small fracti on of S, o r as a possihle o xid atio n of redu ced inorganic substances, Figure 5 . EXA M PLE OF SEDIMENT CH AR ACTERISATI ON Se we r sediment characteri sa tion was partl y don e by model calibrati on using c xpcri mcntul results . T he model pre sented in T abl e I was c alibrated using the OUR me asurements and va ry ing va lues o f XII. Ss. X SII • kllll and K xlI. The least sq uare method was used to find an o ptim um result . T he model paramete rs Y II , 11 11 . Ks und q., were kept constant : Y11=0.55. 1111=5 d 1. Ks= I gC OD/III ' and q ll'= I d I . An e xample is shown in Fig ure 5 . T he resulting fra ctions of hydrolysible matter and biomass arc shown in Ta ble ) .

Aerobic microbial transformations of resuspended sediments

75

Table 3: Initial COD fractions based on results shown in Figure S. Sediment characteristics are 24 g TS/I and 1.4% VS.

0.20 0.16

Z

...in

co 0.12

"'" ~

Organic fractions

0.08

ci.

a

1-

o.~

-

Measu rement . 17.7.95 1 ModeI ..mulatoOn

0.00 0

12

20

36

06

60

72

a.

Tme.h

Figure 5: OUR measurement and corresponding simulation for suspended sewer sediments using the model concept shown in Table 1.

Soluble, mgCOD/ gTS

XS 1 (fast) XS2 (medium) XSJ (slow and inert)

Hydrolysis characteristics K x• k•• d" COD/COD

0.84

XB

s,

Particulate, : mgCOD/ gTS

0.15 2.1 7.3

5.1 0.94

0.51 0.17

10.5

0.34

0.04

The amount of hydrolysible fraction 3 can not be determined by this method. This was partly because organic matter in this fraction was still in excess when the experiments terminated after 100 hours, and partly because fractions with small hydrolysis rate constants - and even inert material - supposedly were present. Fraction 3 was therefore calculated based on a mass balance using a conversion factor 1.4 gCOD/gVS. To determine very low rate constants it is necessary to measure the OUR over a very long time. DISCUSSION The model concept presented simulate the DO consumption and related biotransformation of resuspended sewer sediments well and it was possible to confirm the validity of certain model aspects . Other model aspects need further investigation before the model can be applied to resuspended sewer sediments : The three model variants investigated are shown to simulate the biomass concentration poorly when applied to resuspended sewer sediments. Growth associated product formation in terms of exopolymers, growth dependent maintenance energy requirements and growth of protozoan are known to occur. Introducing such details would probably improve possibilities for calibration of the model. However, as it does not seem possible to quantify these processes independently when working under mixed culture conditions, e.g. in case of sewer sediments, several model parameters need to be determined from calibration based on indirect measurements as the OUR. Hereby the model complexity in terms of model parameters to be determined by calibrat ion would increase. This concept would therefore most likely result in a poor model validation (Larsen, 1996). Also the validation of hydrolysis in suspended sewer sediments was only partly satisfactory as the respiration corresponding to artificially added hydrolysible substrate was extremely small. Only a small part of the suspended sewer sediment investigated was readily biodegradable or rapidly hydrolysible SUbstrate. A considerable part of organic matter in the sediment was observed having a rate constant of app. 1 d'i. Therefore, the oxygen utilisation due to resuspended sewer sediments discharged to receiving waters via overflow structures or wastewater treatment plants can lake place over a long time. The importance of this phenomenon in addition to adsorption and sedimentation of particulate organic matter discharged during CSO events needs further investigation. However, the phenomenon corresponds to previous observations of a delayed DO depletion in streams receiving CSO, (Hvitved-Jacobsen, 1982; Harrernoes, 1982), CONCLUSIONS AND PERSPECTIVES Impacts of resuspended sewer sediments discharged to receiving waters and waste water treatment plants leads to the need of a more detailed characterisation of the organic matter involved than the traditional used gross characteristics: COD, BOD and YS. OUR measurements of sewer sediments in suspension combined

76

J. VOLLERTSEN and THORKILD HVITVED-JACOBSEN

with a corresponding interpretation based on simulation of these measurements with a conceptual model including the aerobic microbial processes biomass growth, maintenance energy requirement and hydrolysis leads to a characterisation in terms of the microbial potential of the sediments. These correspond with the impact of sediments discharged to the receiving waters as well as to the behaviour of sediments transported into the biological part of waste water treatment plants. OUR measurements can be recommended as a valuable methodology for characterisation of suspended sewer sediments in terms of COD-fractions and a related potential for biotransformation. ACKNOWLEDGEMENT Financial support for this research project was provided by the Danish Technical Research Council, the framework programme on "Solids in Sewage Systems". NOTATION lilt bH khn K, Kxn q., S, So I

XB XSn Y"

maximum specitic growth rate for heterotrophic biomass (dO') decay rate constant (d') hydrolysis rate constant. fraction n (dol) saturatron constant for readily biodegradable substrate (g COD m') saturation constant for hydrolysis. traction n (gCOD/gCOD) maintenance energy requirement rate constant (dO') readily biodegradable substrate (g COD m') dissolved oxygen (g O 2 m') tune (d) heterotropmc active biomass (g COD m") hydrolysible substrate. fraction n (g COD rn") yield constant for heterotrophic biomass (g COD biomasslg COD substrate)

REFERENCES Bjerre, H. L.. Hvitved-Jacobsen, T., Teichgraber, B. and Schlegel. S. (1997). Modelling of aerobic wastewater transformations under sewer conditions in the Emscher river. Germany. Accepted for Wat. Env.Res. Bjerre, H. L.. Hvitved-Jacobsen, T., Teichgraber, B. and te Heesen, D. (1995). Experimental procedures characterizing transformations of wastewater organic mailer in the Emscher river, Germany. Wat. Sci. Tech. 31(7). 20t-212. Henze. M .• Grady Jr.. C. P. L.• Gujer, W .• Marais. G. v, R. and Matsuo. T. (1987). Activatedsludge model no. I. Scientific and technicalreportno. I. International Association on Water Pollution Research and Control. Henze, M.. GUJer. W.• Mino, T., Matsuo. T.• Wentzel. M. C. and Marais, G. v, R. (1995). Activatedsludgemodel no. 2. Scientific and technical reportNo.3. International Association on Water Quality. Hvitved-Jacobsen, T. (1982). The impacts of combined sewer overflows on the dissolved oxygen concentration of a river. Wat. Res., 16. 1099-1105. Hvitved-Jacobsen, T., Vollertsen, 1. and Nielsen. P. H. (1998). A process and model concept for microbial wastewater transformations in gravity sewers. Wat. Sci. Tech.•31(7) (this issue). Harremoes, P. (1982). Immediate and delayed oxygen depletion in rivers. Wat. Res.• 16. 1093·1098. Kappeler. J. and Gujer, W. (1992). Estimation of kinetic parameters of heterotrophic biomass under aerobic conditions and characterisation of wastewater for activated sludge modelling. Wat. Sci. Tech., 15(6), 125-139. Kurland. C. G. and Mikkola, R. (1993). The impact of nutritional state on the microevolution of ribosomes. Starvation in Bacteria, Kjelleberg, S. (ed), New York and London: Plenum Press. 1993. pp. 225-237. Larsen, T. (1996). Some remarks on the calibration and validation of numerical water quality models. Unpublished paper. Nielsen. P. H, Jahn, A. and Palmgren, R. (1997). Conceptual model for production and composition of exopolymers in biofilms. Wat. Sci. Tech.• 36(1).11·19. Russel. J. B. and Cook. G. M. (1995). Energetics of bacterial growth: Balance of anabolic and catabolic reactions. Microbiol. Rev. 59( I). 48-62. Sollfrank, U. (1988). Bedeutung organischer Fractionen in kommunalem Abwasser im Hinblick auf die mathematische Modellierung von Belebtschlammsystemen. Dissertation ETH Nr. 8765. Z rich 1988. Standard Methods (1995). Standardmethodsfor the examination of waterand wastewater. 19th edition. Washington 1995. Tempest. D. W. and Neijssel. O. M. (1984). The status ofYATP and maintenance energy as biologically interpretable phenomena. Ann. Rev. Microbiol, 38. 459·86.