CALIBRATION AND VALIDATION OF ACTIVATED SLUDGE MODEL NO. 3 FOR SWISS MUNICIPAL WASTEWATER

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1 PII: S (00) Wat. Res. Vol. 34, No. 14, pp. 3580±3590, Elsevier Science Ltd. All rights reserved Printed in Great Britain /00/$ - see front matter CALIBRATION AND VALIDATION OF ACTIVATED SLUDGE MODEL NO. 3 FOR SWISS MUNICIPAL WASTEWATER G. KOCH M,M.KUÈ HNI, W. GUJER M and H. SIEGRIST* M Swiss Federal Institute for Environmental Science and Technology (EAWAG) and Swiss Federal Institute of Technology (ETH), CH-8600, DuÈ bendorf, Switzerland (First received 13 September 1999; accepted in revised form 14 December 1999) AbstractÐASM3 was tested against experimental data from aerobic and anoxic batches as well as fullscale experiments from various WWTPs treating Swiss municipal wastewater. A set of kinetic and stoichiometric parameters emerged from these tests. The calibrated ASM3 allows the sludge production and the denitri cation capacity to be successfully modeled with a standardized set of parameters. The readily degradable inlet substrate S S,o was estimated from respiration measurements by curve tting. This contradicts the suggestion of the IAWQ task group that S S,o in ASM3 may be approximated by the total soluble COD as determined by 0.45 mm membrane ltration. Aerobic and anoxic respiration of storage products is insigni cant compared to growth on these products Elsevier Science Ltd. All rights reserved Key wordsðactivated Sludge Model No. 3, ASM3, kinetics, stoichiometrics, full-scale experiments, batch experiments NOMENCLATURE A autotrophic organisms C component with soluble and particulate fractions H heterotrophic organisms I inert organic material o input ir internal recirculation ini initial condition NH ammonium nitrogen NO nitrate+nitrate nitrogen, anoxic O, O 2 oxygen, aerobic r return sludge S organic substrate, soluble component STO cell internal storage product X particulate component INTRODUCTION Thanks to today's better scienti c insights into the biological processes of wastewater treatment, the IAWQ Task Group on Mathematical Modeling for Design and Operation of Biological Wastewater Treatment Processes was able to introduce Activated Sludge Model No. 3 (ASM3) (Gujer et al., *Author to whom all correspondence should be addressed. Tel.: ; fax: ; hansruedi.siegrist@eawag.ch 1999). ASM3 corrects some de ciencies of ASM1 (Henze et al., 1987). It includes the storage of organic substrates as a new process and lysis is replaced by an endogenous respiration process. As a result, the hydrolysis becomes less dominant for the rates of oxygen consumption and denitri cation compared to the ASM1 and is now independent of the electron donor. Furthermore, all processes (except for hydrolysis) run at a reduced rate under anoxic as compared to aerobic conditions. The ASM3 also takes smaller anoxic yield coe cients into account. A set of kinetic and stoichiometric parameters for the reliable prediction of the nitri cation and denitri cation rates in wastewater treatment plants (WWTPs) treating Swiss municipal diluted waste water is proposed in this paper. The most important parameters of ASM3 are rst estimated on the basis of batch experiments with activated sludge from di erent WWTPs. For the parameter estimation and the sensitivity analysis, ASM3 was implemented in AQUASIM (Reichert, 1998). Sensitivity analysis are necessary to check, which model parameter can be determined with the aid of the available batch experiments. Based on the multitude of experiments with di erent carbon sources (single substrate and mixed substrate), di erent electron acceptors (nitrate and oxygen) and di erent time constants (long term and short term experiments), the most important model parameters could be esti- 3580

2 Calibration and validation of ASM No mated. The uncertainty of parameter estimation is reduced by simultaneously evaluating all the experiments. With multiple experiments at di erent temperatures or with activated sludge from di erent plants the uncertainty could be further reduced. Nevertheless, it is very likely that the same dynamic behavior can be used to explain several parameter combinations. It should be considered that in this paper only a possible way to calibrate ASM3 is suggested and that the experiments were initially designed to calibrate ASM1 and not ASM3. Further experiments will have to be performed in future, speci cally for calibrating ASM3 more accurately (e.g. heterotrophic storage of organic substrate). ASM3 was then validated for the heterotrophic processes with a wide variety of experimental data from pilot- and full-scale experiments. The experiments were used to calibrate the nitri cation rates and the sludge production. For these parameters a real validation was not possible. The initial sludge composition of all the batch experiments (the heterotrophic biomass X H,ini, the storage products X STO,ini and the slowly degradable substrate X S,ini ) was simultaneously estimated from the simulations of the full-scale experiments. CALIBRATION OF ASM3 WITH BATCH EXPERIMENTS Endogenous respiration under aerobic and anoxic conditions Batch experiments with sludge from the TuÈ f- fenwies pilot plant (Fig. 6) showed a considerably reduced decay rate of the autotrophic and heterotrophic organisms under anoxic compared to aerobic conditions (Siegrist et al., 1999). This e ect can be reproduced in ASM3 by taking into account an aerobic and a slower anoxic endogenous respiration rate for the biomass X H and X A. The decay rates b H,O2 and b H,NO for the heterotrophic biomass at 208C (Table 3) were estimated with AQUASIM from the decrease of the total COD at 14 and 208C, respectively (Fig. 1). The sensitivity analysis shows that both, the decay rate b H and the initial sludge composition X H,ini, are sensitive in these experiments and strongly correlated with each other. Therefore X H,ini was estimated simultaneously from pilot plant steady state simulation. The determined decay rates b H,O2 and b H,NO for the heterotrophic biomass at 208C were 0.30 and 0.10 d 1, respectively. From the decrease of the maximal autotrophic respiration rate also b A,O2 and b A,NO for the autotrophic biomass at 208C were estimated to 0.20 and 0.10 d 1, respectively (Siegrist et al., 1999). Sensitivity analysis with both batch and pilot-scale experiments showed, that the respiration rate b STO for the internal storage products is insigni cant compared to growth on these products and could not be estimated from the batch tests. Therefore, b STO was set equal to the endogenous respiration rate analogous to the original ASM3. Aerobic degradation of acetate A tailing-o of the oxygen-consumption curve is often observed in respiration tests (Fig. 2). This phenomenon can be modeled with the storage products X STO introduced in ASM3. A set of stoichiometric and kinetic parameters for the aerobic storage and growth of the heterotrophic biomass X H could be estimated (Fig. 2) on the basis of aerobic batch experiments with activated sludge from the ZuÈ rich±glatt WWTP (Siegrist et al., 1995) and acetate as the substrate. The parameters Y STO,O2 and k STO are most sensitive after acetate addition during substrate respiration and could be uniquely determined based on the oxygen consumption curve (Fig. 2). Before and after substrate respiration mainly Y H,O2, K STO and m H are sensitive. The saturation constant K S could be determined during the sudden decrease of the oxygen consumptive rate. The initial sludge composition results from the steady state WWTP simulation. For both experiments, the net (true) yield of heterotrophic biomass produced per unit of substrate removed results in Y net; O2 ˆ Y STO; O2 Y H; O2 ˆ Fig. 1. Decrease of activated sludge COD from the TuÈ enwies pilot plant in anoxic and aerobic batch experiments. C COD,ini =3500 gcod m 3, X H,ini =1200 gcod m 3. Best t with b H,O2 =0.30 d 1, b H,NO =0.10 d 1, y T =0.078C 1 (values at T =208C).

3 3582 G. Koch et al. Fig. 2. Heterotrophic oxygen-consumption rate in batch experiments (T = 158C) from 27 November 1992 (left) and 8 December 1992 (right). Acetate addition at and d, X H,ini =1300 and 1100 gcod m 3, respectively. Sludge from the ZuÈ rich±glatt WWTP, pre-aerated for several hours. Best t for both experiments with Y H,O2 =0.80 gx H g 1 X STO, Y STO,O2 =0.72 gx STO g 1 S S, K S =4.0 gcod m 3, m H =3.0 d 1, K STO =0.1 gx H g 1 X STO and k STO =5.0 d 1 (values at T =208C). 0:72 0:80 ˆ 0:58gX H g 1 S S, which is about 8% higher than the value suggested by Gujer et al. (1999) (Table 3). Aerobic degradation of soluble COD from primary sludge acidi cation The characteristic of the oxygen-consumptive curve depends strongly on the available substrate. An aerobic batch experiment with activated sludge from the Neugut±DuÈ bendorf WWTP (Fig. 9) and soluble COD from primary sludge acidi cation as the substrate (Moser-Engeler et al., 1999) was therefore performed (Fig. 3) and con rmed twice by identical experiments. The net yield from the readily degradable fraction of the total soluble COD Y net; O2 ˆ Y STO; O2 Y H; O2 ˆ 0:80 0:80 ˆ 0:64gX H g 1 S S is considerably higher than that observed from the acetate batch and suggested by Gujer et al. (1999) (Table 3). To obtain a good t Fig. 3. Heterotrophic oxygen-consumption rate in the batch experiment from 19 December Substrate addition at d, X H,ini =880 gcod m 3. T =108C. Sludge from the aerobic compartment of the Neugut± DuÈ bendorf WWTP. Best t with Y H,O2 =0.80 gx H g 1 X STO, Y STO,O2 =0.80 gx STO g 1 S S, K S =10 gcod m 3, m H =3.0 d 1, K STO =0.1 gx H g 1 X STO and k STO =11 d 1 (values at T =208C). Some 80% of the soluble COD were volatile fatty acids. of measured data, the aerobic storage rate constant k STO has to be increased by about 100%. The maximum oxygen-consumption rate based on dissolved fermentation products is obviously much higher than that based on single-substrate acetate. This observation is in agreement with the measurements of maximum denitri cation rates on di erent single substrates and mixed fermentation products (Moser-Engeler et al., 1998). Because of the wide variety of substrates in the fermentation products and their individual uptake rates, the saturation constant for substrate S S is greater than that for the single-substrate acetate. It is assumed that fermentation products tend to induce oxygen-consumption curves which are more similar to wastewater than to acetate as a single substrate. Aerobic degradation of COD from wastewater The aerobic yield coe cients for storage and growth as well as the high aerobic storage-rate constant k STO calculated from the batch with fermentation products was con rmed by 14 analyzed respiration curves with sludge and wastewater from the TuÈ enwies pilot plant (eight batch experiments, two of them shown in Fig. 4) and the Neugut± DuÈ bendorf WWTP (six batch experiments). In contrast to the original ASM3 (Gujer et al., 1999) where the readily degradable inlet substrate S S,o may be approximated by the total soluble inlet COD S COD,o as determined by 0.45-mm membrane ltration, the S S,o was estimated from respiration measurements by curve tting (all estimated S S,o and measured S COD,o are summarized in Table 1). Otherwise, completely di erent aerobic yields for the municipal wastewater components (mainly colloidal rather than soluble degradable COD) and di erent fermentation products (mainly fatty acids) as the substrate are required. The level of the basic respiration (respiration after consumption of readily degradable substrate) could only be modeled by a

4 Calibration and validation of ASM No Fig. 4. Typical heterotrophic oxygen-consumption rates for diluted (S S,o =30 gcod m 3, left) and concentrated (S S,o =75 gcod m 3, right) un ltered wastewater. Measurements from 20 November The ratio of sludge to wastewater volume in the batch reactor was 1:3 (left) and 1:4 (right), respectively. T =158C. Activated sludge from the TuÈ enwies pilot plant. Best t for eight experiments in total with Y H,O2 =0.80 gx H g 1 X STO, Y STO,O2 =0.80 gx STO g 1 S S, K S =10 gcod m 3, m H =3.0 d 1, K STO =0.1 gx H g 1 X STO, k STO =13 d 1 and k H =9.0 d 1 (values at T =208C). substantial increase of the hydrolysis rate constant k H. Anoxic degradation of soluble COD from primary sludge acidi cation To quantify the reduction of the anoxic yield and the reduction factor for storage and growth, an anoxic batch experiment with activated sludge from the Neugut-DuÈ bendorf WWTP and soluble COD from primary sludge acidi cation as the substrate was analyzed (Fig. 5). From parameter identi cation with AQUASIM, an anoxic net yield of Y net; NO ˆ Y STO; NO Y H; NO ˆ 0:70 0:65 ˆ 0:46 gx H g 1 S S was estimated, which is about 30% lower than the net aerobic yield. On the basis of thermodynamic calculations, a 15% lower net yield can be predicted (Maurer and Gujer, 1998; Orhon et al., 1996). Copp and Dold (1998) determined an average reduction in net yield of about 38% from batch experiments with di erent organic substrates and biomasses. McClintock et al. (1988) and Purtschert and Gujer (1999) observed a reduction of 45 and 25%, respectively, which clearly also con- rms the signi cant reduction of the anoxic compared with the aerobic yield. The calibrated reduction factor for storage and growth Z NO ˆ 0:50 is slightly lower than that suggested by Gujer et al. (1999). The saturation constant K S for the fermentation products as the substrate was found to be identical under aerobic and anoxic conditions (compare the calibrated parameters in Fig. 3 and Fig. 5). VALIDATION OF ASM3 WITH PILOT AND FULL-SCALE EXPERIMENTS ASM3 is validated with a number of pilot- and full-scale experiments on the basis of the kinetic and stoichiometric parameters determined from batch experiments (Table 3). The sludge production in the various WWTPs is calibrated with the fraction of inert particulate COD X I,o from the waste- Table 1. Wastewater characterization based on 14 respiration experiments with sludge and wastewater from the TuÈ enwies pilot plant and the Neugut±DuÈ bendorf WWTP a WWTP Experiment T (8C) C COD,o (gcsb m 3 ) S COD,o (gcsb m 3 ) S I,o +S S,o (gcsb m 3 ) S I,o +S S,o /S COD,o TuÈ enwies pilot plant 29 October 1996, 10: = TuÈ enwies pilot plant 29 October 1996, 13: = TuÈ enwies pilot plant 30 October 1996, 10: = TuÈ enwies pilot plant 30 October 1996, 13: = TuÈ enwies pilot plant 19 November 1996, 10: = TuÈ enwies pilot plant 19 November 1996, 14: = TuÈ enwies pilot plant 20 November 1996, 10: = TuÈ enwies pilot plant 20 November 1996, 13: = Neugut±DuÈ bendorf 28 February 1996, 16: = Neugut±DuÈ bendorf 3 August 1995, 11: = Neugut±DuÈ bendorf 4 August 1995, 03: = Neugut±DuÈ bendorf 4 August 1995, 10: = Neugut±DuÈ bendorf 18 July 1995, 11: ± 15+70=85 ± Neugut±DuÈ bendorf 21 July 1995, 09: = a C COD,o =total COD inlet concentration (primary e uent), S COD,o =total soluble COD inlet concentration determined by 0.45-mm membrane ltration, S S,o =readily degradable inlet substrate determined by curve tting, S I,o =inert soluble COD inlet concentration (about 0.06C COD,o, Table 4).

5 3584 G. Koch et al. taken from di erent zones of the pilot plant (Fig. 8). The simulation with the calibrated ASM3 corresponds well to the measurements. Fig. 5. Denitri cation rate in the batch experiment from 12 December Substrate addition at d, X H,ini =700 gcod m 3. T = 208C. Sludge from the aerobic compartment of the Neugut±DuÈ bendorf WWTP. Best t with Y H,NO =0.65 gx H g 1 X STO, Y STO,NO = 0.70 gx STO g 1 S S, K S =10 gcod m 3, m H =3.0 d 1, K STO =0.1 gx H g 1 X STO, k STO =11 d 1, Z NO =0.50 (values at T =208C). water (Table 4), the observed nitri cation rate with the maximum autotrophic growth rate m A and the saturation constant for ammonium K A,NH (Table 2). TuÈ enwies pilot plant Tracer experiments showed that the ow characteristic of the TuÈ enwies pilot plant (total volume of activated sludge tank 1.2 m 3 ) can be simulated as a cascade of eight CSTRs, corresponding to the six anoxic and two aerobic compartments. The sludge blanket was characterized with reactors R1 and R10 (Fig. 6). Ammonium and nitrate were added to the in uent to avoid nitrate limitation in the anoxic compartments. A maximum growth rate m A ˆ 1:8 d 1 T ˆ 20^ C and a saturation constant for ammonium K A, NH ˆ 1:0 gn m 3 best ts the measured ammonium concentrations (Fig. 7) and oxygen-consumption rates (results not shown) of the nitri ers. The high m A is probably due to the relatively high air ow in the pilot plant, leading to improved CO 2 stripping and increased ph. A good prediction of the nitrogen elimination can be observed (Fig. 7). Pro les of the heterotrophic oxygen-consumption could be determined with activated sludge samples WWTP Neugut±DuÈbendorf The experimental lane at the Neugut±DuÈ bendorf WWTP treats the wastewater for about 15,000 population equivalents (p.e.) and consists of six reactor compartments (R2±R4 anoxic, R5±R7 aerobic, Fig. 9). The ow regime of the activated sludge tank behaves almost like a cascade of six CSTRs. This could be concluded from tracer experiments. Besides the composite samples from the in uent and e uent, the variation of the denitri cation capacity within the tanks was also observed with on-line nitrate, nitrite and ammonium equipment. To avoid nitrate limitation in the anoxic compartments and to simulate the e ect of the nitrogen-rich supernatant from the sludge digestion that will be built on Neugut in future, nitrogen fertilizer with 25% NH 4 -N, 25% NO 3 -N and 50% urea, respectively, was added to the in uent. A substantial nitrite accumulation occurred in compartment R6 due to the high temperatures prevailing during the investigated period (T = 20± 228C). On the basis of a nitrogen balance over the anoxic zone (R1±R4), the fraction of the denitri ed nitrite b NO2 in the total denitri cation capacity (equation (1)) was about 20%. Due to the signi cantly smaller COD equivalent of nitrite S NO2 compared to nitrate S NO3 i COD, NO2 ˆ 3:43 and i COD; NO3 ˆ 4:57 gcod gn 1, respectively), the denitri cation potential increases. In ASM3, this can be considered in conjunction with a reduced COD equivalent i COD, NO for nitrate plus nitrite (S NO ) within the composition matrix (equation (2)), leading to increased stoichiometric coe cients n j,sno and therefore to about 8% higher anoxic storage and growth rates. b NO2 ˆ Qo S NO2, o Q ir S NO2, R6 Q r S NO2, R7 Q o S NO, o Q ir S NO, R6 Q r S NO, R7 ˆ 0:20 1 Table 2. Maximum autotrophic growth rate (T =208) and saturation constant for ammonium from pilot and full-scale experiments a WWTP Experiment m A [d 1 ] K A,NH [gn m 3 ] Remarks TuÈ enwies pilot plant 11±12 March No chemical precipitation Neugut±DuÈ bendorf WWTP 26±27 August No chemical precipitation Neugut±DuÈ bendorf WWTP 3±4 February 1998 b No chemical precipitation ZuÈ rich±werdhoè lzli WWTP 8 March 1998 c Fe(II) addition in aerobic zone ZuÈ rich±werdhoè lzli WWTP 2 December 1993 c Fe(II) addition in aerobic zone ZuÈ rich±werdhoè lzli WWTP 4±5 December Fe(II) addition in aerobic zone ZuÈ rich±glatt WWTP 9±10 November Fe(II) addition in anoxic zone a b A,O2 =0.20 d 1. b Experiments not shown. c Experiments discussed in Siegrist et al. (2000).

6 Calibration and validation of ASM No Fig. 6. Flow scheme and operating conditions of the TuÈ enwies pilot plant. The total sludge retention time y X,tot (incl. R1 and R10) was 17 d. The oxygen concentration in the reactors R8 and R9 was controlled. i COD, NO ˆ b NO2 i COD, NO2 1 b NO2 i COD, NO3 4:34 gcod gn 1 Š 2 With a maximum growth rate m A ˆ 0:9 d 1 (T = 208C) and a saturation constant for ammonium K A, NH ˆ 1:0 gn m 3, the modeled ammonium pro- le (Fig. 10) and autotrophic oxygen-consumption rates (results not shown) t the measurements best. With the kinetic and stoichiometric parameters calculated from the batch experiments, the heterotrophic oxygen-consumption rates (results not shown) as well as the total denitri cation rate (Fig. 10) are slightly underestimated. WWTP ZuÈrich±WerdhoÈlzli The ZuÈ rich±werdhoè lzli WWTP (600,000 p.e.) is split up into two lanes, north and south. Both lanes have six parallel activated-sludge tanks ( m 3 ) and six rectangular clari ers with transverse ow. The activated sludge tanks have a 28% anoxic volume (Fig. 11). The Kjeldahl-nitrogen load of the digester supernatant is about 20% of the load in the primary e uent. Here, ASM3 was tested with three experiments (8 March 1988, before installation of anoxic zones; 2 December 1993; 4±5 December 1996) performed at the ZuÈ rich±werdhoè lzli WWTP with respect to process optimization in the water lane. The ow scheme and the simulation results with the calibrated ASM3 of the rst two experiments are discussed in Siegrist et al. (2000). Figure 12 shows the simulation of the third experiment with the calibrated ASM3 (best reproduction of the nitri cation capacity with m A =1.0 d 1 and K A, NH ˆ 1:0 gn m 3 at T =208C). The measured and modeled denitri cation rates correlate well without any adjustments of the calibrated ASM3. The same experiment could also successfully be modeled with a slightly modi ed ASM1 (Koch et al., 1999). WWTP ZuÈrich±Glatt The activated sludge system of the WWTP ZuÈ r- ich±glatt (110,000 p.e.) consists of four parallel lanes ( m 3 ) and four circular secondary Fig. 7. Modeled (lines) and measured (dots) ammonium (left) and nitrate+nitrite (right) concentrations in R7 and in the e uent of the secondary clari er on 11±12 March The average total COD and ammonium inlet concentrations during the previous three weeks were 290 gcod m 3 and 38 gn m 3 (ammonium dosage).

7 3586 G. Koch et al. Fig. 8. Modeled and measured pro les of the actual heterotrophic oxygen-consumption rate. The time axis corresponds to the hydraulic retention time of a sludge package in the TuÈ enwies pilot plant (space time). clari ers with central inlets. The experimental lane had a 33% anoxic volume (Fig. 13, see also Siegrist et al., 1995). The Kjeldahl-nitrogen load of the reject water from digester supernatant is about 15% of the load in the primary e uent. With the set of parameters obtained from the batch experiments, the modeled nitrate pro les reproduce the measurements well (Fig. 14). The maximum growth rate of the nitri es m A has to be increased to 2.0 d 1 for a best t, which is substantially higher than generally assumed. The relatively high estimated saturation constant for ammonium K A, NH ˆ 2:0 gn m 3 partly compensates the high maximum growth rate. Adjustment of ASM3 after the pilot and full-scale experiments The calibrated ASM3, validated with pilot and Fig. 9. Flow scheme and operating conditions of the Neugut DuÈ bendorf WWTP (experimental lane). The total sludge retention time y X,tot (including the sludge blanket R1) was 20 d. The oxygen concentration in the reactors R6 and R7 was controlled. Fig. 10. Modeled and measured ammonium (left) and nitrate+nitrite (right) concentration in di erent reactors on 26±27 August The average total COD and ammonium inlet concentrations in the previous three weeks were 320 gcod m 3 and 15.7 gn m 3 (without N-dosage) respectively. The strong variation in nitrogen dosage in the previous week was considered in the simulation. T = 218C. See Table 4 for the COD inlet fractions and Table 3 for the model parameters.

8 Calibration and validation of ASM No Table 3. Calibrated kinetic and stoichiometric parameters at T =208 with exponential temperature dependence y T in parentheses Symbol Unit ASM3 (Gujer et al., 1999) Calculated values Kinetic parameters Hydrolysis rate constant k H d (0.04) 9.0 (0.04) Hydrolysis saturation constant K X gx S g 1 X H Heterotrophic organisms X H Aerobic storage rate constant k STO gs S g 1 X H d (0.07) 12 (0.07) Anoxic reduction factor for growth/storage Z NO ± Saturation/inhibition constant for oxygen S O K O go 2 m Saturation/inhibition constant for S NO K NO gn m Saturation constant for substrate S S K S gcod m Saturation constant for storage K STO gx STO g 1 X H Heterotrophic maximum aerobic growth rate m H d (0.07) 3.0 (0.07) Saturation constant for ammonium S NH K NH gn m Saturation constant for bicarbonate S HCO K HCO mol m Aerobic endogenous respiration rate of X H b H,O2 d (0.07) 0.3 (0.07) Anoxic endogenous respiration rate of X H b H,NO d (0.07) 0.10 and 0.15 (0.07) (from batch and full-scale experiments respectively) Aerobic respiration rate for X STO b STO,O2 d a Anoxic respiration rate for X STO b STO,NO d a Autotrophic organisms (nitri ers) X A Autotrophic maximum growth rate m A d (0.105) (0.105) Ammonium substrate saturation constant for nitri ers K A,NH gn m Oxygen substrate saturation constant for nitri ers K A,O go 2 m Bicarbonate saturation constant for nitri ers K A,HCO mol m Aerobic endogenous respiration of nitri ers b A,O2 d (0.105) 0.20 (0.105) Anoxic endogenous respiration of nitri ers b A,NO ± 0.05 (0.105) 0.10 (0.105) Stoichiometric parameters Production of X I in endogenous biomass respiration f XI gx I g 1 X H Aerobic yield of stored products per S S Y STO,O2 gx STO g 1 S S Anoxic yield of stored products per S S Y STO,NO gx STO g 1 S S Aerobic yield of heterotrophic biomass growth on X STO Y H,O2 gx H g 1 X STO Anoxic yield of heterotrophic biomass growth on X STO Y H,NO gx H g 1 X STO Yield of autotrophic biomass per g NO 3 -N Y A gcod gn Nitrogen content of S I i NSI gn gcod Nitrogen content of S S i NSS gn gcod Nitrogen content of X I i NXI gn gcod Nitrogen content of X S i NXS gn gcod Nitrogen content of X H and X A i NBM gn gcod a b STO could not be calibrated with the experiments and is not sensitive to the change in respiration rates. Analogous to the original ASM3 (Gujer et al., 1999). b STO was set equal to the endogenous respiration rate b H. full-scale experiments, reproduces the observed nitrate+nitrite pro les quite well. Nevertheless, the denitri cation capacity is systematically underestimated in all the experiments. From a sensitivity analysis with AQUASIM (Reichert, 1998) performed with the TuÈ enwies experiment from 11±12 March 1997 after model calibration, the most sensitive parameters with respect to the concentrations S NH and S NO in the di erent tanks could be estimated. The parameters most sensitive to the change in denitri cation capacity are the anoxic yields (Y STO,NO, Y H,NO ), the inlet substrates X S,o and S S,o and the endogenous respiration rate b H. By changing only b H,NO as the most sensitive kinetic parameter for reproducing the S NO pro les of the plant, the modeled concentrations of the batch experiments would change only slightly. Therefore b H,NO is suitable for model adjustment. With a 50% higher anoxic endogenous respiration rate (b H,NO =0.15 instead of 0.10 d 1 and the anoxic reduction factor 0.50 instead of 0.33, respectively) the denitri cation rates and thus the modeled nitrate plus nitrite pro les of all experiments produced a better t (modeled pro les not shown). With this Table 4. Fractions of total COD inlet concentrations (primary e uent) WWTP Experiment S S,o S I,o X I,o X S,o X STO,o X H,o X A,o TuÈ enwies pilot plant 11±12 March Neugut±DuÈ bendorf WWTP 26±27 August Neugut±DuÈ bendorf WWTP 3±4 February ZuÈ rich±werdhoè lzli WWTP 8 March 1988 a ZuÈ rich±werdhoè lzli WWTP 2 December 1993 a ZuÈ rich±werdhoè lzli WWTP 4±5 December ZuÈ rich±glatt WWTP 9±10 November a Experiments discussed in Siegrist et al. (2000).

9 3588 G. Koch et al. Fig. 11. Flow scheme and operating conditions of the ZuÈ rich±werdhoè lzli WWTP (experimental lane). The total sludge retention time y X,tot (including the sludge blanket R1 and the inlet channel R7, Koch et al., 1999) was 16 d. The oxygen concentration in the reactors R4 to R6 was controlled. adjustment, b H,NO loses its physical meaning, including any uncertainties in the model parameters and the structure of ASM3. To t the total inlet nitrogen C TN,o (equation (3)) and that total nitrogen content of the activated sludge i N,CSB (equation (4)), i NXS and i NXI must be 0:03 gn gx S 1 (Table 3). and 0:04 gn gx 1 I C TN, o ˆ i NXI X I, o i NXS X S, o i NBM, respectively X A, o X H, o S NH i NSI S I, in i NSS S S, in S NO gn m 3 Š 3 i NCSB ˆ inxi X I i NXS X S i NBM X A X H i CSBTSS X TS gn gcsb 1 Š From ammonium pro les and the autotrophic oxygen-consumption rate during peak ow, the maximum growth rate m A and the saturation constant for ammonium K A,NH were estimated in all fullscale experiments (summarized in Table 2). The 4 most sensitive kinetic parameters m A, b A,O2 and K A,NH enable a good t of the measured and modeled ammonium pro les to be obtained in all experiments. Nevertheless, the tted maximum growth rates of the nitri ers vary strongly in the investigated WWTPs. It is not clear whether the inhibition e ects to the nitri es in the di erent plants due to Fe(II), digester supernatant, wastewater composition, etc. or the general uncertainties in parameter estimation (especially if y X; aer ma; 1 net are responsible for the wide variation in m A. The tted m A and K A,NH correspond to a net growth rate of m A, net ˆ 0:25 d 1 with a standard deviation of 20.1 d 1 at T =108C for 4.0 mm alkalinity, 3.0 go 2 m 3 and 10 gnh 4 -N m 3, which is similar to the values given by Gujer (1986). CONCLUSIONS ASM3 was tested against a wide variety of experimental data. A set of kinetic and stoichiometric parameters is proposed to describe the sludge production, the nitri cation and denitri cation ca- Fig. 12. Modeled and measured ammonium (left) and nitrate+nitrite (right) concentration in di erent reactors on 4±5 December The average total COD and ammonium inlet concentrations during the previous three weeks were 200 gcod m 3 and 17 gn m 3, respectively. T =148C. See Table 4 for the COD inlet fractions and Table 3 for the model parameters.

10 Calibration and validation of ASM No Fig. 13. Flow scheme and operating conditions of the ZuÈ rich±glatt WWTP (experimental lane). The total sludge retention time y X,tot (including the sludge blanket R1) was 15 d. The oxygen concentration in the reactors R4 and R5 was controlled. pacity. Aerobic and anoxic batch experiments as well as pilot- and full-scale experiments from di erent WWTPs were successfully modeled with the calibrated ASM3. To further improve the prediction of denitri cation capacity in full-scale experiments, the anoxic endogenous respiration rate b H,NO should be increased by 50% compared to the value obtained from speci c decay experiments in the batch reactors. This adjustment leads to b H,NO becoming less physically meaningful, including now uncertainties in the model parameters and structure. The respiration rate on internal storage products is signi cant compared to growth on these products. This was concluded from sensitivity analyses with both batch and full-scale experiments. Therefore, this process was set equal to the endogenous respiration rate. Readily degradable inlet substrate S S,o was estimated from respiration measurements by curve tting. This contradicts the idea of the IAWQ Task Group, who assumed that S S,o may be approximated by the total soluble COD as determined by 0.45-mm membrane ltration. This de nition would lead to di erent heterotrophic yields to be selected for the simulation of batch experiments with wastewater and single substrates, respectively. Because of the poor correlation between soluble inlet COD and readily degradable inlet COD, respiration tests are still recommended for WWTP simulation to decrease model uncertainty. The maximum autotrophic growth rate m A resulting from parameter estimation with full-scale data during peak ow varies greatly from one plant to another. It is not clear whether the nitri ers in some plants are partly inhibited by the Fe(II), digester supernatant or the wastewater composition. In principle, uncertainties in parameter estimation or model structure may also play an important role in the wide range of the tted m A. The parameters of the nitri ers may be estimated more accurately on the basis of inhibition tests and additional batch experiments with activated sludge from di erent WWTPs. In any case, for unknown wastewater the kinetics of nitri cation should be investigated under di erent operation conditions in order to obtain relief simulation results. Obviously, ASM3 as well as ASM1 is capable to describe the dynamic behavior in common WWTPs satisfactorily. In situations where for example the storage of readily degradable substrate is dominant (e.g. batch tests or WWTPs treating industrial waste water with a high amount of COD) or for Fig. 14. Modeled and measured ammonium (left) and nitrate nitrite (right) concentration in di erent reactors on 9±10 November The average total COD and ammonium inlet concentrations during the previous three weeks were 180 gcod m 3 and 15 gn m 3, respectively. T =158C. See Table 4 for COD inlet fractions and Table 3 for the model parameters.

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