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Application of the IAWQ activated sludge model to nutrient removal process
~
Pergamon
Wal. SCI. Tech. Vol. 3S. No.8. pp. 111-118. 1997. ~ 1997 IAWQ. Pubhshed by Elsevier Science Ltd Prinled in Greal Bnlain.
PU: S0273-1223(97)00157-1
0273-1223/97 $17'00 + 0-00
APPLICATION OF THE IAWQ ACTIVATED SLUDGE MODEL TO NUTRIENT REMOVAL PROCESS T. Mino*, D. C. San Pedro**, S. Yamamoto* and T. Matsuo* ... Department of Urban Engineering, The University ofTokyo. 7-3-1 Hongo. Bunkyo. Tokyo 113, Japan .. Department of Chemistry, Central Luzon State University, Murio. Nueva Ecija. The Philippines
ABSTRACT IAWQ Activated Sludge Model No.2. with the parameter values recommended by the IAWQ Task Group. was capable of predicting the behavior of a nutrient removal activated sludge pilot plant in Japan rather weIl except for the profile ofNRi-N and NOx-N. The model was calibrated for the process and procedures for the calibration of the model are proposed. By applying the proposed calibration procedures, Activated Sludge Model No.2 has been satisfactorily calibrated to simulate the steady state behavior of the process in terms of organic matter and nitrogen removal. But. sometimes, selection of the specific growth rate of nitrifiers gives very sensitive effect on nitrate concentration in the effluent. It is indicated that the rate of anaerobic processes is affected by hydrolysis of particulate organic or slowly biodegradable substrates. and the hydrolysis can be the rate limiting step of the removal of organic matters or. sometimes, nitrogen and phosphorus. At the moment. it is rather risky to estimate the anaerobic hydrolysis rate, the anaerobic substrate uptake rate by phosphate accumulating organisms and the PHA yield with respect to polyphosphate utilization simultaneously only by using static data. The present case shows how relevant calibration procedures can be developed with limited static data. © 1997 IAWQ. Published by Elsevier Science Ltd
KEYWORDS Activated sludge model; calibration; denitrification; enhanced biological phosphate removal; hydrolysis; nitrification; nutrient removal; simulation.
INTRODUCTION The IAWQ Task Group on Mathematical Modeling of Activated Sludge Processes proposed Activated Sludge Model No.1 (ASM I) in 1987 (Henze et aI, 1987). This model is designed to simulate the behaviors of different activated sludge processes removing organic matters and nitrogen. The revised version. Activated Sludge Model No.2 (ASM 2). has been recently completed (Henze et al. 1994). which can simulate the biological phosphate removal process as weIl as the organic matter and nitrogen removal processes. These models may be used for design. monitoring and control. process analysis. etc. For these purposes. precise calibration of model is essential. In the present paper. ASM 2 is calibrated for a nutrient removal activated sludge plant in Japan and procedures for the calibration of the model are proposed. Some important aspects related to the calibration of the model are discussed. JWST 35:1-l
III
T. MINO ~t al.
1\2
IAWQ ACTIVATED SLUDGE MODEL NO.2 IAWQ Activated Sludge Model No.2 (hereafter referred to as ASM 2) is designed to simulate organic matter. nitrogen and phosphate removal in activated sludge processes and composed of 9 soluble and 10 particulate components, and 19 reaction processes. All organic components are expressed in kg COD I m3. Nitrogenous and phosphorus compounds are expressed in kg P and N 1m3 , respectively. A matrix is given in the ASM 2 Report (Henze et al, 1994) to express the stoichiometric relationship among these components in the relevant processes. Each process has its own rate equation, which is also summarized in the report (Henze etal,1994). In the present model, the biological structure of activated sludge and its functions are assumed as follows: The model contains inert organic components, SI (soluble) and XI (particulate), which do not react at all once produced. Activated sludge biomass is composed of three microbiological groups; heterotrophs, autotrophs and phosphate accumulating organisms (PADs). Particulate or "slowly biodegradable" organic substrates (Xs) can be utilized by these microorganisms only after they are hydrolyzed to soluble or "readily biodegradable" organic substrates (SF or SM. There are three hydrolysis processes in ASM 2 under three different electron acceptor conditions, namely, anaerobic, anoxic and aerobic conditions. Heterotrophs (XH) can do fermentation in which fermentable substrates (SF) may be converted to fermentation product like acetate (SM. They can aerobically grow on either SF or SA using oxygen (SOl) as the electron acceptor. They can also grow on SF or SA under anoxic conditions using nitrate or nitrite (S/'/03) as the electron acceptor, or denitrify on SF or SA, through which SN03 may be converted to nitrogen gas (SN2). Autotrophs (X AIJf) nitrify, or grow autotrophically without consuming any organic matters but converting ammonia nitrogen (SNH4) to SN03 under aerobic conditions using oxygen (S02) as the electron acceptor. PADs (XPAO) can take up only SA under anaerobic conditions and store it in the form of an organic storage material, polyhydroxyalkanoates (PH A, XpJIA}. No other organic substrates can be utilized by PADs. The energy necessary for the SA uptake is supplied through the breakdown of a high energy phosphate polymer, polyphosphate (Xpp). As the result of Xpp breakdown, therefore, orthophosphate (SPQ4) is released to the bulk solution under the anaerobic conditions. PADs grow on XPHA under aerobic conditions using S02 as the electron acceptor. These PADs' characteristics are taken from relevant literature review (Wentzel et al, 1990; Satoh et al, 1992; Mino et al 1993). All biomass is subjected to the lysis process, in which biomass is converted to either XI or Xs. In order to account for chemical precipitation I dissolution of phosphate, relevant two chemical processes are further added to the model. By ASM 2, behaviors of various activated sludge processes with different configurations and wastewaters can be simulated under either dynamic or steady state conditions. Users of ASM 2 can delete any of the above mentioned processes, if they are not needed for their own specific purposes. Although it has rather high qualitative predictability, precise calibration is essential in order to be applicable to more quantitative usage like design and control of activated sludge processes.
METHODOLOGIES General Calibration of ASM 2 is attempted by using static data obtained from a domestic sewage treatment pilot plant in Japan which was operated for enhanced biological phosphate removal (Fukase, T., 1986). The software used for the simulation is Activated Sludge sIMulation program (AS 1M) Ver. 2.2 (a simulation program on MS• DOS, provided by Prof. W. Gujer, EAWAG, Switzerland).
Plant Description The pilot plant was designed to investigate suitable operatin& conditions f<;>r organic matt~r and phosphate removal from domestic sewage. It was composed of 4 anaerobiC ~d 3 aer~blc completely nuxed tank reactors in series as shown in Fig. 1. By changing the number of reactors ill use ":Ith a fixed wastewater flow rate of 100 m3/d, anaerobic or aerobic retention time were changed. Each anaerobIC tank had a volume of 2.3 m3 and each aerobic tank had a volume of 4. 6 m3 . Seven experimental runs were conducted, where the anaerobic and aerobic retention times were varied in the ranges of 1.1 - 2.2 hours and 2.2 -3.3 hours, respectively. The return sludge ratio was varied in the range between 7 - 100%, but the average value of 25% was used for simulation. Static data of the influent, the effluent from the last anaerobic tank and the final effluent were collected and the average values are shown in Table I. These are the data sets used for the present calibration.
IAWQ Activated Sludge Model
113
Aerobic Tanks
Anaerobic Tanks
rir.-r.
Sedimentation Tanks
Influe:n:.t. .
Return Sludge
Excess Sludge
'----'----'--Air
Fig.l- Process Configuration Table 1 - Steady State Data from Activated Sludge Operation with Domestic Sewage (mgIL) Run No I-I 6.3 SRT No. of Anaerobic Tank 2 (Anaerobic Retention Time, hrs' 1.1 No. of Aerobic Tank 2 2.2 (Aerobic Retention Time, hrs: MLSS 2870 MLVSS 2290 T-eOD 207 S-COD 108 T-P 4.87 Influent P04-P 3.56 T-N 34.0 24.1 NH4-N S-COD 46.9 Last Anaerobic P04-P 18.7 Tank S-COD 34.3 0.03 P04-P T-N 24.5 Effluent 17.3 NH4-N 5.20 NOx-N Phosphorus Content of Sludge 4.69 (Last Aerobic 'hnk)
1-2 4.3 2 1.1 2 2.2 1860 1460 204 105 5.05 3.01 31.3 22.4 47.7 23.2 32.2 0.05 20.2 10.4 7.40
1-3 2.5 2 1.1 2 2.2 1270 1040 218 105 5.00 3.48 32.4 24.7 54.5 20.6 34.6 0.15 22.4 16.9 5.28
2-1 5.6 2 1.1 3 3.3 1030 818 159 72.4 4.20 2.54 25.3 18.8 38.3 7.43 29.7 1.99 12.7 0.84 11.2
2-4 7.7 4 2.2 3 3.3 2530 2150 242 115 4.86 3.79 44.6 32.8 59.0 21.2 39.8 0.03 32.3 24.9 5.43
2-5 5.9 4 2.2 3 3.3 1970 1640 228 112 4.69 3.48 43.5 32.7 57.3 20.8 46.1 0.44 27.9 18.0 10.0
3-1 4.7 3 1.7 2 2.2 2040 1740 226 102 4.59 3.43 41.0 30.6 50.5 18.0 44.4 0.40 28.1 25.3 2.73
5.41
5.15
3.6
4.3
4.1
4.2
Initial Parameter Settin!: The saturation concentration of oxygen was assumed to be 9 mgIL. The dissolved oxygen was assumed to be
oand 6 mgIL in the anaerobic and aerobic reactors, respectively. The temperature was not regularly measured
during the 16 months operation of the pilot plant. In the simulation, the temperature was set at 20 C for all the cases, although the actual temperature should have varied in the range of 15 - 25 C.
Wastewater Characterization Influent Total- and Soluble-COD was divided into the relevant COD fractions in the following way:
T. MIND til al.
114
= = = =
S, Effluent SCOD SF = (SCOD - SI> • 0.6 SA (SCOD - SI) • 0.4 (Assuming SA : SF 2 : 3) XAUT XPAO XPHA Xpp 0 XI (l'COD - SCOD)· 0.139 (Assuming XI : Xs : XH I : 5 : 1.2) Xs = (l'COD - SCOD) • 0.694 XH = (l'COD - SCOD) • 0.167
=
=
= =
=
The ratios of SA:SF and Xt:XS:XH were selected after "Typical wastewater composition" shown in the ASM 2 Report (Henze et al, 1994). As a result of the simulation conducted. it was found that the composition of soluble COD did not affect the calibration results very much. This is because the rate constant of fennentation process (conversion of SF to SA>. qFe, is large enough to supply sufficient amount of SA in the anaerobic phase. The composition of particulate COD. or the ratio of XI. affects the total S5 concentration rn the reactors. but the simulation results showed that the present ratio was reasonable. Inorganic parameters for the influent were decided as follows:
=
= =
S02 SNOx SN2 0 SP04 .. Measured influent P04-P SNH4 = Measured influent N~-N
The Nitrogen and Phosphorus contents of COD components (iNSt, iNXS. ipXH etc.) are parameters to be decided depending on the wastewater characteristics. In the present simulation. the default values of these parameters were simply applied without any calibration. Due to this. there was some inconsistency between the influent organic nitrogenlphosphorus concentrations measured and those in the model. This inconsistency did not significantly affect the values of the calibrated parameters after all. In general. however, the N and P contents of COD can affect the N and P balance in the system, and hence affect the estimation of kinetic parameters related to N and P metabolisms. Careful detennination may be necessary.
RESULTS Simulation by the Parameters recommended by the fAWO Task Group The results of simulation using the kinetic and stoichiometric parameters recommended by the IAWQ task Group (Henze et ai, 1994) are shown in Table 2. In this table, the sum of SF. SA and SI corresponds to the measured S-eOD. The model, with the default values, was able to predict the behavior of the process rather well except for NH4-N and NOx-N as shown in Table 2. Table 2 - Results of Simulation Using Default Parameters
SF SA SI SNH4 SNOx SP04
End of Anaerobic End of Anaerobic (Measured) (Simulated) 2.33 47.7 14.6 (S-COD) 32.0 20.1 0.0 23.2 22.8
-
SN2
4.34
XI XS XH XPAO XPP XPHA
707 82 1249 656
XAUT
64
70 96
-
-
-
-End of Aerobic (Simulated) 0.58 0.Q2 32.0
End of Aerobic (Measured)
16.9 0.Q2
1.2
10.4 7.4 0.05
4.63
.
694
-
44
1236 660 85 19 96
32.2 (S-COD)
-
-
-
IAWQ Activated Sludge Model
liS
Calibration Procedures In order to obtain better fitting simulation results, the following calibration procedures were introduced. 1. Adjustment of effluent NH4-N and NOx-N by changing specific growth rate of autotrophs (mAl!!') : When NOx-N. formed through nitrification. is carried over to the anaerobic zone with the return sludge. denitrification takes place there and PHA production by PAOs should be adversely affected. Therefore. kinetic parameters related to nitrification must be calibrated before the calibration for PAa s parameters is done. Since the saturation coefficient, KNH4. seems to have smaIl effects on nitrification in the present range of NH4-N. mAl!!' was tuned so that the observed SNH4 and SNo. could be achieved in the model simulation. 2. Adjustment of soluble COD in the anaerobic phase by changing the rate constant for the anaerobic PHA storage (qPttA>: The parameter qPHA directly determines the SA uptake in the anaerobic phase. So, this parameter was calibrated next by adjusting soluble COD (SA+SF+S.) at the end of anaerobic phase. 3. Adjustment of P04-P release in the anaerobic phase by changing the ratio of released phosphorus to PHA produced (YPQ4) : It is often reported that YPQ4 varies significantly case by case. The exact reasons for this variation still remain to be clarified. It is important to identify an appropriate YPQ4 for the system to which users of the present model are going to apply it. By adjusting SPQ4 concentration at the end of anaerobic phase. suitable YPQ4 values were determined for each run. Table 3 - Calibrated Parameters Default SRT No of anaerobic Tank No of Aerobic Tank ,uAUT qPHA
YP04 qPP
-
I-I 6.3 2
1-2
1-3
2-1
2-4
2-5
3-1
Average
7.7 5.9 4.3 2.5 5.6 4.7 4 2 4 2 2 3 2 2 3 3 3 2 2 I 0.623 0.738 0.988 0.670 0.802 0.615 0.728 3 2.44 3.90 4.60 4.80 2.59 2.25 2.85 0.15-0.4 0.205 0.340 0.315 0.190 0.14 0.220 0.175 0.55 0.56 0.475 I.5 1.00 1.15 0.41 1.12
-
-
-
0.738 3.22 0.229 0.752
Table 4 - An Example of Steady State Simulation Result Using Calibrated Parameters (Run 1-2) End of Anaerobic (Measured)
SF SA SI SNH4 SNOx
End of Anaerobic (Simulated) 2.87 11.7 32.0 22.4 0.0
SP04 SN2 XI XS XH XPAD XPP XPHA XAUT
End of Aerobic (Measured)
-
End of Aerobic (Simulated) 0.51 0.02 32.0 10.5 9.53
24.0
23.2
0.05
0.05
2.47
-
2.73
696 83 1I08 818 63 78 55
47.7 (S-COD)
-
-
682 45 lIOI 824 85 14 55
32.2 (S-eoO) 10.4 7.4
-
T. MIND ttl al.
116
4. Adjustment of effluent SPQ4 by changing polyphosphate accumulation rate by PAOs (qpp) : After adjusting the phosphate release in the anaerobic phase. it is necessary to adjust the phosphate uptake in the subsequent aerobic phase. which was done by changing qpp. These four steps are not independent to each other and one can affect others. So. the calibration was done not in a step-by-step manner but in such a way that the calibrated parameters were fed back to the steps before to re-calibrate parameters which had been estimated in the steps before.
Results of Calibration Calibration of relevant parameters was carried out following the above mentioned procedures. A summary of the calibrated parameters is given in Table 3. Table 4 shows an example of results of a steady state simulation using the calibrated parameters (Run 1-2). By applying the present procedures. Activated Sludge Model No.2 has been satisfactorily calibrated to simulate the steady state behavior of the process investigated.
DISCUSSION Hydrolysis Rate of Slowly Biodel:radable Substrate. Xs When the anaerobic PHA storage process is calibrated in step 2 of the above procedures. the rate constants for hydrolysis and fermentation in the anaerobic phase (Kh*hFc and qF. respectively. hFe is a reduction factor for anoxic hydrolysis) were fIxed. and qPHA was estimated. In the model, SA is supplied by fermentation of SF. which is produced from slowly biodegradable substrate. Xs. through hydrolysis. Therefore. it must be noted that the estimation of qPHA is more or less affected by qF and Kh*hFc. In ASM 2, the reduction factor for anoxic hydrolysis, hFc. is assumed to be o. I. This means that the rate of hydrolysis under anaerobic conditions is assumed to be only 10 % of that under aerobic conditions. Mino et al ( 1994) and San Pedro et al (1994) indicated that the rates of hydrolysis of starch and azocasein are not very much affected by the electron acceptor conditions and that they are hydrolyzed at similar rates under anaerobic. anoxic and aerobic conditions. Contradictory to the recommendation of ASM 2, the hydrolysis rate under the anaerobic conditions seems to be as high as that under aerobic conditions. If we set a hFe value much higher than the default one. qPHA should be estimated to be correspondingly higher than the value obtained in the present calibration. The hydrolysis rate of Xs can affect the calibration not only of qPHA but also of other parameters like PHA yield with respect to polyphosphate utilization. YPQ4. and denitrifIcation rate constant, mH*hNOJ' because PHA storage and denitrifIcation are processes which, directly or indirectly. depend on the supply of the organic substrate through the hydrolysis of Xs. In other words, the hydrolysis process can be the rate limiting step of the removal of organic matters or, sometimes. nitrogen and phosphorus. At the moment. it is rather risky to estimate Kh*hFc, qF. qPHA and YP04 as a whole only by using static data.
Nitrification Process The simulated effluent N0J-N was found to be very sensitive to mAl!!" in case of steady state calibration. although the calibrated mAl!!" does not vary very much. The effluent N0J-N can be adjusted by changing two parameters. mAl!!" and the saturation coefficient of nitrifIcation process. KNH4. In steady state calibrations using static data like the present case. KNH4 does not affect the steady state concentration of N0J-N very much. But it may strongly affect how fast the nitrifiers respond to a given condition in a dynamic situation. So. the observed high sensitivity of effluent N0J-N to mAl!!" seems to be a possible limitation of the calibration with static data.
IAWQ Activated Sludge Model
117
Enhanced Biological Phosphate Removal QJantitative prediction of phosphate removal is rather difficult because of the difference in the actual range of phosphate concentrations in enhanced biological phosphate removal processes and the desired level of effluent phosphate. But qualitative explanation of the behavior of phosphate accumulating organisms is possible by the model. Calibrated YPQ4 values in Table 3 have large diviation as expected, reflecting the experimental observation that the ratio of consumed organic substrates to released phosphate during the anaerobic PHA storage varies depending on wastewater characteristics. type of sludge or other unknown reasons. Usually. stoichiometric factors in mathematical models should not be changed case by case. Preferably. they have to be theoretically defmed or experimentally determined with acceptable accuracy. Since YPOl is an unstable stoichiometric factor, it is recommended that YPOl should not be considered as constant but that it should be calibrated for individual
cases.
In step 4 of the calibration procedures. qpp was determined by using the average values of SPOl at the end of the aerobic zone only. because data of SPQ4 profiles along with the wastewater stream or their temporal changes are not available in the present case. Kinetic parameters must be calibrated based on the "curve fitting" concept and. therefore. the estimated values of qpp in Table 3 may carry some inaccuracy.
Predictability of Model The present calibration was carried out with static data of the influent, the effluent from the anaerobic zone and the fmal effluent Since most of reaction rates are concentration dependent (Monod kinetics are applied) and processes are complicatedly related to each other. it should be noted that kinetic parameters may have a certain unavoidable inaccuracy. We need to have spacially or temporally dynamic data for more accurate and reliable calibration. namely either the profiles of variables along the stream of the process or the changes of variables with time. However. available data are more or less limited in actual cases. Appropriate procedures for model calibration should be developed according to the availability of data. The present case shows how relevant calibration procedures can be developed with limited static data. Although some limitations of model calibration with static or steady state data were found, the overall predictability of ASM 2 seems satisfactory after proper calibration. Mathematical models of activated sludge can be a powerful tool to understand the reaction mechanisms of different biomass and to predict the behaviors of the process under different situations. Accumulation of calibration studies will be further necessary with data from different activated sludge processes.
ACKNOWLEDGEMENT The authours would like to express their thanks to Dr. Tetsuro Fukase of Kurita Water Industries for providing with the plant operation data used in the present simulation. REFERENCES Fukase. T. (1986). A SlUdy on Phosphate Removal from Waslewater by the Anaerobic Aerobic Process. phD. thesis. University of Tokyo (in Japanese). Henze, M.• Grady. C. P. L. Jr.• Gujer, W.. M:ll'ais, O. v. R. and Mmsuo, T. (1987). Activated Sludge Model No. I. IAWPRC Scientific and Technical Report No. J,IAWPRC, London.ISSN 101O-707X. Henze. M.. Gujer. W.. MiRO. T.• Matsuo, T.. Wentzel. M. and Marais. G. v. R. (1995). Activaled Sludge Model No. 2.IAWQ Scientific and Technical Report. No. J JAWQ. London. Mino. T.. Satoh. H. and M.'IlSuo. T. (1994). Metabolism of different bocleria1 populations in enhanced biological phosphate removal processes. Wat. Sci. Tech., 29 (7). 67·70. Mino. Too San Pedro, D. C. and Matsuo, T. (1995). Estimation of the Rate of Slowly Biodegradable COD (SBCOD) under Anaerobic Anoxic and Aerobic Conditions by Experiments Uisng Starch as Model Substrate. Wat. Sci. Tech.. 31 (2).95•
103.
San Pedro. D. C.. Mino. T. and Matsuo. T. (1994). EvaJualioo of the Rate of Hydrolysis of SloWly Biodegradable COD (SBCOD) using SlarCh as Substrate under Anaerobic. Anoxic and Aerobic Conditions. Wat. Sci. Tech•• 30 (II), 191-199.
118
T.
MINO~ral.
Saloh. H.• Mino. T. and Matsuo. T. (1992). Uptake of organic substrates and acc;umulalion of polyhydroxya1kanoales linked widl glycolysis of intrneellular carbohydrates under anaerobic condition in the biological excess phosphate removal processes. War. Sci. Tech.• Ui (5/6).933-942. Wentzel. M. C.• Dold. P. L.. Ekarna. G. A. and M,ums. G. v. R. (1990). Processes and Modelling of Nilrifacation Denitrification Biological Excess Phosphorus Removal Syslems - A Review. Wal. Sci. Tech.. 25. No.6. p59-82.
Pergamon
Wal. SCI. Tech. Vol. 3S. No.8. pp. 111-118. 1997. ~ 1997 IAWQ. Pubhshed by Elsevier Science Ltd Prinled in Greal Bnlain.
PU: S0273-1223(97)00157-1
0273-1223/97 $17'00 + 0-00
APPLICATION OF THE IAWQ ACTIVATED SLUDGE MODEL TO NUTRIENT REMOVAL PROCESS T. Mino*, D. C. San Pedro**, S. Yamamoto* and T. Matsuo* ... Department of Urban Engineering, The University ofTokyo. 7-3-1 Hongo. Bunkyo. Tokyo 113, Japan .. Department of Chemistry, Central Luzon State University, Murio. Nueva Ecija. The Philippines
ABSTRACT IAWQ Activated Sludge Model No.2. with the parameter values recommended by the IAWQ Task Group. was capable of predicting the behavior of a nutrient removal activated sludge pilot plant in Japan rather weIl except for the profile ofNRi-N and NOx-N. The model was calibrated for the process and procedures for the calibration of the model are proposed. By applying the proposed calibration procedures, Activated Sludge Model No.2 has been satisfactorily calibrated to simulate the steady state behavior of the process in terms of organic matter and nitrogen removal. But. sometimes, selection of the specific growth rate of nitrifiers gives very sensitive effect on nitrate concentration in the effluent. It is indicated that the rate of anaerobic processes is affected by hydrolysis of particulate organic or slowly biodegradable substrates. and the hydrolysis can be the rate limiting step of the removal of organic matters or. sometimes, nitrogen and phosphorus. At the moment. it is rather risky to estimate the anaerobic hydrolysis rate, the anaerobic substrate uptake rate by phosphate accumulating organisms and the PHA yield with respect to polyphosphate utilization simultaneously only by using static data. The present case shows how relevant calibration procedures can be developed with limited static data. © 1997 IAWQ. Published by Elsevier Science Ltd
KEYWORDS Activated sludge model; calibration; denitrification; enhanced biological phosphate removal; hydrolysis; nitrification; nutrient removal; simulation.
INTRODUCTION The IAWQ Task Group on Mathematical Modeling of Activated Sludge Processes proposed Activated Sludge Model No.1 (ASM I) in 1987 (Henze et aI, 1987). This model is designed to simulate the behaviors of different activated sludge processes removing organic matters and nitrogen. The revised version. Activated Sludge Model No.2 (ASM 2). has been recently completed (Henze et al. 1994). which can simulate the biological phosphate removal process as weIl as the organic matter and nitrogen removal processes. These models may be used for design. monitoring and control. process analysis. etc. For these purposes. precise calibration of model is essential. In the present paper. ASM 2 is calibrated for a nutrient removal activated sludge plant in Japan and procedures for the calibration of the model are proposed. Some important aspects related to the calibration of the model are discussed. JWST 35:1-l
III
T. MINO ~t al.
1\2
IAWQ ACTIVATED SLUDGE MODEL NO.2 IAWQ Activated Sludge Model No.2 (hereafter referred to as ASM 2) is designed to simulate organic matter. nitrogen and phosphate removal in activated sludge processes and composed of 9 soluble and 10 particulate components, and 19 reaction processes. All organic components are expressed in kg COD I m3. Nitrogenous and phosphorus compounds are expressed in kg P and N 1m3 , respectively. A matrix is given in the ASM 2 Report (Henze et al, 1994) to express the stoichiometric relationship among these components in the relevant processes. Each process has its own rate equation, which is also summarized in the report (Henze etal,1994). In the present model, the biological structure of activated sludge and its functions are assumed as follows: The model contains inert organic components, SI (soluble) and XI (particulate), which do not react at all once produced. Activated sludge biomass is composed of three microbiological groups; heterotrophs, autotrophs and phosphate accumulating organisms (PADs). Particulate or "slowly biodegradable" organic substrates (Xs) can be utilized by these microorganisms only after they are hydrolyzed to soluble or "readily biodegradable" organic substrates (SF or SM. There are three hydrolysis processes in ASM 2 under three different electron acceptor conditions, namely, anaerobic, anoxic and aerobic conditions. Heterotrophs (XH) can do fermentation in which fermentable substrates (SF) may be converted to fermentation product like acetate (SM. They can aerobically grow on either SF or SA using oxygen (SOl) as the electron acceptor. They can also grow on SF or SA under anoxic conditions using nitrate or nitrite (S/'/03) as the electron acceptor, or denitrify on SF or SA, through which SN03 may be converted to nitrogen gas (SN2). Autotrophs (X AIJf) nitrify, or grow autotrophically without consuming any organic matters but converting ammonia nitrogen (SNH4) to SN03 under aerobic conditions using oxygen (S02) as the electron acceptor. PADs (XPAO) can take up only SA under anaerobic conditions and store it in the form of an organic storage material, polyhydroxyalkanoates (PH A, XpJIA}. No other organic substrates can be utilized by PADs. The energy necessary for the SA uptake is supplied through the breakdown of a high energy phosphate polymer, polyphosphate (Xpp). As the result of Xpp breakdown, therefore, orthophosphate (SPQ4) is released to the bulk solution under the anaerobic conditions. PADs grow on XPHA under aerobic conditions using S02 as the electron acceptor. These PADs' characteristics are taken from relevant literature review (Wentzel et al, 1990; Satoh et al, 1992; Mino et al 1993). All biomass is subjected to the lysis process, in which biomass is converted to either XI or Xs. In order to account for chemical precipitation I dissolution of phosphate, relevant two chemical processes are further added to the model. By ASM 2, behaviors of various activated sludge processes with different configurations and wastewaters can be simulated under either dynamic or steady state conditions. Users of ASM 2 can delete any of the above mentioned processes, if they are not needed for their own specific purposes. Although it has rather high qualitative predictability, precise calibration is essential in order to be applicable to more quantitative usage like design and control of activated sludge processes.
METHODOLOGIES General Calibration of ASM 2 is attempted by using static data obtained from a domestic sewage treatment pilot plant in Japan which was operated for enhanced biological phosphate removal (Fukase, T., 1986). The software used for the simulation is Activated Sludge sIMulation program (AS 1M) Ver. 2.2 (a simulation program on MS• DOS, provided by Prof. W. Gujer, EAWAG, Switzerland).
Plant Description The pilot plant was designed to investigate suitable operatin& conditions f<;>r organic matt~r and phosphate removal from domestic sewage. It was composed of 4 anaerobiC ~d 3 aer~blc completely nuxed tank reactors in series as shown in Fig. 1. By changing the number of reactors ill use ":Ith a fixed wastewater flow rate of 100 m3/d, anaerobic or aerobic retention time were changed. Each anaerobIC tank had a volume of 2.3 m3 and each aerobic tank had a volume of 4. 6 m3 . Seven experimental runs were conducted, where the anaerobic and aerobic retention times were varied in the ranges of 1.1 - 2.2 hours and 2.2 -3.3 hours, respectively. The return sludge ratio was varied in the range between 7 - 100%, but the average value of 25% was used for simulation. Static data of the influent, the effluent from the last anaerobic tank and the final effluent were collected and the average values are shown in Table I. These are the data sets used for the present calibration.
IAWQ Activated Sludge Model
113
Aerobic Tanks
Anaerobic Tanks
rir.-r.
Sedimentation Tanks
Influe:n:.t. .
Return Sludge
Excess Sludge
'----'----'--Air
Fig.l- Process Configuration Table 1 - Steady State Data from Activated Sludge Operation with Domestic Sewage (mgIL) Run No I-I 6.3 SRT No. of Anaerobic Tank 2 (Anaerobic Retention Time, hrs' 1.1 No. of Aerobic Tank 2 2.2 (Aerobic Retention Time, hrs: MLSS 2870 MLVSS 2290 T-eOD 207 S-COD 108 T-P 4.87 Influent P04-P 3.56 T-N 34.0 24.1 NH4-N S-COD 46.9 Last Anaerobic P04-P 18.7 Tank S-COD 34.3 0.03 P04-P T-N 24.5 Effluent 17.3 NH4-N 5.20 NOx-N Phosphorus Content of Sludge 4.69 (Last Aerobic 'hnk)
1-2 4.3 2 1.1 2 2.2 1860 1460 204 105 5.05 3.01 31.3 22.4 47.7 23.2 32.2 0.05 20.2 10.4 7.40
1-3 2.5 2 1.1 2 2.2 1270 1040 218 105 5.00 3.48 32.4 24.7 54.5 20.6 34.6 0.15 22.4 16.9 5.28
2-1 5.6 2 1.1 3 3.3 1030 818 159 72.4 4.20 2.54 25.3 18.8 38.3 7.43 29.7 1.99 12.7 0.84 11.2
2-4 7.7 4 2.2 3 3.3 2530 2150 242 115 4.86 3.79 44.6 32.8 59.0 21.2 39.8 0.03 32.3 24.9 5.43
2-5 5.9 4 2.2 3 3.3 1970 1640 228 112 4.69 3.48 43.5 32.7 57.3 20.8 46.1 0.44 27.9 18.0 10.0
3-1 4.7 3 1.7 2 2.2 2040 1740 226 102 4.59 3.43 41.0 30.6 50.5 18.0 44.4 0.40 28.1 25.3 2.73
5.41
5.15
3.6
4.3
4.1
4.2
Initial Parameter Settin!: The saturation concentration of oxygen was assumed to be 9 mgIL. The dissolved oxygen was assumed to be
oand 6 mgIL in the anaerobic and aerobic reactors, respectively. The temperature was not regularly measured
during the 16 months operation of the pilot plant. In the simulation, the temperature was set at 20 C for all the cases, although the actual temperature should have varied in the range of 15 - 25 C.
Wastewater Characterization Influent Total- and Soluble-COD was divided into the relevant COD fractions in the following way:
T. MIND til al.
114
= = = =
S, Effluent SCOD SF = (SCOD - SI> • 0.6 SA (SCOD - SI) • 0.4 (Assuming SA : SF 2 : 3) XAUT XPAO XPHA Xpp 0 XI (l'COD - SCOD)· 0.139 (Assuming XI : Xs : XH I : 5 : 1.2) Xs = (l'COD - SCOD) • 0.694 XH = (l'COD - SCOD) • 0.167
=
=
= =
=
The ratios of SA:SF and Xt:XS:XH were selected after "Typical wastewater composition" shown in the ASM 2 Report (Henze et al, 1994). As a result of the simulation conducted. it was found that the composition of soluble COD did not affect the calibration results very much. This is because the rate constant of fennentation process (conversion of SF to SA>. qFe, is large enough to supply sufficient amount of SA in the anaerobic phase. The composition of particulate COD. or the ratio of XI. affects the total S5 concentration rn the reactors. but the simulation results showed that the present ratio was reasonable. Inorganic parameters for the influent were decided as follows:
=
= =
S02 SNOx SN2 0 SP04 .. Measured influent P04-P SNH4 = Measured influent N~-N
The Nitrogen and Phosphorus contents of COD components (iNSt, iNXS. ipXH etc.) are parameters to be decided depending on the wastewater characteristics. In the present simulation. the default values of these parameters were simply applied without any calibration. Due to this. there was some inconsistency between the influent organic nitrogenlphosphorus concentrations measured and those in the model. This inconsistency did not significantly affect the values of the calibrated parameters after all. In general. however, the N and P contents of COD can affect the N and P balance in the system, and hence affect the estimation of kinetic parameters related to N and P metabolisms. Careful detennination may be necessary.
RESULTS Simulation by the Parameters recommended by the fAWO Task Group The results of simulation using the kinetic and stoichiometric parameters recommended by the IAWQ task Group (Henze et ai, 1994) are shown in Table 2. In this table, the sum of SF. SA and SI corresponds to the measured S-eOD. The model, with the default values, was able to predict the behavior of the process rather well except for NH4-N and NOx-N as shown in Table 2. Table 2 - Results of Simulation Using Default Parameters
SF SA SI SNH4 SNOx SP04
End of Anaerobic End of Anaerobic (Measured) (Simulated) 2.33 47.7 14.6 (S-COD) 32.0 20.1 0.0 23.2 22.8
-
SN2
4.34
XI XS XH XPAO XPP XPHA
707 82 1249 656
XAUT
64
70 96
-
-
-
-End of Aerobic (Simulated) 0.58 0.Q2 32.0
End of Aerobic (Measured)
16.9 0.Q2
1.2
10.4 7.4 0.05
4.63
.
694
-
44
1236 660 85 19 96
32.2 (S-COD)
-
-
-
IAWQ Activated Sludge Model
liS
Calibration Procedures In order to obtain better fitting simulation results, the following calibration procedures were introduced. 1. Adjustment of effluent NH4-N and NOx-N by changing specific growth rate of autotrophs (mAl!!') : When NOx-N. formed through nitrification. is carried over to the anaerobic zone with the return sludge. denitrification takes place there and PHA production by PAOs should be adversely affected. Therefore. kinetic parameters related to nitrification must be calibrated before the calibration for PAa s parameters is done. Since the saturation coefficient, KNH4. seems to have smaIl effects on nitrification in the present range of NH4-N. mAl!!' was tuned so that the observed SNH4 and SNo. could be achieved in the model simulation. 2. Adjustment of soluble COD in the anaerobic phase by changing the rate constant for the anaerobic PHA storage (qPttA>: The parameter qPHA directly determines the SA uptake in the anaerobic phase. So, this parameter was calibrated next by adjusting soluble COD (SA+SF+S.) at the end of anaerobic phase. 3. Adjustment of P04-P release in the anaerobic phase by changing the ratio of released phosphorus to PHA produced (YPQ4) : It is often reported that YPQ4 varies significantly case by case. The exact reasons for this variation still remain to be clarified. It is important to identify an appropriate YPQ4 for the system to which users of the present model are going to apply it. By adjusting SPQ4 concentration at the end of anaerobic phase. suitable YPQ4 values were determined for each run. Table 3 - Calibrated Parameters Default SRT No of anaerobic Tank No of Aerobic Tank ,uAUT qPHA
YP04 qPP
-
I-I 6.3 2
1-2
1-3
2-1
2-4
2-5
3-1
Average
7.7 5.9 4.3 2.5 5.6 4.7 4 2 4 2 2 3 2 2 3 3 3 2 2 I 0.623 0.738 0.988 0.670 0.802 0.615 0.728 3 2.44 3.90 4.60 4.80 2.59 2.25 2.85 0.15-0.4 0.205 0.340 0.315 0.190 0.14 0.220 0.175 0.55 0.56 0.475 I.5 1.00 1.15 0.41 1.12
-
-
-
0.738 3.22 0.229 0.752
Table 4 - An Example of Steady State Simulation Result Using Calibrated Parameters (Run 1-2) End of Anaerobic (Measured)
SF SA SI SNH4 SNOx
End of Anaerobic (Simulated) 2.87 11.7 32.0 22.4 0.0
SP04 SN2 XI XS XH XPAD XPP XPHA XAUT
End of Aerobic (Measured)
-
End of Aerobic (Simulated) 0.51 0.02 32.0 10.5 9.53
24.0
23.2
0.05
0.05
2.47
-
2.73
696 83 1I08 818 63 78 55
47.7 (S-COD)
-
-
682 45 lIOI 824 85 14 55
32.2 (S-eoO) 10.4 7.4
-
T. MIND ttl al.
116
4. Adjustment of effluent SPQ4 by changing polyphosphate accumulation rate by PAOs (qpp) : After adjusting the phosphate release in the anaerobic phase. it is necessary to adjust the phosphate uptake in the subsequent aerobic phase. which was done by changing qpp. These four steps are not independent to each other and one can affect others. So. the calibration was done not in a step-by-step manner but in such a way that the calibrated parameters were fed back to the steps before to re-calibrate parameters which had been estimated in the steps before.
Results of Calibration Calibration of relevant parameters was carried out following the above mentioned procedures. A summary of the calibrated parameters is given in Table 3. Table 4 shows an example of results of a steady state simulation using the calibrated parameters (Run 1-2). By applying the present procedures. Activated Sludge Model No.2 has been satisfactorily calibrated to simulate the steady state behavior of the process investigated.
DISCUSSION Hydrolysis Rate of Slowly Biodel:radable Substrate. Xs When the anaerobic PHA storage process is calibrated in step 2 of the above procedures. the rate constants for hydrolysis and fermentation in the anaerobic phase (Kh*hFc and qF. respectively. hFe is a reduction factor for anoxic hydrolysis) were fIxed. and qPHA was estimated. In the model, SA is supplied by fermentation of SF. which is produced from slowly biodegradable substrate. Xs. through hydrolysis. Therefore. it must be noted that the estimation of qPHA is more or less affected by qF and Kh*hFc. In ASM 2, the reduction factor for anoxic hydrolysis, hFc. is assumed to be o. I. This means that the rate of hydrolysis under anaerobic conditions is assumed to be only 10 % of that under aerobic conditions. Mino et al ( 1994) and San Pedro et al (1994) indicated that the rates of hydrolysis of starch and azocasein are not very much affected by the electron acceptor conditions and that they are hydrolyzed at similar rates under anaerobic. anoxic and aerobic conditions. Contradictory to the recommendation of ASM 2, the hydrolysis rate under the anaerobic conditions seems to be as high as that under aerobic conditions. If we set a hFe value much higher than the default one. qPHA should be estimated to be correspondingly higher than the value obtained in the present calibration. The hydrolysis rate of Xs can affect the calibration not only of qPHA but also of other parameters like PHA yield with respect to polyphosphate utilization. YPQ4. and denitrifIcation rate constant, mH*hNOJ' because PHA storage and denitrifIcation are processes which, directly or indirectly. depend on the supply of the organic substrate through the hydrolysis of Xs. In other words, the hydrolysis process can be the rate limiting step of the removal of organic matters or, sometimes. nitrogen and phosphorus. At the moment. it is rather risky to estimate Kh*hFc, qF. qPHA and YP04 as a whole only by using static data.
Nitrification Process The simulated effluent N0J-N was found to be very sensitive to mAl!!" in case of steady state calibration. although the calibrated mAl!!" does not vary very much. The effluent N0J-N can be adjusted by changing two parameters. mAl!!" and the saturation coefficient of nitrifIcation process. KNH4. In steady state calibrations using static data like the present case. KNH4 does not affect the steady state concentration of N0J-N very much. But it may strongly affect how fast the nitrifiers respond to a given condition in a dynamic situation. So. the observed high sensitivity of effluent N0J-N to mAl!!" seems to be a possible limitation of the calibration with static data.
IAWQ Activated Sludge Model
117
Enhanced Biological Phosphate Removal QJantitative prediction of phosphate removal is rather difficult because of the difference in the actual range of phosphate concentrations in enhanced biological phosphate removal processes and the desired level of effluent phosphate. But qualitative explanation of the behavior of phosphate accumulating organisms is possible by the model. Calibrated YPQ4 values in Table 3 have large diviation as expected, reflecting the experimental observation that the ratio of consumed organic substrates to released phosphate during the anaerobic PHA storage varies depending on wastewater characteristics. type of sludge or other unknown reasons. Usually. stoichiometric factors in mathematical models should not be changed case by case. Preferably. they have to be theoretically defmed or experimentally determined with acceptable accuracy. Since YPOl is an unstable stoichiometric factor, it is recommended that YPOl should not be considered as constant but that it should be calibrated for individual
cases.
In step 4 of the calibration procedures. qpp was determined by using the average values of SPOl at the end of the aerobic zone only. because data of SPQ4 profiles along with the wastewater stream or their temporal changes are not available in the present case. Kinetic parameters must be calibrated based on the "curve fitting" concept and. therefore. the estimated values of qpp in Table 3 may carry some inaccuracy.
Predictability of Model The present calibration was carried out with static data of the influent, the effluent from the anaerobic zone and the fmal effluent Since most of reaction rates are concentration dependent (Monod kinetics are applied) and processes are complicatedly related to each other. it should be noted that kinetic parameters may have a certain unavoidable inaccuracy. We need to have spacially or temporally dynamic data for more accurate and reliable calibration. namely either the profiles of variables along the stream of the process or the changes of variables with time. However. available data are more or less limited in actual cases. Appropriate procedures for model calibration should be developed according to the availability of data. The present case shows how relevant calibration procedures can be developed with limited static data. Although some limitations of model calibration with static or steady state data were found, the overall predictability of ASM 2 seems satisfactory after proper calibration. Mathematical models of activated sludge can be a powerful tool to understand the reaction mechanisms of different biomass and to predict the behaviors of the process under different situations. Accumulation of calibration studies will be further necessary with data from different activated sludge processes.
ACKNOWLEDGEMENT The authours would like to express their thanks to Dr. Tetsuro Fukase of Kurita Water Industries for providing with the plant operation data used in the present simulation. REFERENCES Fukase. T. (1986). A SlUdy on Phosphate Removal from Waslewater by the Anaerobic Aerobic Process. phD. thesis. University of Tokyo (in Japanese). Henze, M.• Grady. C. P. L. Jr.• Gujer, W.. M:ll'ais, O. v. R. and Mmsuo, T. (1987). Activated Sludge Model No. I. IAWPRC Scientific and Technical Report No. J,IAWPRC, London.ISSN 101O-707X. Henze. M.. Gujer. W.. MiRO. T.• Matsuo, T.. Wentzel. M. and Marais. G. v. R. (1995). Activaled Sludge Model No. 2.IAWQ Scientific and Technical Report. No. J JAWQ. London. Mino. T.. Satoh. H. and M.'IlSuo. T. (1994). Metabolism of different bocleria1 populations in enhanced biological phosphate removal processes. Wat. Sci. Tech., 29 (7). 67·70. Mino. Too San Pedro, D. C. and Matsuo, T. (1995). Estimation of the Rate of Slowly Biodegradable COD (SBCOD) under Anaerobic Anoxic and Aerobic Conditions by Experiments Uisng Starch as Model Substrate. Wat. Sci. Tech.. 31 (2).95•
103.
San Pedro. D. C.. Mino. T. and Matsuo. T. (1994). EvaJualioo of the Rate of Hydrolysis of SloWly Biodegradable COD (SBCOD) using SlarCh as Substrate under Anaerobic. Anoxic and Aerobic Conditions. Wat. Sci. Tech•• 30 (II), 191-199.
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T.
MINO~ral.
Saloh. H.• Mino. T. and Matsuo. T. (1992). Uptake of organic substrates and acc;umulalion of polyhydroxya1kanoales linked widl glycolysis of intrneellular carbohydrates under anaerobic condition in the biological excess phosphate removal processes. War. Sci. Tech.• Ui (5/6).933-942. Wentzel. M. C.• Dold. P. L.. Ekarna. G. A. and M,ums. G. v. R. (1990). Processes and Modelling of Nilrifacation Denitrification Biological Excess Phosphorus Removal Syslems - A Review. Wal. Sci. Tech.. 25. No.6. p59-82.