S. Abdul-Talib*, Z. Ujang**, J. Vollertsen*** and T. Hvitved-Jacobsen*** *Faculty of Civil Engineering, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia (E-mail:
[email protected]) **Institute of Environmental and Water Resources Management, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia (E-mail:
[email protected]) ***Department of Environmental Engineering, Institute of Life Sciences, Aalborg University, Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark (E-mail:
[email protected];
[email protected]) Abstract A two-stage anoxic transformation process, involving growth of biomass utilizing two types of different electron acceptors, namely nitrate and nitrite, has been observed. The present water quality modules established for sewer processes cannot account for the two-stage process. This paper outlines the development of a model concept that enables the two-stage anoxic transformation process to be simulated. The proposed model is formulated in a matrix form that is similar to the Activated Sludge Models and Sewer Process Model matrices. The model was successfully applied to simulate changes in nitrate and nitrite concentrations during anoxic transformations in the bulkwater phase of municipal wastewater. Keywords Anoxic transformation model; in-sewer processes; nitrate utilization rate; nitrite utilization rate
Water Science & Technology Vol 52 No 3 pp 181–189 Q IWA Publishing 2005
Model concept for nitrate and nitrite utilization during anoxic transformation in the bulk water phase of municipal wastewater under sewer conditions
Introduction
Significant progress has been made in developing fundamental knowledge on kinetics of microbial transformations during transport of wastewater in sewer networks. A model concept has been established for transformations occurring in sewer networks under aerobic and anaerobic conditions (Vollertsen et al., 2002). Investigations on insewer transformation processes in the bulkwater phase under anoxic conditions have been initiated by Abdul-Talib et al. (2002). These investigations were directed towards studies on kinetics of anoxic transformation processes in the bulk water phase of municipal wastewater. These studies have established that anoxic transformation of wastewater occurs in two stages with nitrate and nitrite serving as electron acceptors. These observations do not fit with the existing water quality modules in sewer modelling describing anoxic transformations used in the AEROSEPT model (Matos, 1992; Mourato et al., 2003) and the modified ASM No. 3 (Huisman, 2001). Thus, the objective of this paper is to introduce a model concept for simulating nitrate and nitrite utilization during the two-stage anoxic transformation process in municipal wastewater under sewer conditions.
Materials and methods
A series of tests were conducted to study kinetics of anoxic transformation process in raw wastewater taken from the sewerage monitoring station at Frejlev, Denmark. Detailed description of the sampling location has been described by Abdul-Talib et al. (2002).
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S. Abdul-Talib et al. Figure 1 Batch reactor used for determining nitrate utilization rate
Nitrate and nitrite Utilization Rate Test
Nitrate and nitrite utilization rates (NUR) were determined using batch reactors shown in Figure 1. Detail of experimental procedures used to determine NUR has been described in Abdul-Talib et al. (2002). In total, tests on 12 different wastewater samples were conducted, five experiments were performed under conditions of excess electron acceptor and excess electron donor while the remaining seven experiments were performed under conditions of limited electron acceptor and excess electron donor. For each experiment, four reactors were used simultaneously. Results
A typical variation of nitrate and nitrite concentrations during the anoxic transformation process is shown in Figure 2. Anoxic transformation was observed to occur in two stages in all samples. In Stage I, nitrate was utilized with an accumulation of nitrite, while in Stage II nitrite was utilized after nitrate had been depleted. Since the accumulation of nitrite did not equal the initial concentration of nitrate, it can be deduced that nitrite was utilized simultaneously with nitrate during stage I. Having observed the experimental evidence of the two-stage anoxic transformation process, there is a need to develop a model that can simulate the nitrate and nitrite utilization rates. The proposed model was developed in a manner that is compatible
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Figure 2 Typical variation of nitrate and nitrite concentrations during anoxic transformation in the bulkwater phase
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Figure 3 Model concept on anoxic transformation in the bulk water phase of municipal wastewater under sewer conditions
with the existing WATS model developed by Aalborg University (Vollertsen et al., 2002).
Model development
The two-stage anoxic transformation process is schematically shown in Figure 3. It is suggested that anoxic growth occurs first on nitrate and nitrite and then on nitrite alone. In raw wastewater, the electron donors required for the transformation processes were obtained from the readily biodegradable substrate, SS, already present or from SS that was hydrolysed from fast hydrolysable substrate, XS,fast and slow hydrolysable substrate, XS,slow. An anoxic transformation matrix, similar to Activated Sludge Models (ASM) and the Wastewater Aerobic/anaerobic Transformations in Sewers model (WATS) was developed to model the observed two-stage anoxic transformation process. The matrix relating various anoxic transformation processes to the biomass, XBw, electron donors and electron acceptors cannot be directly written as the units used for electron donors and electron acceptors are different. Nitrate concentrations, SNO3 and nitrite concentrations, SNO2 were measured in g NO3-Nm23 and NO2-Nm23 respectively while XBw was measured in g CODm23. A common unit, such as electron equivalent (e-eq.), must be adopted for both electron acceptor and donor so that the stoichiometry can be correctly specified in the anoxic transformation matrix (Abdul-Talib et al., 2004). The transformation of 1 mole of nitrate undergoing reduction to nitrite involved 2 moles of electrons while the transformation of 1 mole of nitrite undergoing reduction to nitrogen involved 3 moles of electrons. The oxidation of 1 mole of the electron donor, measured in terms of oxygen demand, involved 4 moles of electrons. The derivation of the conversion factors is summarized in Table 1. Table 2 gives the matrix describing the processes, components and stoichiometry for anoxic transformations of organic matter in sewers that can be written. Monod type expressions, similar to those used in the Activated Sludge Models and the WATS model Table 1 Conversion factors used to establish stoichiometry for anoxic transformation process in the bulk water phase of municipal wastewater under sewer conditions Processes
Change in XBw (COD)4 e-eq./32 g O2
2 2 Change in NO2 3 (NO3 to NO2 ) 2 e-eq./14 g NO3-N
ð2e 2 eq:=14 g NO3 2 NÞ ¼ 1:14 ð4 e 2 eq:=32 g O2 Þ
2 Change in NO2 2 (NO2 to N2) 3 e-eq./14 g NO3-N
ð3 e 2 eq:=14 g NO2 2 NÞ ¼ 1:71 ð4 e 2 eq:=32 g O2 Þ 183
Anoxic growth on nitrite in Stage II
3 Nomenclature Symbol SNO3 SNO2 N2 SS XBw mHNO3I
Note: The factors 1.14 and 1.71 have units of [g N (m3)] [g COD (m3)]. Only processes in the bulk water phase utilizing the electron acceptors are shown.
KO2 KS YH
KNO2#
KNO3#
mHNO2II
mHNO2I
Anoxic growth on nitrite in Stage I
2
Description Concentration of nitrate Concentration of nitrite Molecular nitrogen Readily biodegradable substrate Heterotrophic active biomass Max. specific growth rate for XBw, on nitrate in Stage I Max. specific growth rate for XBw, on nitrite in Stage I Max. specific growth rate for XBw, on nitrite in Stage II Half saturation constant for nitrate during Stages I and II Half saturation constant for nitrite during Stages I and II Half saturation constant for oxygen Half saturation constant for SS Yield const. for heterotrophic biomass under anoxic conditions
Anoxic growth on nitrate in Stage I
1
Process
Component j
Table 2 Model concept for nitrate and nitrite utilization in the bulk water phase under sewer conditions
1 2 YH 1:14Y H
SNO2
2
1 2 YH 1:14Y H 1 2 YH 2 1:71Y H 1 2 YH 2 1:71Y H
þ
[g O2m23] [g CODm23] [g CODm23]/[g CODm23]
[g NO2-Nm23]
[g NO3-Nm23]
[d21]
[d21]
Units [g NO3-Nm23] [g NO2-Nm23] ppm [g CODm23] [g CODm23] [d21]
2
SNO3
1 N2
3
1 2 YH 1:71Y H 1 2 YH þ 1:71Y H þ
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Equation 3
Equation 2
Equation 1
Process rate
described by Henze (2000) and Vollertsen et al. (2002), were assumed to govern growth processes. With this assumption, process rate Equations 1 –3 describing the anoxic growth on nitrate in Stage I, the anoxic growth on nitrite in Stage I and the anoxic growth on nitrite in Stage II respectively can be written. K O2 SNO3 SS X Bw K O2 þ SO2 K NO3 I þ SNO3 K S þ SS
ð1Þ
mH;NO2 ;I
K O2 SNO3 SNO2 SS X Bw K O2 þ SO2 K NO3 I þ SNO3 K NO2 I þ SNO2 K S þ SS
ð2Þ
K NO3 II K O2 SNO3 SNO2 SS X Bw K NO3 II þ SNO3 K O2 þ SO2 K NO3 I þ SNO3 K NO2 II þ SNO2 K S þ SS
ð3Þ
mH;NO2 ;II
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mH;NO3 ;I
All measurements on NUR in this study were observed under conditions of excess substrates and free of oxygen. Several simplifications of terms used in Equations 1 to 3 arising from these conditions are summarized in Table 3. Making the appropriate simplifications given in Table 3 to Equations 1 to 3, the simplified Equations 4 to 6 were written. mHNO3I
SNO3 X Bw K NO3I þ SNO3
ð4Þ
mHNO2I
SNO3 SNO2 X Bw K NO3I þ SNO3 K NO2 I þ SNO2
ð5Þ
K NO3II SNO2 X Bw K NO3 II þ SNO3 K NO2 II þ SNO2
ð6Þ
mHNO 2 II
Thus by applying the simplified process rate Equation 4, c.f. Table 2, the change in nitrate concentration over time during the anoxic transformation process can be expressed as Equation 7. Similarly, an expression for the change in nitrite concentration can be written as Equation 8. Both differential equations were solved and written in finite difference form, given by Equations 9 and 10. These equations enable variations of SNO3 and SNO2 during anoxic transformations under conditions of excess substrate to be simulated, provided that the model parameters can be evaluated. dSNO3 1 2 YH SNO3 X Bw ¼2 mHNO3 I dt 1:14Y H K NO3 I þ SNO3
ð7Þ
Table 3 Simplifications to the process rate equations associated with conditions imposed during anoxic transformation process Conditions Rate Equations
Excess substrate
Absence of O2
Equations 1, 2 and 3
SS K S þSS
K O2 K O2 þSO2
¼1
¼1 185
25.5 51.0 50.0 47.0 92.5 30.0 48.0 43.0 82.0 54.0 65.0 34.0
#XBw [g CODm23]
2.00 0.97 (0.15) 0.97 (0.06) 1.27 (0.25) 2.30 1.17 (0.21) 0.60 (0.09) 0.67 (0.06) 0.38 (0.03) 0.72 (0.04) 0.62 (0.02) 1.03 (0.06)
mHNO3I [d21]
0.4 0.32 (0.26) 0.12 (0.03) 0.7 (0.62) 1.27 (0.46) 0.33 (0.12) 0.27 (0.14) 0.28 (0.08) 0.25 (0.05) 0.52 (0.16) 0.35 (0.17) 0.70 (0.17)
mHNO2I [d21]
2.67 (0.06) 0.97 (0.06) 1.20 (0.35) 2 (0.2) 1.03 (0.25) 1.57 (0.67) 0.88 (0.13) 1.3 1.12 (0.24) 1.37 (0.45) 1.33(0.45) 2.53 (0.46)
mHNO2II [d21]
0.8 0.6 0.6 0.8 0.8 1.0 1.0 0.8 0.8 0.8 0.8 0.8
p KNO3I [gNO3-Nm23]
0.5 0.5 0.5 0.3 0.4 0.4 0.2 0.5 0.3 0.5 0.5 0.5
KNO3II [gNO3-Nm23]
0.3 0.3 0.3 0.3 0.4 0.4 0.3 0.3 0.3 0.2 0.2 0.2
KNO2I [gNO2-Nm23]
Note: Numbers in brackets give the std. dev. of the average values determined from three measured NURs. Where no brackets are shown, std. dev. are zero. # determined directly from OUR experiments. p determined from laboratory experiments described by Abdul-Talib (2002). YH for all samples taken as 0.37 [g CODm23]/[g CODm23], determined from laboratory experiments described in Abdul-Talib (2002).
L01_150800 L02_180800 L03_210800 L04_250800 L05_260800 L06_290800 L07_310800 H01_280600 H02_300600 H03_030700 H04_050700 H05_070700
Samples
Table 4 Model parameters used in simulating measured nitrate and nitrite concentrations during anoxic transformation processes
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0.3 0.3 0.3 0.3 0.4 0.4 0.3 0.3 0.3 0.2 0.2 0.2
KNO2II [gNO2-Nm23]
SNO3 ðtÞ ¼ SNO3 ðt 2 1Þ 2
1 2 YH YH
1 SNO3 ðtÞ mHNO3 I X Bw ðtÞ Dt 1:14 K NO3I þ SNO3 ðtÞ
1 2 YH YH
ð9Þ
1 SNO3 ðtÞ mHNO3 I X Bw ðtÞ 1:14 K NO3I þ SNO3 ðtÞ 1 SNO3 ðtÞ SNO2 ðtÞ mHNO2I X Bw ðtÞ 2 1:71 K NO3 I þ SNO3 ðtÞ K NO2I þ SNO2 ðtÞ 1 K NO3 II SNO2 ðtÞ mHNO2II X Bw ðtÞ Dt 2 1:71 K NO3II þ SNO3 ðtÞ K NO2 II þ SNO2 ðtÞ
SNO2 ðtÞ ¼ SNO2 ðt 2 1Þ þ
ð8Þ
Figure 4 Typical simulation of the measured NUR using the anoxic transformation model
S. Abdul-Talib et al.
dSNO2 1 2 YH SNO3 X Bw ¼þ mHNO3I dt 1:14Y H K NO3I þ SNO3 1 2 YH SNO3 SNO2 X Bw 2 mHNO2I 1:71Y H K NO3I þ SNO3 K NO2 I þ SNO2 1 2 YH K NO3 II SNO2 2 X Bw mHNO2II 1:71Y H K NO3II þ SNO3 K NO2II þ SNO2
ð10Þ
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The model parameters were either determined directly through experiments or implicitly by curve fitting during calibration of the model. Parameters determined directly include the heterotrophic biomass, XBw, the anoxic yield constant for heterotrophic biomass, YH, and the half saturation constant for nitrate, KNO3I, while the remaining parameters were determined through curve fitting. Detailed description of procedures to determine all the model parameters is given by Abdul-Talib (2002). Table 4 gives the recommended values of the model parameters in simulating the observed nitrate and nitrite utilization during anoxic transformation processes. Typical results as shown in Figure 4 indicate that the proposed model is able to simulate the observed two-stage anoxic transformation processes well. At present, full integration into the WATS model cannot be implemented as the transformation model proposed in this study only handles transformations in the bulk water, focusing on utilisation of nitrate and nitrite during the two-stage anoxic transformation process. A complete anoxic transformation model would require process rate equations to describe: i. nitrate utilisation for growth in bulk water during Stage I ii. nitrite utilisation for growth in bulk water during Stages I and II iii. nitrate utilisation for growth in biofilm during Stage I iv. nitrite utilisation for growth in biofilm during Stages I and II v. SS utilisation for growth in bulk water and biofilm during Stages I and II vi. hydrolysis of XS,fast and XS,slow into SS during Stages I and II vii. growth of biomass in bulk water and biofilm during Stages I and II viii. maintenance energy requirement for the biomass in bulk water and biofilm.
Concluding remarks
The proposed model was developed and formulated in a matrix form that is compatible with the ASM and WATS models. Model parameters were established either directly through specially designed laboratory experiments or through curve fitting, during model calibration. The model is capable of simulating nitrate and nitrite concentrations during anoxic transformation process in the bulkwater phase of municipal wastewater.
References
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Abdul-Talib, S. (2002). In Sewer Processes: Transformation of organic matter under anoxic conditions. PhD Thesis, unpublished, Faculty of Civil Engineering, Universiti Teknologi Malaysia. Abdul-Talib, S., Hvitved-Jacobsen, T., Vollertsen, J. and Ujang, Z. (2002). Anoxic transformation of wastewater organic matter in sewers – process kinetics, model concept and wastewater treatment potential. Wat. Sci. Tech., 45(3), 53 – 60. Abdul-Talib, S., Ujang, Z., Vollertsen, J. and Hvitved-Jacobsen, T. (2004). Electron transfer rates and energy releases during denitrification of municipal wastewater. In Water Environmental Management Series: Advancement on water and wastewater- Application in the Tropics, IWA Publishing, London, United Kingdom. Henze, M. (ed.) (2000). The activated sludge models (1, 2, 2 d and 3). IWA Scientific and Technical Report, IWA Publishing, London, United Kingdom, by the IWA Task Group on Mathematical Modelling for Design and Operation of Biological Wastewater Treatment. Huisman, J.L. (2001). Transport and transformation processes in combined sewers. PhD Thesis, Swiss Federal Institute of Technology, Zurich, Switzerland, unpublished.
Matos, J.S. (1992). Aerobiose e septicidade em sistemas de drenagem de aguas residuas. Lisboa. PhD Thesis (in Portuguese). Mourato, S., Matos, J.S., Almeida, M. and Hvitved-Jacobsen, T. (2003). Modelling in-sewer pollutant degradation processes in the Costa do Estoril sewer system. Wat. Sci. Tech., 47(4), 93 – 100. Vollertsen, J., Hvitved-Jacobsen, T., Ujang, Z. and Abdul-Talib, S. (2002). Integrated design of sewers and wastewater treatment plants. Wat. Sci. Tech., 46(9), 11 – 20.
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