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ABSTRACT Laboratory scale experiments operated by the EBPR (enhanced biological phosphorus removal) process with coagulant addition were carried out to clarify the effects of coagulant on phosphorus uptake and release as well as the stabilization of phosphorus removal efficiency in the EBPR process using sequencing batch reactors. Phosphorus was removed without Fe addition, however, standard deviation of effluent PO4-P concentration was high and the phosphorus removal performance was generally unstable. Phosphorus removal was stable with the Fe addition. The PO4-P concentrations at the end of the anaerobic phase with Fe addition were lower than those concentrations without Fe addition. The phosphorus uptake rates at the aerobic phase with Fe addition were higher than those concentrations without Fe addition. These indicated that coagulant addition could decrease the phosphorus concentration at the end of anaerobic phase and enhance the phosphorus uptake rate during aerobic phase. These contributed to the stabilization of the phosphorus removal in the EBPR process. Addition of Fe with Fe/P molar ratio of 0.2 was enough to stabilize the phosphorus removal efficiency. The molar ratio was extremely small compared to the usual amount of the coagulant added in the activated sludge process.
Journal of Water and Environment Technology, Vol. 8, No.4, 2010 Address correspondence to Iori Mishima, Water Environment Group, Water and Geo-Environment Division, Center for Environmental Science in Saitama, Email: mishima.iori@pref.saitama.lg.jp Received May 10, 2010, Accepted August 20, 2010. - 383 - Effect of Coagulant on Phosphorus Uptake and Release in EBPR Process Iori MISHIMA*, Jun NAKAJIMA** *Water Environment Group, Water and Geo-Environment Division, Center for Environmental Science in Saitama, 914 Kamitanadare, Kazo 347-0115, Japan **Department of Environmental Systems Engineering, Faculty of Science and Engineering, Ritsumeikan Univ., Nojihigashi 1-1-1, Kusatsu 525-8577, Japan ABSTRACT Laboratory scale experiments operated by the EBPR (enhanced biological phosphorus removal) process with coagulant addition were carried out to clarify the effects of coagulant on phosphorus uptake and release as well as the stabilization of phosphorus removal efficiency in the EBPR process using sequencing batch reactors. Phosphorus was removed without Fe addition, however, standard deviation of effluent PO 4 -P concentration was high and the phosphorus removal performance was generally unstable. Phosphorus removal was stable with the Fe addition. The PO 4 -P concentrations at the end of the anaerobic phase with Fe addition were lower than those concentrations without Fe addition. The phosphorus uptake rates at the aerobic phase with Fe addition were higher than those concentrations without Fe addition. These indicated that coagulant addition could decrease the phosphorus concentration at the end of anaerobic phase and enhance the phosphorus uptake rate during aerobic phase. These contributed to the stabilization of the phosphorus removal in the EBPR process. Addition of Fe with Fe/P molar ratio of 0.2 was enough to stabilize the phosphorus removal efficiency. The molar ratio was extremely small compared to the usual amount of the coagulant added in the activated sludge process. Keywords: activated sludge process, coagulation, EBPR, stabilization of phosphorus removal. INTRODUCTION The EBPR (enhanced biological phosphorus removal) process has been widely used to remove phosphorus from wastewater. The process can simultaneously remove nitrogen with the combination of nitrification and denitrification reaction. However, phosphorus removal efficiency in the EBPR process sometimes decreases due to the lack of strict anaerobic condition caused by the decrease of BOD load. Therefore, it is necessary to stabilize the phosphorus removal efficiency by introducing some improvements in this process. On the other hand, the activated sludge process with coagulant addition shows stable phosphorus removal efficiency. However, the high coagulant cost and the generation of large amount of excess sludge are the disadvantages of this process. It is generally known that phosphorus removal with coagulant results from the formation of metal phosphate salts as follows (Metcalf and Eddy, 1991), Me 3+ + PO 4 3- → MePO 4 (1) This equation shows that 1 mole of metal coagulant will fundamentally combine with 1 mole of phosphorus. However, the coagulant addition of more than 1 molar ratio is required because some parts of coagulant ions combine with hydroxide ions to form hydroxide compounds. In addition, Fe 3+ , Al 3+ and PO 4 3- ions are not dominant at neutral Journal of Water and Environment Technology, Vol. 8, No.4, 2010 - 384 - pH (Packham,1962; Rittmann and McCarty, 2001; Mishima and Nakajima, 2003a). Therefore, the added coagulant ions would become insoluble particulates and accumulated in the activated sludge process (Nakajima and Mishima, 2004). On the other hand, the smaller amounts of coagulant dosage sometimes result in a good efficiency of phosphorus removal in the EBPR process. Moreover, supplementary coagulant is sometimes added to the EBPR process to stabilize the efficiency of phosphorus removal (Janssen et al., 2002). Mishima and Nakajima (2003b, c) reported that the performance of phosphorus removal was kept high and stable when large amounts of coagulant remained in the sludge even after the cease of coagulant addition. Therefore, the combination of the biological phosphorus removal by EBPR process and the chemical phosphorus removal by the coagulant addition would remove phosphorus economically and efficiently. However, the optimum amount of coagulant to be added to the combination process where biological phosphorus uptake and release occurs has not yet been clarified. In this study, laboratory-scale experiments operated by the EBPR process with coagulant addition were carried out and the stabilization of phosphorus removal efficiency was investigated. The objective of this study is to clarify the effect of coagulant on phosphorus uptake and release as well as the stabilization of phosphorus removal efficiency in the EBPR process from the viewpoint of the coagulant content in the activated sludge. MATERIALS AND METHODS Experimental apparatus Two laboratory-scale sequencing batch reactors (SBR1 and SBR2) were operated in a cycle of 8 h, which consisted of 5 phases; 1 h influent, 3 h anaerobic, 2.5 h aerobic, 1 h settling and 0.5 h discharge. The duration of the aerobic phase was shorter compared with the anaerobic phase, and these conditions were the setting to expect instability of phosphorus removal. The volume of each reactor was 6 L. Two liters of synthetic wastewater mainly containing acetate and glucose (components are listed in Table 1) was flowed into each reactor during the influent phase. The concentrations of influent BOD, nitrogen and phosphorus were 200 mg/L, 50 mg/L and 5mg/L, respectively. Two liters of the supernatant was discharged during the discharge phase. The activated sludge taken from an existing wastewater treatment plant was seeded in each reactor and cultured for about 3 months. The mixed liquor, with a volume of 100 mL or 200 mL, was removed as the excess sludge every day. Both reactors were operated for about 130 days, at a temperature of 25°C. Iron was added into the reactor as coagulant by electrolysis (Dobolyi, 1978; Moriizumi et al., 1999, 2000, 2001). Two iron electrodes (cathode and anode) were placed in the reactor and direct current was allowed to elute iron ions during aerobic phase. Ferrous ion was produced from the cathode to the bulk and oxidized to ferric ion by dissolved oxygen. Ferric ion could combine with phosphorus, therefore phosphorus removal from wastewater would be accomplished. By controlling the amount of electric current, the Fe/P molar ratio in SBR1 was set at 0, 0.2, 0.5 and 1.0 for Run1-1, Run 1-2, Run 1-3 and Run1-4, respectively. In SBR2, the Fe/P molar ratio was set at 0 and 0.2 for Run2-1 Journal of Water and Environment Technology, Vol. 8, No.4, 2010 - 385 - Table 1 - Synthetic wastewater CH 3 COONa∙3H 2 O 270 mg/L C 6 H 12 O 6 200 mg/L KH 2 PO 4 22 mg/L (NH 4 ) 2 SO 4 230 mg/L NaCl 3.3 mg/L CaCl 2 ∙2H 2 O 2.2 mg/L MgSO 4 ∙7H 2 O 2.1 mg/L NaHCO 3 220 mg/L Table 2 - Operational conditions in SBR1 and SBR2 Period Run1-1 Run1-2 Run1-3 Run1-4 Operational time (d) 0-35 45-75 79-111 116-126 Fe/P molar ratio - 0.2 0.5 1.0 Period Run2-1 Run2-2 Operational time (d) 0-35 45-75 Fe/P molar ratio - 0.2 76-126 - SBR1 SBR2 Run2-3 and Run2-2, respectively, and after that, Fe addition was stopped for Run2-3. Details of the experiments are shown in Table 2. Analytical methods The mixed liquor and effluent were taken from each reactor 2 times in a week and mixed liquor suspended solids (MLSS), mixed liquor volatile suspended solids (MLVSS), total phosphorus (T-P) and total iron (T-Fe) of the activated sludge were measured. The pH, suspended solids (SS), dissolved biochemical oxygen demand (D-BOD) (1 time in a week), T-P, PO 4 -P (3 times in a week), total nitrogen (T-N) and T-Fe concentrations in the effluent were also measured. The mixed liquor was also taken from each reactor during anaerobic phase and aerobic phase. It was immediately filtered and the PO 4 -P and NO 3 -N in the filtrate were measured. After digestion of organics using nitric acid and hydrogen peroxide, T-Fe was determined by inductively coupled plasma emission spectrometry (SPS4000, Seiko, Japan). Quantitative analyses of the other parameters were measured according to JIS K0102. RESULTS AND DISCUSSION Summary of Experimental Results The mean and standard deviation of MLSS, SRT, effluent SS, D-BOD and T-N are shown in Table 3. By the removal of the activated sludge as the excess sludge, MLSS of each SBR were maintained approximately 3000 mg/L and SRT was about 30 days in each reactor. Effluent SS and D-BOD were less than 20 mg/L and 3 mg/L, respectively. Effluent T-N mainly consisted of NO 3 -N was 13.5 mg/L and 9.50 mg/L, in SBR1 and SBR2, respectively. These results showed that the removal of the organics and nitrogen were effectively done in each reactor and Fe addition had no negative effect on them. Journal of Water and Environment Technology, Vol. 8, No.4, 2010 - 386 - Table 3 - Experimental results for the activated sludge and effluent in SBR1 and SBR2 SBR1 3000 ± 380 32 ± 11 17 ± 8.4 1.9 ± 1.7 13.5 ± 2.86 SBR2 3100 ± 460 31 ± 9.9 18 ± 5.0 2.2 ± 2.5 9.50 ± 4.29 Activated sludge Effluent D-BOD (mg/L) T-N (mg/L)MLSS (mg/L) SS (mg/L)SRT (day) 0 1 2 3 4 5 0 20406080100120 time (day) Effluent PO 4 -P (mg/L) Run1-1 Run1-2 Run1-3 Run1-4 Fig. 1 - Change in PO 4 -P concentration in the effluent of SBR1 Phosphorus removal in SBR1 The PO 4 -P concentration in the effluent of SBR1 in Run1-1 to Run1-4 is shown in Fig. 1. In Run1-1, the mean and standard deviation of PO 4 -P concentration were 1.44 ± 1.12 mg/L and PO 4 -P concentration was sometimes below 0.5 mg/L. These indicate that phosphorus was well removed without coagulant addition because of the occurrence of the EBPR. High standard deviation, however, implied that phosphorus removal was generally unstable. In Run1-2, the mean and standard deviation of PO 4 -P concentration was 0.09 ± 0.08 mg/L. The maximum PO 4 -P concentration in this run was 0.31 mg/L and phosphorus was considerably removed. In addition, the phosphorus removal was stable. In Run1-3 and Run1-4, effluent PO 4 -P concentration was always below 0.1 mg/L and phosphorus was extensively removed. These indicated that phosphorus removal was enhanced and stabilized by Fe addition. The typical results of the PO 4 -P concentration in the filtrate during the anaerobic phase and the aerobic phase in Run1-1 to Run1-4 are shown in Fig. 2. In the anaerobic phase, PO 4 -P concentration increased to a certain value and remained constant in each run. In Run1-1, PO 4 -P concentration in the anaerobic phase was higher than that in the other runs and the maximum PO 4 -P concentration at the end of the anaerobic phase was higher than 25 mg/L. PO 4 -P concentration at the end of anaerobic phase in Run1-2 and Run1-3 was about 10 mg/L, and about 7 mg/L in Run1-4. At the aerobic phase, PO 4 -P concentration was decreased. Finally, PO 4 -P concentration at the aerobic phase became 0.24 mg/L, 0.04 mg/L and 0.04 mg/L in Run1-2, Run1-3 and Run1-4, respectively. However, PO 4 -P concentration at the end of the aerobic phase of Run1-1 remained more than 1.0 mg/L. It was reported that the NO 3 -N remaining during the anaerobic phase controlled the phosphorus release from the activated sludge bacteria and the phosphorus removal efficiency in the EBPR process deteriorated (Kuba et al., 1993; Kuba et al., 1994; Arnz Journal of Water and Environment Technology, Vol. 8, No.4, 2010 - 387 - 0 10 20 30 0 50 100 150 200 250 300 PO 4 -P in the filtra te (mg/L) time (min) Run1-1 (14days) Run1-2 (56days) Run1-3 (89days) Run1-4 (119days) Ana erobic pha se Aerobic pha se Fig.2 - Change in PO 4 -P concentration in the anaerobic and aerobic phase et al., 2001). In this experiment, nitrogen was removed efficiently as mentioned above according to the ratio of the discharge to total volume in the SBR system. Although NO 3 -N of about 1 mg/L remained at the beginning of the anaerobic phase, NO 3 -N was immediately removed by denitrification within 10 minutes. So, it was thought that the remaining NO 3 -N did not have an effect on the phosphorus removal efficiency. On the other hand, it was reported that the competition of PAOs (poly-phosphate accumulating organisms) and GAOs (glycogen-accumulating organisms) occurred in the EBPR process to utilize organics and the phosphorus removal efficiency deteriorated (Tasli, et al., 1997; Mino et al., 1998). Moreover, it was reported that the enhanced biological phosphorus removal ability deteriorated in SBR using acetate and glucose (Cech and Hartman, 1990). Acetate and glucose were fed to the activated sludge in this experiment, therefore it was thought that both PAOs and GAOs were cultured in the SBR tank and the competition of acetate uptake by PAOs and GAOs occurred. So, it was thought the phosphorus removal was deteriorated and unstable. The stabilization of phosphorus removal was supposed to be influenced by Fe addition which had indirect effects on the biological phosphorus uptake and release during the anaerobic and aerobic phases. Therefore, three key points of phosphorus behavior namely, the initial phosphorus release rate (v rel ), phosphorus concentration at the end of the anaerobic phase (C max ) and phosphorus uptake rate constant at the aerobic phase (k upt ), were estimated and discussed. The v rel was calculated as the gradient of phosphorus increase during first 10 minutes in anaerobic phase while k upt was calculated as the rate constant of phosphorus decrease based on the first order reaction during 2.5 h in aerobic phase. The effect of Fe addition on biological phosphorus uptake and release was discussed by comparing these indices. Fig. 3 shows the average and standard deviation of v rel , C max and k upt which values were calculated from 7, 9, 9 and 3 data in Run1-1, Run1-2, Run1-3 and Run1-4, respectively. It was observed that v rel was almost constant around 0.8 kg/m 3 /d and there were no significant differences on v rel between Run1-1 to Run1-4. On the other hand, C max in Run1-2 to Run1-4 was lower than that in Run1-1 and significant difference between Run1-1 and Run1-2 to Run1-4 was observed. The value of k upt in Run1-1 was in the same range as that of Run1-2 and there was no significant difference between Run1-1and Run1-2. However, k upt in Run1-3 and Run1-4 Journal of Water and Environment Technology, Vol. 8, No.4, 2010 - 388 - 0.0 0.5 1.0 1.5 v rel (kg/m 3 /d) 0 5 10 15 20 25 30 C max (mg/L) 0 40 80 120 160 200 k upt (1/d) Fig.3 - v rel (left), C max (center) and k upt (right) in Run1-1 - Run1-4 was obviously higher than that in Run1-1 and Run1-2 and there was significant difference among these runs. These results indicated that Fe addition could decrease the phosphorus concentration at the end of the anaerobic phase and enhance the phosphorus uptake rate during the aerobic phase. The Fe content in the activated sludge (CFe; mg/g) in Run1-1 (without Fe addition), Run1-2 (Fe/P=0.2), Run1-3 (Fe/P=0.5) and Run1-4 (Fe/P=1.0) are shown in Fig. 4. CFe in Run1-1 to Run1-4 was 1.8 ± 0.5 mg/g, 35 ± 4.1 mg/g, 66 ± 11 mg/g and 112 ± 12 mg/g, respectively, and CFe in each run was stable. With Fe addition by iron electrolysis method, CFe was increased. The relationship between CFe and C max , k upt is shown in Fig. 5. The value of C max was immediately decreased and k upt was gradually increased due to the increase of CFe. Mishima and Nakajima (2003a) reported that the insoluble and accumulated coagulant in the activated sludge removed soluble phosphorus from wastewater. It was thought that the added Fe accumulated in the activated sludge in the reactor and removed the soluble phosphorus in this experiment. There were 2 kinds of phosphorus in the activated sludge which were biologically taken by the activated sludge bacteria and chemically combined with Fe. The former can be released in the anaerobic condition but the latter cannot be done. The chemically combined phosphorus in the activated sludge was increased by the Fe addition. It was, therefore, implied that C max was slightly decreased and k upt was increased by the accumulated Fe in the activated sludge. This indicated that Fe in the activated sludge contributed to the stabilization of phosphorus removal. Phosphorus removal in SBR2 The PO 4 -P concentration in the effluent of SBR2 in Run2-1 to Run2-3 is shown in Fig. 6. In Run2-1, the mean and standard deviation of PO 4 -P concentration were 0.48 ± 0.63 mg/L and phosphorus was well removed without coagulant addition. However, PO 4 -P concentration was sometimes increased to about 2 mg/L and phosphorus removal performance was generally unstable. The PO 4 -P concentration was always below 0.2 mg/L and the phosphorus removal was stable in Run2-2. These indicated that Fe addition of the Fe/P molar ratio of 0.2 could stabilize the phosphorus removal and the amount of coagulant to be required for this purpose was very small compared to the usual coagulant addition of the Fe/P molar ratio of more than 1. Addition of Fe was stopped in Run2-3 and the PO 4 -P concentration before 90 days was less than 1.0 mg/L. Journal of Water and Environment Technology, Vol. 8, No.4, 2010 - 389 - 0 20 40 60 80 100 120 140 Run1-1 Run1-2 Run1-3 Run1-4 CFe (mg/g) 0 50 100 150 200 0 5 10 15 20 25 30 0 20406080100120140 kupt (1/d) C max (mg/L) CFe (m g/g) Fig.4 - CFe in Run1-1 to Run1-4 Fig.5 - Relationship between CFe and C max , k upt 0 1 2 3 4 5 0 20 40 60 80 100 120 time (day) Effluent PO 4 -P (mg/L) Run2-1 Run2-2 Run2-3 Fig.6 - Change in PO 4 -P concentration in the effluent of SBR2 However, the PO 4 -P concentration was increased to 4 mg/L and the phosphorus removal performance was deteriorated. Mishima and Nakajima (2003b) reported that CFe could be calculated by equation (2), where u was the amount of Fe to be added into the activated sludge (mg/d), S was the total amount of SS in the aeration tank (g) and CFe 0 was CFe at the beginning of the operation (mg/g). CFe = u · SRT / S + (CFe 0 - u·SRT / S)·exp(-t / SRT) (2) The change of CFe in Run2-3 and the relationship between CFe and k upt in Run2-3 are shown in Fig. 7 and Fig. 8, respectively. In Fig. 7, CFe was decreased exponentially after the addition of Fe was stopped. The calculated CFe using equation (2) agreed with the measured value, which indicated that CFe could be controlled by using equation (2). The value of k upt was decreased corresponding to the decrease of CFe (Fig. 8). As mentioned above, phosphorus removal performance was decreased after 90 days. It was reported that the amount of coagulant in the activated sludge was decreased by the removal of excess sludge after the cease of coagulant addition (Mishima and Nakajima, 2003c). On the other hand, the relationship between CFe and v rel , C max is shown in Fig. 9 and a clear tendency of v rel and C max against CFe was not observed in Run2-3. Therefore, it can be thought that the amount of Fe in the activated sludge was decreased by the removal of excess sludge, and k upt was decreased, then the phosphorus removal Journal of Water and Environment Technology, Vol. 8, No.4, 2010 - 390 - 0 10 20 30 70 90 110 130 CFe (m g/g) time (d) Measured Calculated with equation (2) Run2-3 0 10 20 30 40 50 60 0 10 20 30 k upt (1/d) CFe (m g/g) Fig. 7 - Change in CFe in Run2-3 Fig.8 - Relationship between CFe and k upt 0 5 10 15 20 25 0.0 0.2 0.4 0.6 0.8 1.0 0102030 C max (mg/L) v rel (kg/m 3 /d) CFe (m g/g) v rel C max Fig. 9 - Relationship between CFe and v rel , C max in Run2-3 performance decreased. From the stoichiometric point of view based on equation (1), the Fe addition of the Fe/P molar ratio of 0.2 could remove a maximum of 1 mg/L of phosphorus from the influent phosphorus concentration of 5 mg/L. However, the phosphorus concentration in effluent was less than 0.2 mg/L in Run2-2. On the other hand, Fe content in the SBR tank was about 22 mg/g in Run2-2. In this case, the Fe/P molar ratio of the Fe concentration in the SBR tank to the phosphorus concentration in influent was about 7. Therefore, enough Fe was accumulated in the activated sludge and they would contribute to the stabilization of biological phosphorus removal. These indicated that the stabilization of phosphorus removal could be achieved by the control of coagulant content in the activated sludge. The phosphorus removal performance fluctuated when Fe was not added in Run2-1 and Run2-3. The change of phosphorus concentration in the effluent was different between these runs. It was thought that one reason for this phenomenon was the shift of microbial community. Further research will be required to clarify this shift due to coagulant addition using the molecular biological technique. Journal of Water and Environment Technology, Vol. 8, No.4, 2010 - 391 - CONCLUSIONS Laboratory-scale experiments operated by the EBPR process with the coagulant addition were carried out and the stabilization of phosphorus removal efficiency was investigated. Although phosphorus was removed biologically, phosphorus removal performance was generally unstable without coagulant addition. The coagulant addition could decrease the phosphorus concentration at the end of the anaerobic phase and enhance the phosphorus uptake rate during the aerobic phase. Phosphorus removal was stabilized by the coagulant addition and the amount of coagulant to be required for this purpose was very small compared to the usual amount of the coagulant added in the activated sludge process. The stabilization of phosphorus removal can be achieved by the control of coagulant content in the activated sludge. REFERENCES Arnz P., Arnold E. and Wilderer P. A. (2001). 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Tasli R., Artan N. and Orhon D. (1997). The influence of different substrates on enhanced biological phosphorus removal in a sequencing batch reactor, Wat. Sci. Tech., 35(1), 75-80. . Technology, Vol. 8, No.4, 2010 - 386 - Table 3 - Experimental results for the activated sludge and effluent in SBR1 and SBR2 SBR1 3000 ± 380 32 ± 11 17 ± 8. 4 1.9. Environment Technology, Vol. 8, No.4, 2010 - 388 - 0.0 0.5 1.0 1.5 v rel (kg/m 3 /d) 0 5 10 15 20 25 30 C max (mg/L) 0 40 80 120 160 200 k upt (1/d) Fig.3