- Email: [email protected]
O) process treating low strength wastewater
Desalination 249 (2009) 822–827
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Performance evaluation of a modified anaerobic/anoxic/oxic (A2/O) process treating low strength wastewater Jie Fan a,⁎, Tao Tao a, Jing Zhang a, Gui-lin You b,1 a b
School of Environmental Science and Engineering, Huazhong University of Science and Technology, 430074, Wuhan, China Anqing Wastewater Treatment Plant, 246003, Anqing, China
a r t i c l e
i n f o
Article history: Accepted 3 March 2009 Available online 4 October 2009 Keywords: Modified A2/O process Nitrogen and phosphorus removal Low strength wastewater
a b s t r a c t A full scale modified A2/O process which combined pre-anoxic selector and the staging strategy treating low strength wastewater was investigated. In South China, domestic wastewater is always low in strength due to the high level of groundwater and setting of septic tank at the beginning of wastewater collection system. The results suggested that inadequate denitrification could result in deterioration of phosphorus removal. In addition, influent phosphorus concentration had effect on phosphorus removal. The pre-anoxic selector in modified A2/O process changed the distribution of nitrogen denitrified in different tanks. Characteristics of 3stage aeration tanks were also studied. The simplified design of rectangular aeration tank could also perform as plug flow as conventional channel aeration tank. In 3-stage aeration tanks, mixed liquid suspended solid (MLSS) increased from one tank to another, while specific oxygen uptake rate (SOUR) of sludge, chemical oxygen demand (COD) and total phosphorus (TP) removal rate decreased, however ammonia nitrogen (NH3-N) and nitrate nitrogen (NO3-N) reaction rate remained constant. Furthermore, high MLSS concentration was not suitable for treating low strength wastewater. Waste sludge discharge could improve removal efficiency of COD, NH3-N, and TP. Without waste sludge discharge, nitrite accumulated in settler. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Wastewater containing excess nitrogen and phosphorus lead to eutrophication. In order to reduce such pollution in receiving waters, biological nutrient removal (BNR) processes have been widely used in the existing wastewater treatment plant (WWTP) for integrated removal of carbon, nitrogen and phosphorus due to its economic advantages compared with chemical treatment methods. The BNR processes include the five-stage Bardenpho process, the University of Cape Town (UCT) process, the anaerobic–anoxic–oxic (A2/O) process, etc. Among these processes, the most commonly used process is the A2/O process [1]. It is a single-sludge suspended growth system incorporating anaerobic, anoxic, and oxic zone in sequence. In the oxic zone, nitrification occurs and the produced nitrate is recycled to the anoxic zone, where denitrification takes place. The return sludge is recycled from the secondary settler to the anaerobic zone where organic carbon is take up by phosphate-accumulating organisms (PAOs), accompanied by the release of phosphate into wastewater. But, there is a conflict in such a system. Nitrate nitrogen in return sludge has a negative effect on anaerobic phosphorus release. To solve this problem, a pre-anoxic zone before anaerobic zone was set to reduce nitrate nitrogen that is entering anaerobic zone. Domenec et al.
[2] stated that anaerobic/anoxic selector could result in an increase in soluble orthophosphate release. Paolo [3] stated that initial anoxic contact zones could control filament bulking. Furthermore, some researchers [4–6] state that stage configuration of tanks can form concentration gradient and promote reaction rate that is a key to improve performance. In this study, the oxic zone consisted of 3 rectangular aeration tanks. In South China, the domestic wastewater is always low in strength due to the high level of groundwater and setting of septic tank at the beginning of wastewater collection system. The performance of a BNR system is strongly affected by the characteristics of influent wastewater. For low strength wastewater, the shortage of organic carbon may affect nutrient removal. A modified A2/O process consisted of pre-anoxic/anaerobic/anoxic/3-stage oxic tanks was applied in Anqing WWTP to treat low strength wastewater. The main objectives of this study were to evaluate: (1) nutrient removal performance of modified A2/O process typical of treating low strength influent; (2) role of pre-anoxic selector; (3) staging strategy of oxic tank; and (4) trial for optimizing operation. 2. Materials and methods 2.1. Wastewater treatment plant
⁎ Corresponding author. Tel.: +86 27 87792152; fax: +86 27 87792104. E-mail address: [email protected] (J. Fan). 1 Tel./fax: +86 556 5527684. 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.03.015
The WWTP studied was designed to treat a flow of 12 × 104m3/d, 80% of domestic wastewater and 20% of industry wastewater. A
J. Fan et al. / Desalination 249 (2009) 822–827
823
Fig. 1. Schematic diagram of the Anqing WWTP.
complete description of the configuration was shown in Fig. 1, 90% of influent was fed to anaerobic tank while 10% to pre-anoxic selector. Oxic zone consisted of 3 identical rectangular tanks, 24.8 × 38.6 m of each, whose rectangular design was few in practice. Aeration was provided by pressurized air passing through porous membrane aerators. The return sludge from the settler was recycled to pre-anoxic selector. A summary of operating conditions was presented in Table 1. The characteristics of influent wastewater were shown in Table 2. Considering low concentration of influent nitrogen and economic operation, the nitrate recirculation ratio was controlled at a low level.
aeration tank 1 and 2 with 10 min interval. Samples were filtered before conductivity measurement through which the concentration of sodium chloride could be analyzed based on conductivity–concentration standard curve. Finally, a residence time distribution (RTD) curve could be drawn with the outlet concentration data. Nondimensional variance (σθ2) was used to describe the mixing condition. σθ2 indicated flow was near PFR (σθ2 = 0) or CSTR (σθ2 = 1). σθ2 was calculated according to Eqs. (1)–(4)[7]. CðtÞ
EðtÞ =
ð1Þ
∞
∑ CðtÞΔt 0
2.2. Tracer test Tracer test was to describe the flow behavior in aeration tanks. Sodium chloride was used as tracer. Sodium chloride of 20 kg was previously diluted in tap water of 400 L, and quickly injected to the inlet of aeration tank 1. Samples were collected from the outlet of
∞
t=
∑ tEðtÞΔt
0 ∞
2 σt
Table 1 Operating conditions of this WWTP. Parameter Flow rate (m3/h) HRT (h)
Nitrate recirculation ratio Sludge recycle ratio SRT (d) MLSS (mg/L) DO (mg/L) F/M (kg BOD5/kg MLSS d)
ð2Þ
0 ∞
∑ EðtÞΔt 2
∑ t EðtÞΔt =
0
∞
∑ EðtÞΔt
2 −t
ð3Þ
0
Value Pre-anoxic Anaerobic Anoxic Oxic Settling 0.5 1.0 15 1200 3–4 0.12
1871 0.5 1.5 2.0 6.5 3.6
σ2 2 σθ = t2 t
ð4Þ
2.3. Specific oxygen uptake rate (SOUR) test 400 mL of mixed liquid was collected in each aeration tank, respectively, aerated adequately and washed by distilled water in order to
Table 2 Characteristics of influent wastewater. Parameter
Average concentration (mg/L)
COD BOD5 TN TP NH3-N NO3-N NO2-N SS pH
185 52 16 1.56 9.96 2.58 0.011 86 7.61
Fig. 2. SOUR test vessel.
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J. Fan et al. / Desalination 249 (2009) 822–827
Table 3 Removal of COD, NH3-N, TN, TP, and SS.
Influent (mg/L) Effluent (mg/L) Removal efficiency (%)a a
COD
NH3-N
TN
TP
SS
172 32.4 81 ± 2.0
10.79 0.32 97 ± 3.4
16.35 12.30 25 ± 9.1
1.34 0.62 53 ± 6.5
74 15 80 ± 3.5
mean ± standard deviations, sample number = 30.
consume the residual carbon source in wastewater, and then 200 mL of influent wastewater was added. The vessel was full and airtight, and then stirred. Dissolved oxygen (DO) was measured (Fig. 2), and then SOUR could be calculated according to Eq. (5). SOUR =
DOO −DOt MLVSS⋅t
ð5Þ
2.4. Analytical methods According to standard methods [8], chemical oxygen demand (COD), mixed liquor suspended solid (MLSS), mixed liquor volatile suspended solid (MLVSS), NH3-N, NO3-N, NO2-N, TN, TP, and PO43−-P were measured. DO and temperature were measured using a Cellox 325-3 oxygen probe. pH was measured using PB-10 meter. Conductivity was measured using DDSJ-308A conductivity meter.
Fig. 4. Effluent NO3−-N and TP.
1 mg/L. The TP removal ranged between 25 and 43% for influent TP below 1mg/L, and 35–72% for influent TP above 1 mg/L. It indicated that influent TP concentration had an impact on phosphorus removal. Ros M [9] stated that P removal was related to P concentration in the original wastewater. Chang H A et al. [10] stated that along with decreased P/COD, phosphate-accumulating metabolism was weakened.
3.2. Role of pre-anoxic selector
Table 3 summarized the influent and effluent concentrations of COD, NH3-N, TN, TP, and SS, and their average removal efficiencies were 81, 97, 25, 53, and 80%, respectively. The TN removal was very low. Considering the influent BOD5/COD was about 0.3, it was necessary to estimate whether carbon source was adequate for denitrification. The effect of influent COD/TN on TN removal was shown in Fig. 3. When influent COD/TN increased from 10 to 15, TN removal was almost the same, so it demonstrated that TN removal was not limited by carbon source. The low TN removal was mainly due to the low nitrate recirculation ratio (nitrate recirculation ratio was 0.5). Fig. 4 showed the effluent NO3−-N and TP. It could be seen that along with increase of effluent NO3−-N, the effluent TP also increased. This was because nitrate nitrogen in return sludge could also increase as effluent NO3−-N increased. In anaerobic zone, nitrate nitrogen competed carbon source with phosphorus release, which would cause the inadequate release of phosphate, and then PAOs function of phosphate release and uptake was affected. In this way, inadequate denitrification of the process led to deterioration of phosphorus removal. On the other hand, it could be seen from Fig. 5 that TP removal increased when influent TP increased from below 1 mg/L to above
The return sludge recycled from settler first entered the selector where denitrification occurred, and then entered anaerobic tank. The removal of nitrate and nitrite nitrogen by selector was shown in Fig. 6. Average removal efficiency of nitrate and nitrite nitrogen was 35%, and 30%, respectively. But the nitrate removal was more stable than nitrite removal, and the nitrate removal efficiency ranged between 21% and 54% while nitrite removal efficiency ranged between 16% and 69%.The pre-anoxic selector weakened the negative effect of NO3-N, and NO2-N on phosphorus release, because the presence of nitrate and nitrite did not permit anaerobic conditions to prevail. The ammonia nitrogen and pH along the reactor were shown in Fig. 7. It could be seen that there was a rapid decrease of ammonia nitrogen and pH in pre-anoxic selector caused by dilution. The pH of pre-anoxic selector was 7.1 (optimum pH for denitrification was between 7.5 and 7.8) which to some extent limited the denitrification performance in selector. According to mass balance, the mass of nitrogen denitrified in different tanks were calculated and presented in Table 4. Compared with results reported by Xu [11] which adopted conventional A2/O process, the ratio of nitrogen denitrified in anoxic tank had little difference, however, ratio of nitrogen denitrified in anaerobic tank decreased from 48.7% to 4.8%. It indicated that denitrification was shared by pre-anoxic selector, thus the anaerobic tank could focus on phosphorus release.
Fig. 3. Effect of influent COD/TN on TN removal.
Fig. 5. Effect of influent TP on TP removal.
3. Results and discussion 3.1. Nutrient removal
J. Fan et al. / Desalination 249 (2009) 822–827
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Fig. 6. Removal of NO3-N, and NO2-N by selector.
rates both decreased, from 11 mg/L h to 6.6 mg/L h, and 0.15 mg/L h to 0.05 mg/L h, respectively. However, NO3-N and NH3-N reaction rates remained constant. That was, with staging, heterotrophic reaction rate decreased whereas autotrophic reaction rate was unchanged. 3.4. Effect of MLSS on nutrient removal
Fig. 7. Typical variation of ammonia nitrogen and pH along the reactor.
3.3. Characteristics of 3-stage aeration tank MLSS concentration of each aeration tank was shown in Fig. 8. Average MLSS concentration of aeration tanks was 1270, 1336, and 1830 mg/L, respectively. This uneven mass distribution was also mentioned by Pai TY [12] and You SJ [13]. This phenomenon may be caused by liquid mixing condition. The mixing condition in aeration tank was described by σθ2 shown in Fig. 9. The σθ2 of aeration tank 1 and 2 was 0.31, and 0.25, which indicated that with staging the flow pattern was closer to ideal plug flow (σθ2 = 0). The changed mixing condition changed the biomass distribution. Moreover, the σθ2 in each aeration tank was near plug flow instead of CSTR. It indicated that the simplified design of rectangular aeration tank (L = 1.58 B) could also perform as plug flow reactor as conventional channel aeration tank (L ≥ 5 – 10 B), The reason why it could perform as plug flow might be due to its aeration system shown in Fig. 10. SOUR of each aeration tank shown in Fig. 11 was 11.57, 8.47, and 7.44 mgO2/gMLSS h, respectively. The consumed oxygen consisted of oxidation of external carbon (in the wastewater), internal carbon (in the sludge), and nitrification. Considering the same volume of added wastewater, the oxygen consumed for oxidation of external carbon and nitrification were the same, thus the difference of consumed oxygen might be caused by internal carbon, i.e. oxidation of polyβ-hydroxybutyrate (PHB). The COD, TP, NO3-N, and NH3-N concentrations of each aeration tank were shown in Fig. 12. With staging, COD and PO43−-P removal
MLSS concentration had a direct impact on process performance. Germain et al. [14] stated that MLSS was found to be the main parameter controlling the oxygen transfer. Gulnur [15] stated that MLSS concentration, influent BOD5 and temperature were the most influential factors on nitrification performance. In order to select a suitable MLSS concentration for operation, two MLSS concentrations were selected for trial, 1200 mg/L (in March) and 3500 mg/L (in April), controlled by different discharge amounts of wasted sludge. Water temperature of the two months was over a range of 12–16°C, and 14– 18 °C, respectively. Variation caused by little temperature difference was ignored. Nutrient removal performance under the two different MLSS concentrations was compared in Fig. 13. When MLSS concentration increased from 1200 to 3500 mg/L, COD, and NH3-N removal slightly increased from 77% to 80%, and from 97% to 98%, whereas TN
Fig. 8. MLSS concentration in each aeration tank.
Table 4 The mass of nitrogen denitrified in different tanks.
Modified A2/O A2/O[11]
Selector
Anaerobic
Anoxic
94 kg/d (44.8%) –
10 kg/d (4.8%) 540 mg/d (48.7%)
106 kg/d (50.5%) 568 mg/d (51.3%)
( ): the ratio of its denitrified nitrogen to total denitrified nitrogen.
Fig. 9. RTD curve of aeration tank 1 and 2.
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J. Fan et al. / Desalination 249 (2009) 822–827
Fig. 10. Configuration of aeration tank.
Fig. 13. Effect of MLSS on nutrient removal.
Fig. 11. SOUR of every aeration tank.
removal decreased from 17% to 13%, and TP removal decreased from 55% to 32%, thus the optimum MLSS was 1200 mg/L. High MLSS concentration indicated long sludge retention time (SRT) which was disadvantageous for phosphorus removal. Besides, BOD5 sludge ratio decreased from 0.12 to 0.05 kg BOD5/kg MLSS d led to the shortage of carbon source which may result in starvation and death of PAOs, and then phosphorus released by the death of PAOs would increase the phosphorus in the effluent. Shehab [16] and Converti [17] stated that a decrease in suspended solids concentration was shown to improve TP removal. 3.5. Effect of without sludge discharge on nutrient removal In December 2007, MLSS increased very slowly and it demonstrated that growth of microorganisms was equal to bacteriolysis, so waste sludge was not discharged in order to maintain MLSS at 1200 mg/L in aeration tank. In March 2008, waste sludge was discharged. The temperature of the two months was the same, 12–16 °C, so data of the two months were selected to analyze the effect of sludge discharge on nutrient removal. The nutrient removal of the two months was compared in Fig. 14. When waste sludge was discharged, process performance was improved. COD removal increased from 70% to 77%, TP removal increased from 31% to 55%, NH3-N removal increased from 91% to 97%, while TN removal was not obviously
Fig. 14. Effect of waste sludge discharge on nutrient removal.
influenced. The performance difference between with and without waste sludge discharge was possibly due to that as SRT increased, sludge mainly consisted of inert material whereas active biomass and stored substrate became very low[18]. Under the condition of without sludge discharge, SRT was longer than with sludge discharge. Longer SRT was theoretically favored for nitrobacteria whose generation time was long, but the decreased NH3-N removal indicated that the condition of without sludge discharge was not actually favored for nitrobacteria. The extra-cellular polymers (ECP) of activated sludge increased resulting in decrease of nitrobacteria activity [19]. Nitrogen variation in settler with/without sludge discharge was shown in Fig. 15. Sludge discharge caused a nitrogen variation in settler. It was obvious to notice that there was a nitrite accumulation under the condition of without sludge discharge. This phenomenon did not occur under the condition of with sludge discharge. Without sludge discharge, the sludge bed height in settler was higher than with sludge discharge, which led to low DO in sludge bed resulting in partial nitrification. With sludge discharge, the low DO condition in sludge bed was destroyed by sludge discharge pump, and nitrite was all converted to nitrate.
Fig. 12. COD, PO4 3−-P, NO3-N, and NH3-N of each aeration tank.
J. Fan et al. / Desalination 249 (2009) 822–827
[3]
[4] [5]
[6] [7] [8]
[9] Fig. 15. Nitrogen variation in settler with/without waste sludge discharge.
4. Conclusions
[10]
[11] [12]
In this study, a full scale modified A2/O process was evaluated for treating low strength wastewater (BOD/TN = 3, BOD/TP = 26). Influent TP concentration had effect on TP removal, the results showed that when influent TP was below 1mg/L average TP removal decreased by 23% compared with influent TP above 1mg/L. The nitrate recycle ratio was controlled very low which caused high concentration of effluent NO3-N and also effluent TP. Pre-anoxic selector reduced NO3-N and NO2-N by 35%, and 30%, respectively. However, the removal of NO3-N was more stable than NO2N. Selector led to a decline of nitrogen denitrified in anaerobic tank compared with a conventional A2/O process. Anaerobic tank in modified A2/O process was only responsible for phosphorus release instead of combination function of phosphorus release and denitrification. The simplified design of rectangular aeration tank (L = 1.58 B) could also perform as plug flow reactor as conventional channel aeration tank. With staging, MLSS in aeration tank increased from one tank to another, and this might be due to a change of flow mixing condition. SOUR of sludge, COD and PO43−-P removal rates in 3-stage aeration tank were degressive whereas NO3−-N, and NH3-N reaction rates were constant. High MLSS concentration was not suitable for treating low strength wastewater, because the decreased BOD-sludge loading would lead to shortage of carbon source and deterioration of TP removal. MLSS concentration had little effect on COD, NH3-N, and TN removal. Discharge of waste sludge improved COD, NH3-N, and TP removal by 7%, 6%, and 24%. References [1] Metcalf, Eddy (Eds.), Wastewater Engineering: Treatment, Disposal, Reuse, 3rd, McGraw-Hill, New York, 1991. [2] J. Domenec, A.A. Mitch, M. Matina, et al., Effects of anaerobic selector hydraulic retention time on biological foam control and enhanced biological phosphorus
[13] [14] [15]
[16]
[17]
[18]
[19]
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removal in a pure-oxygen activated sludge system, Water Environment Research 79 (5) (2007) 472–478. Paolo Madoni, D. Davoli, Microorganisms responsible for foaming in a full-scale activated sludge plant running with initial aerobic or anoxic contact zones, Bioresource Technology 60 (1996) 43–49. Sedlak (Ed.), Phosphorus and Nitrogen Removal from Municipal Wastewater, The soap and detergent association, New York, 1991. LourdesR. Mayor, J.V. Camacho, F.F. Morales, Operational optimization of pilot scale biological nutrient removal at the Ciudad Real (Spain) domestic wastewater treatment plant, Water, Air, and Soil Pollution 152 (2004) 279–296. S.E. Scuras, A. Jobbagy, C.P. Grady, Optimization of activated sludge reactor configuration: kinetic considerations, Water Research 35 (18) (2001) 4277–4284. Levenspiel Octave (Ed.), Chemical Reaction Engineering, 3rd edition, Wiley, New York, 1998. APHA, AWWA (Eds.), Standard Methods for the Examination of Water and Wastewater, American Public Health Association/American Water Works Association, Washington, 1995. M. Ros, J. Vrtovsed, The study of nutrient balance in sequencing batch reactor wastewater treatment, Acta Chimica Slovenica 51 (4) (2004) 779–785. H.A. Chang, H.D. Park, J.K. Park, Enhanced biological phosphorus removal performance and microbial population changes at high organic loading rates, Journal of Environmental Engineering-ASCE 133 (10) (2007) 962–969. Wei-Feng Xu, Research on ASM2D simulation and mechanism of biological nutrient removal, PhD dissertation of Tongji University, 2006. T.Y. Pai, Y.P. Tsai, Y.J. Chou, et al., Microbial kinetic analysis of three different types of EBNR process, Chemosphere 55 (2004) 109–118. S.J. You, C.F. Ouyang, Simultaneous wastewater nutrient removal by a novel hybrid bioprocess, Journal of Environmental Engineering 131 (2005) 883–891. E. Germain, F. Nelles, A. Drews, et al., Biomass effects on oxygen transfer in membrane bioreactors, Water Research 41 (2007) 1038–1044. C. Gulnur, S.J. Majeed, Development of a correlation to study parameters affecting nitrification in a domestic wastewater treatment plant, Chemical Technology and Biotechnology 83 (2008) 299–308. O. Schehab, R. Derninger, F. Porta, et al., Optimizing phosphorus removal at the Ann Arbor wastewater treatment plant, Water Science and Technology 34 (1996) 493–499. A. Converti, M. Rovatti, M. D. Borghi, Biological removal of phosphorus from wastewater by alternating aerobic and anaerobic conditions, Water Research 29 (1995) 263–269. M. Sperandio, M.C. Espinosa, Modelling an aerobic submerged membrane bioreactor with ASM models on a large range of sludge retention time, Desalination 231 (2008) 82–90. Hong-yan Li, G.M. Chun, Y. Qing-xiang, Activity of nitrifiers and metabolized products in a membrane bioreactor MBR under the condition of non-sludge discharge, Environmental Science 25 (2004) 89–91.
Glossary Symbols and abbreviations A2/O: Anaerobic–anoxic–oxic MLSS: Mixed liquid suspended solid concentration, mg/L MLVSS: Mixed liquid volatile suspended solid concentration, mg/L SOUR: Specific oxygen uptake rate, mgO2/gMLSS h RTD: Residence time distribution E(t): Exit-age distribution function, s− 1 t : Average residence time, s σt 2: Variance, s2 σθ2: Nondimensional variance SRT: Sludge retention time, d
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Performance evaluation of a modified anaerobic/anoxic/oxic (A2/O) process treating low strength wastewater Jie Fan a,⁎, Tao Tao a, Jing Zhang a, Gui-lin You b,1 a b
School of Environmental Science and Engineering, Huazhong University of Science and Technology, 430074, Wuhan, China Anqing Wastewater Treatment Plant, 246003, Anqing, China
a r t i c l e
i n f o
Article history: Accepted 3 March 2009 Available online 4 October 2009 Keywords: Modified A2/O process Nitrogen and phosphorus removal Low strength wastewater
a b s t r a c t A full scale modified A2/O process which combined pre-anoxic selector and the staging strategy treating low strength wastewater was investigated. In South China, domestic wastewater is always low in strength due to the high level of groundwater and setting of septic tank at the beginning of wastewater collection system. The results suggested that inadequate denitrification could result in deterioration of phosphorus removal. In addition, influent phosphorus concentration had effect on phosphorus removal. The pre-anoxic selector in modified A2/O process changed the distribution of nitrogen denitrified in different tanks. Characteristics of 3stage aeration tanks were also studied. The simplified design of rectangular aeration tank could also perform as plug flow as conventional channel aeration tank. In 3-stage aeration tanks, mixed liquid suspended solid (MLSS) increased from one tank to another, while specific oxygen uptake rate (SOUR) of sludge, chemical oxygen demand (COD) and total phosphorus (TP) removal rate decreased, however ammonia nitrogen (NH3-N) and nitrate nitrogen (NO3-N) reaction rate remained constant. Furthermore, high MLSS concentration was not suitable for treating low strength wastewater. Waste sludge discharge could improve removal efficiency of COD, NH3-N, and TP. Without waste sludge discharge, nitrite accumulated in settler. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Wastewater containing excess nitrogen and phosphorus lead to eutrophication. In order to reduce such pollution in receiving waters, biological nutrient removal (BNR) processes have been widely used in the existing wastewater treatment plant (WWTP) for integrated removal of carbon, nitrogen and phosphorus due to its economic advantages compared with chemical treatment methods. The BNR processes include the five-stage Bardenpho process, the University of Cape Town (UCT) process, the anaerobic–anoxic–oxic (A2/O) process, etc. Among these processes, the most commonly used process is the A2/O process [1]. It is a single-sludge suspended growth system incorporating anaerobic, anoxic, and oxic zone in sequence. In the oxic zone, nitrification occurs and the produced nitrate is recycled to the anoxic zone, where denitrification takes place. The return sludge is recycled from the secondary settler to the anaerobic zone where organic carbon is take up by phosphate-accumulating organisms (PAOs), accompanied by the release of phosphate into wastewater. But, there is a conflict in such a system. Nitrate nitrogen in return sludge has a negative effect on anaerobic phosphorus release. To solve this problem, a pre-anoxic zone before anaerobic zone was set to reduce nitrate nitrogen that is entering anaerobic zone. Domenec et al.
[2] stated that anaerobic/anoxic selector could result in an increase in soluble orthophosphate release. Paolo [3] stated that initial anoxic contact zones could control filament bulking. Furthermore, some researchers [4–6] state that stage configuration of tanks can form concentration gradient and promote reaction rate that is a key to improve performance. In this study, the oxic zone consisted of 3 rectangular aeration tanks. In South China, the domestic wastewater is always low in strength due to the high level of groundwater and setting of septic tank at the beginning of wastewater collection system. The performance of a BNR system is strongly affected by the characteristics of influent wastewater. For low strength wastewater, the shortage of organic carbon may affect nutrient removal. A modified A2/O process consisted of pre-anoxic/anaerobic/anoxic/3-stage oxic tanks was applied in Anqing WWTP to treat low strength wastewater. The main objectives of this study were to evaluate: (1) nutrient removal performance of modified A2/O process typical of treating low strength influent; (2) role of pre-anoxic selector; (3) staging strategy of oxic tank; and (4) trial for optimizing operation. 2. Materials and methods 2.1. Wastewater treatment plant
⁎ Corresponding author. Tel.: +86 27 87792152; fax: +86 27 87792104. E-mail address: [email protected] (J. Fan). 1 Tel./fax: +86 556 5527684. 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.03.015
The WWTP studied was designed to treat a flow of 12 × 104m3/d, 80% of domestic wastewater and 20% of industry wastewater. A
J. Fan et al. / Desalination 249 (2009) 822–827
823
Fig. 1. Schematic diagram of the Anqing WWTP.
complete description of the configuration was shown in Fig. 1, 90% of influent was fed to anaerobic tank while 10% to pre-anoxic selector. Oxic zone consisted of 3 identical rectangular tanks, 24.8 × 38.6 m of each, whose rectangular design was few in practice. Aeration was provided by pressurized air passing through porous membrane aerators. The return sludge from the settler was recycled to pre-anoxic selector. A summary of operating conditions was presented in Table 1. The characteristics of influent wastewater were shown in Table 2. Considering low concentration of influent nitrogen and economic operation, the nitrate recirculation ratio was controlled at a low level.
aeration tank 1 and 2 with 10 min interval. Samples were filtered before conductivity measurement through which the concentration of sodium chloride could be analyzed based on conductivity–concentration standard curve. Finally, a residence time distribution (RTD) curve could be drawn with the outlet concentration data. Nondimensional variance (σθ2) was used to describe the mixing condition. σθ2 indicated flow was near PFR (σθ2 = 0) or CSTR (σθ2 = 1). σθ2 was calculated according to Eqs. (1)–(4)[7]. CðtÞ
EðtÞ =
ð1Þ
∞
∑ CðtÞΔt 0
2.2. Tracer test Tracer test was to describe the flow behavior in aeration tanks. Sodium chloride was used as tracer. Sodium chloride of 20 kg was previously diluted in tap water of 400 L, and quickly injected to the inlet of aeration tank 1. Samples were collected from the outlet of
∞
t=
∑ tEðtÞΔt
0 ∞
2 σt
Table 1 Operating conditions of this WWTP. Parameter Flow rate (m3/h) HRT (h)
Nitrate recirculation ratio Sludge recycle ratio SRT (d) MLSS (mg/L) DO (mg/L) F/M (kg BOD5/kg MLSS d)
ð2Þ
0 ∞
∑ EðtÞΔt 2
∑ t EðtÞΔt =
0
∞
∑ EðtÞΔt
2 −t
ð3Þ
0
Value Pre-anoxic Anaerobic Anoxic Oxic Settling 0.5 1.0 15 1200 3–4 0.12
1871 0.5 1.5 2.0 6.5 3.6
σ2 2 σθ = t2 t
ð4Þ
2.3. Specific oxygen uptake rate (SOUR) test 400 mL of mixed liquid was collected in each aeration tank, respectively, aerated adequately and washed by distilled water in order to
Table 2 Characteristics of influent wastewater. Parameter
Average concentration (mg/L)
COD BOD5 TN TP NH3-N NO3-N NO2-N SS pH
185 52 16 1.56 9.96 2.58 0.011 86 7.61
Fig. 2. SOUR test vessel.
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J. Fan et al. / Desalination 249 (2009) 822–827
Table 3 Removal of COD, NH3-N, TN, TP, and SS.
Influent (mg/L) Effluent (mg/L) Removal efficiency (%)a a
COD
NH3-N
TN
TP
SS
172 32.4 81 ± 2.0
10.79 0.32 97 ± 3.4
16.35 12.30 25 ± 9.1
1.34 0.62 53 ± 6.5
74 15 80 ± 3.5
mean ± standard deviations, sample number = 30.
consume the residual carbon source in wastewater, and then 200 mL of influent wastewater was added. The vessel was full and airtight, and then stirred. Dissolved oxygen (DO) was measured (Fig. 2), and then SOUR could be calculated according to Eq. (5). SOUR =
DOO −DOt MLVSS⋅t
ð5Þ
2.4. Analytical methods According to standard methods [8], chemical oxygen demand (COD), mixed liquor suspended solid (MLSS), mixed liquor volatile suspended solid (MLVSS), NH3-N, NO3-N, NO2-N, TN, TP, and PO43−-P were measured. DO and temperature were measured using a Cellox 325-3 oxygen probe. pH was measured using PB-10 meter. Conductivity was measured using DDSJ-308A conductivity meter.
Fig. 4. Effluent NO3−-N and TP.
1 mg/L. The TP removal ranged between 25 and 43% for influent TP below 1mg/L, and 35–72% for influent TP above 1 mg/L. It indicated that influent TP concentration had an impact on phosphorus removal. Ros M [9] stated that P removal was related to P concentration in the original wastewater. Chang H A et al. [10] stated that along with decreased P/COD, phosphate-accumulating metabolism was weakened.
3.2. Role of pre-anoxic selector
Table 3 summarized the influent and effluent concentrations of COD, NH3-N, TN, TP, and SS, and their average removal efficiencies were 81, 97, 25, 53, and 80%, respectively. The TN removal was very low. Considering the influent BOD5/COD was about 0.3, it was necessary to estimate whether carbon source was adequate for denitrification. The effect of influent COD/TN on TN removal was shown in Fig. 3. When influent COD/TN increased from 10 to 15, TN removal was almost the same, so it demonstrated that TN removal was not limited by carbon source. The low TN removal was mainly due to the low nitrate recirculation ratio (nitrate recirculation ratio was 0.5). Fig. 4 showed the effluent NO3−-N and TP. It could be seen that along with increase of effluent NO3−-N, the effluent TP also increased. This was because nitrate nitrogen in return sludge could also increase as effluent NO3−-N increased. In anaerobic zone, nitrate nitrogen competed carbon source with phosphorus release, which would cause the inadequate release of phosphate, and then PAOs function of phosphate release and uptake was affected. In this way, inadequate denitrification of the process led to deterioration of phosphorus removal. On the other hand, it could be seen from Fig. 5 that TP removal increased when influent TP increased from below 1 mg/L to above
The return sludge recycled from settler first entered the selector where denitrification occurred, and then entered anaerobic tank. The removal of nitrate and nitrite nitrogen by selector was shown in Fig. 6. Average removal efficiency of nitrate and nitrite nitrogen was 35%, and 30%, respectively. But the nitrate removal was more stable than nitrite removal, and the nitrate removal efficiency ranged between 21% and 54% while nitrite removal efficiency ranged between 16% and 69%.The pre-anoxic selector weakened the negative effect of NO3-N, and NO2-N on phosphorus release, because the presence of nitrate and nitrite did not permit anaerobic conditions to prevail. The ammonia nitrogen and pH along the reactor were shown in Fig. 7. It could be seen that there was a rapid decrease of ammonia nitrogen and pH in pre-anoxic selector caused by dilution. The pH of pre-anoxic selector was 7.1 (optimum pH for denitrification was between 7.5 and 7.8) which to some extent limited the denitrification performance in selector. According to mass balance, the mass of nitrogen denitrified in different tanks were calculated and presented in Table 4. Compared with results reported by Xu [11] which adopted conventional A2/O process, the ratio of nitrogen denitrified in anoxic tank had little difference, however, ratio of nitrogen denitrified in anaerobic tank decreased from 48.7% to 4.8%. It indicated that denitrification was shared by pre-anoxic selector, thus the anaerobic tank could focus on phosphorus release.
Fig. 3. Effect of influent COD/TN on TN removal.
Fig. 5. Effect of influent TP on TP removal.
3. Results and discussion 3.1. Nutrient removal
J. Fan et al. / Desalination 249 (2009) 822–827
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Fig. 6. Removal of NO3-N, and NO2-N by selector.
rates both decreased, from 11 mg/L h to 6.6 mg/L h, and 0.15 mg/L h to 0.05 mg/L h, respectively. However, NO3-N and NH3-N reaction rates remained constant. That was, with staging, heterotrophic reaction rate decreased whereas autotrophic reaction rate was unchanged. 3.4. Effect of MLSS on nutrient removal
Fig. 7. Typical variation of ammonia nitrogen and pH along the reactor.
3.3. Characteristics of 3-stage aeration tank MLSS concentration of each aeration tank was shown in Fig. 8. Average MLSS concentration of aeration tanks was 1270, 1336, and 1830 mg/L, respectively. This uneven mass distribution was also mentioned by Pai TY [12] and You SJ [13]. This phenomenon may be caused by liquid mixing condition. The mixing condition in aeration tank was described by σθ2 shown in Fig. 9. The σθ2 of aeration tank 1 and 2 was 0.31, and 0.25, which indicated that with staging the flow pattern was closer to ideal plug flow (σθ2 = 0). The changed mixing condition changed the biomass distribution. Moreover, the σθ2 in each aeration tank was near plug flow instead of CSTR. It indicated that the simplified design of rectangular aeration tank (L = 1.58 B) could also perform as plug flow reactor as conventional channel aeration tank (L ≥ 5 – 10 B), The reason why it could perform as plug flow might be due to its aeration system shown in Fig. 10. SOUR of each aeration tank shown in Fig. 11 was 11.57, 8.47, and 7.44 mgO2/gMLSS h, respectively. The consumed oxygen consisted of oxidation of external carbon (in the wastewater), internal carbon (in the sludge), and nitrification. Considering the same volume of added wastewater, the oxygen consumed for oxidation of external carbon and nitrification were the same, thus the difference of consumed oxygen might be caused by internal carbon, i.e. oxidation of polyβ-hydroxybutyrate (PHB). The COD, TP, NO3-N, and NH3-N concentrations of each aeration tank were shown in Fig. 12. With staging, COD and PO43−-P removal
MLSS concentration had a direct impact on process performance. Germain et al. [14] stated that MLSS was found to be the main parameter controlling the oxygen transfer. Gulnur [15] stated that MLSS concentration, influent BOD5 and temperature were the most influential factors on nitrification performance. In order to select a suitable MLSS concentration for operation, two MLSS concentrations were selected for trial, 1200 mg/L (in March) and 3500 mg/L (in April), controlled by different discharge amounts of wasted sludge. Water temperature of the two months was over a range of 12–16°C, and 14– 18 °C, respectively. Variation caused by little temperature difference was ignored. Nutrient removal performance under the two different MLSS concentrations was compared in Fig. 13. When MLSS concentration increased from 1200 to 3500 mg/L, COD, and NH3-N removal slightly increased from 77% to 80%, and from 97% to 98%, whereas TN
Fig. 8. MLSS concentration in each aeration tank.
Table 4 The mass of nitrogen denitrified in different tanks.
Modified A2/O A2/O[11]
Selector
Anaerobic
Anoxic
94 kg/d (44.8%) –
10 kg/d (4.8%) 540 mg/d (48.7%)
106 kg/d (50.5%) 568 mg/d (51.3%)
( ): the ratio of its denitrified nitrogen to total denitrified nitrogen.
Fig. 9. RTD curve of aeration tank 1 and 2.
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Fig. 10. Configuration of aeration tank.
Fig. 13. Effect of MLSS on nutrient removal.
Fig. 11. SOUR of every aeration tank.
removal decreased from 17% to 13%, and TP removal decreased from 55% to 32%, thus the optimum MLSS was 1200 mg/L. High MLSS concentration indicated long sludge retention time (SRT) which was disadvantageous for phosphorus removal. Besides, BOD5 sludge ratio decreased from 0.12 to 0.05 kg BOD5/kg MLSS d led to the shortage of carbon source which may result in starvation and death of PAOs, and then phosphorus released by the death of PAOs would increase the phosphorus in the effluent. Shehab [16] and Converti [17] stated that a decrease in suspended solids concentration was shown to improve TP removal. 3.5. Effect of without sludge discharge on nutrient removal In December 2007, MLSS increased very slowly and it demonstrated that growth of microorganisms was equal to bacteriolysis, so waste sludge was not discharged in order to maintain MLSS at 1200 mg/L in aeration tank. In March 2008, waste sludge was discharged. The temperature of the two months was the same, 12–16 °C, so data of the two months were selected to analyze the effect of sludge discharge on nutrient removal. The nutrient removal of the two months was compared in Fig. 14. When waste sludge was discharged, process performance was improved. COD removal increased from 70% to 77%, TP removal increased from 31% to 55%, NH3-N removal increased from 91% to 97%, while TN removal was not obviously
Fig. 14. Effect of waste sludge discharge on nutrient removal.
influenced. The performance difference between with and without waste sludge discharge was possibly due to that as SRT increased, sludge mainly consisted of inert material whereas active biomass and stored substrate became very low[18]. Under the condition of without sludge discharge, SRT was longer than with sludge discharge. Longer SRT was theoretically favored for nitrobacteria whose generation time was long, but the decreased NH3-N removal indicated that the condition of without sludge discharge was not actually favored for nitrobacteria. The extra-cellular polymers (ECP) of activated sludge increased resulting in decrease of nitrobacteria activity [19]. Nitrogen variation in settler with/without sludge discharge was shown in Fig. 15. Sludge discharge caused a nitrogen variation in settler. It was obvious to notice that there was a nitrite accumulation under the condition of without sludge discharge. This phenomenon did not occur under the condition of with sludge discharge. Without sludge discharge, the sludge bed height in settler was higher than with sludge discharge, which led to low DO in sludge bed resulting in partial nitrification. With sludge discharge, the low DO condition in sludge bed was destroyed by sludge discharge pump, and nitrite was all converted to nitrate.
Fig. 12. COD, PO4 3−-P, NO3-N, and NH3-N of each aeration tank.
J. Fan et al. / Desalination 249 (2009) 822–827
[3]
[4] [5]
[6] [7] [8]
[9] Fig. 15. Nitrogen variation in settler with/without waste sludge discharge.
4. Conclusions
[10]
[11] [12]
In this study, a full scale modified A2/O process was evaluated for treating low strength wastewater (BOD/TN = 3, BOD/TP = 26). Influent TP concentration had effect on TP removal, the results showed that when influent TP was below 1mg/L average TP removal decreased by 23% compared with influent TP above 1mg/L. The nitrate recycle ratio was controlled very low which caused high concentration of effluent NO3-N and also effluent TP. Pre-anoxic selector reduced NO3-N and NO2-N by 35%, and 30%, respectively. However, the removal of NO3-N was more stable than NO2N. Selector led to a decline of nitrogen denitrified in anaerobic tank compared with a conventional A2/O process. Anaerobic tank in modified A2/O process was only responsible for phosphorus release instead of combination function of phosphorus release and denitrification. The simplified design of rectangular aeration tank (L = 1.58 B) could also perform as plug flow reactor as conventional channel aeration tank. With staging, MLSS in aeration tank increased from one tank to another, and this might be due to a change of flow mixing condition. SOUR of sludge, COD and PO43−-P removal rates in 3-stage aeration tank were degressive whereas NO3−-N, and NH3-N reaction rates were constant. High MLSS concentration was not suitable for treating low strength wastewater, because the decreased BOD-sludge loading would lead to shortage of carbon source and deterioration of TP removal. MLSS concentration had little effect on COD, NH3-N, and TN removal. Discharge of waste sludge improved COD, NH3-N, and TP removal by 7%, 6%, and 24%. References [1] Metcalf, Eddy (Eds.), Wastewater Engineering: Treatment, Disposal, Reuse, 3rd, McGraw-Hill, New York, 1991. [2] J. Domenec, A.A. Mitch, M. Matina, et al., Effects of anaerobic selector hydraulic retention time on biological foam control and enhanced biological phosphorus
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Glossary Symbols and abbreviations A2/O: Anaerobic–anoxic–oxic MLSS: Mixed liquid suspended solid concentration, mg/L MLVSS: Mixed liquid volatile suspended solid concentration, mg/L SOUR: Specific oxygen uptake rate, mgO2/gMLSS h RTD: Residence time distribution E(t): Exit-age distribution function, s− 1 t : Average residence time, s σt 2: Variance, s2 σθ2: Nondimensional variance SRT: Sludge retention time, d