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Simultaneous Bioenergy (CH4) Production and Nitrogen Removal in a Combined Upflow Anaerobic Sludge Blanket and Aerobic Membrane Bioreactor† Yingyu An,‡,§ Fenglin Yang,‡ Fook Sin Wong,§ and Hwee Chuan Chua*,§,| School of EnVironmental and Biological Science and Technology, Dalian UniVersity of Technology, Dalian 116024, China, Institute of EnVironmental Science and Engineering, Nanyang Technological UniVersity, InnoVation Centre, Block 2, 637723, Singapore, and School of Applied Science, Republic Polytechnic, 738964, Singapore ReceiVed May 27, 2007. ReVised Manuscript ReceiVed July 30, 2007
In this study, the effect of the aeration rate on simultaneous methanogenesis and denitrification was investigated in a combined system consisting of an oxygen-limited upflow anaerobic sludge blanket (UASB) and aerobic membrane bioreactor (MBR). Different oxygen concentrations in the UASB were achieved by recycling the mixed liquor of the aerobic MBR into the bottom of the UASB. The dissolved oxygen (DO) of the aerobic MBR mixed liquor was determined by the aeration rate in the aerobic MBR. Quantitative analysis of biogas production and composition and total organic carbon (TOC) and total nitrogen (TN) removal performances at different aeration rates in the aerobic MBR were carried out. Results showed that TOC and TN removal performances were improved in the combined system compared to a strictly anaerobic UASB. TOC removal efficiency up to 98% after membrane filtration was achieved. TN removal efficiency was related to the aeration rate in the aerobic MBR, which was limited by the nitrification efficiency at a low aeration rate and improved at aeration rates above 2.5 L/min. Biogas production and composition were also investigated. Results showed that a low DO concentration and degradation of NOx-–N through the denitrification process did not affect the methane production. A slight increase in methane production because of the improvement in the TOC removal rate and decrease of CO2 at a low aeration rate was observed. However, methane production and the ratio decreased at a high aeration rate as a result of more nitrogen and CO2 being produced via the processes of denitrification and oxidation, respectively. In this study, above 50% methane in the biogas and 80% TN and 98% TOC removal efficiencies at the aeration rates between 2.5 and 5.0 L/min in the aerobic MBR could be successfully achieved. On the basis of these results, it was suggested that simultaneous methanogenesis and denitrification could be employed as a bioenergy production technology using a combined UASB and aerobic MBR.
Introduction A significant advantage of anaerobic technology over the aerobic system is to produce methane, which can be reused as a renewable energy source for a favorable energy balance. However, the residual nitrogen compounds and odor emissions from an anaerobic effluent were considered as the main negative factors to prevent anaerobic bioenergy production technology from widespread practical application. Hence, investigations on simultaneous methanogenesis and nitrogen removal have been given more attention during the last decade. Some researchers have found that simultaneous methanogenesis and denitrification happened under oxygen-limited conditions, although all anaerobic bacteria only exhibit limited tolerance to low oxygen levels.1–5 On the basis of the findings, they concluded that the † Presented at the International Conference on Bioenergy Outlook 2007, Singapore, April 26–27, 2007. * To whom correspondence should be addressed. E-mail:
[email protected]. ‡ Dalian University of Technology. § Nanyang Technological University. | Republic Polytechnic. (1) Zitomer, D. H. Water Res. 1998, 32, 669–676. (2) Rolfe, R. D.; Hentges, D. J.; Campbell, B. J.; Arret, J. T. Appl. EnViron. Microb. 1978, 36, 306–313. (3) Marvin, L. S.; Susan, M. B. Plant Physiol. 1982, 69, 161–165.
DO concentration has a critical effect on both the activities of methanogens and denitrifiers, thereby establishing the basic theory on it.1,6,7 However, few researchers have focused on the effect of aeration rate in the aerobic bioreactor (dissolved oxygen concentration in circulation water) on the simultaneous methanogenesis and denitrification using a combined system of upflow anaerobic sludge blanket (UASB) and aerobic membrane bioreactor (MBR). Zhang et al.8 applied a system consisting of an UASB reactor and an aerobic MBR to integrate methanogenesis with simultaneous nitrogen removal for the treatment of a high-strength wastewater containing high ammonium concentration. The aim of this paper is to study the effect of the aeration rate in the aerobic MBR on system performance and practicability. The methane production and nitrogen removal process in an oxygen(4) Jared, R. L.; Breznak, J. A. Appl. EnViron. Microb. 1996, 62, 3620– 3631. (5) Kiener, A.; Leisinger, T. Appl. Microbiol. Biol. 1983, 4, 305–312. (6) Windey, K.; Bo, I. D.; Verstraete, W. Water Res. 2005, 39, 4512– 4520. (7) Shin, J.-H.; Lee, S.-M.; Jung, J.-Y.; Chung, Y.-C.; Noh, S.-H. Process Biochem. 2005, 40, 3769–3376. (8) Zhang, D.-J.; Liu, P.-L.; Long, T.-R.; Verstraete, W. Process Biochem. 2005, 40, 541–547.
10.1021/ef7002816 CCC: $40.75 2008 American Chemical Society Published on Web 09/22/2007
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Figure 2. DO values at different aeration rates in the aerobic MBR.
Figure 1. Schematic of the laboratory-scale experimental setup. Table 1. Operating Conditions for This Study period
blank
HRT (h) (UASB/MBR) temperature (°C) (UASB/MBR) operating period (days) aeration rate (L/min) (MBR) feed rate (mL/min) recirculation ratio (%) upflow velocity (m/h) TOC/TN (g/g) OLR [g of TOC (g of VSS)–1 day–1] VLR (g of TN m–3 day–1) permeate flux (L m–2 h–1)
run 1
run 2
run 3
run 4
3.5/5 ∼30–32/∼28–30 0–25
26–47
48–74
75–104
104–120
0
1.0
2.5
5.0
8.0
20 0 0.23
400 1.14 ∼3.3–4.0 0.3–0.4 ∼0.16–0.29 12
limited UASB coupled with aerobic MBR will be investigated and described. Experimental Section Laboratory-Scale Experimental Setup. The laboratory-scale experimental setup is shown in Figure 1. It consisted of two parts, the UASB and aerobic MBR. The UASB was a cylindrical tank with a conical-shaped bottom (8.2 cm internal diameter, 70.0 cm total height, and 4.2 L working volume). A three-phase separator was installed at the top of the tank to separate the biogas from the mixed liquor. It was also used to retain suspended particles in the reactor. An overflow line was situated above the three-phase separator, which was connected to the aerobic MBR (6 L working volume). A flat-sheet membrane module [polyvinylidene difluoride (PVDF), hydrophilic, pore size of 0.22 µm, Millipore, Billerica, MA] was placed into the aerobic MBR, and a suction pump removed the permeate continuously from the aerobic MBR. The aerobic MBR mixed liquor was recirculated to the bottom of the UASB by a peristaltic pump (Masterflex, Cole-Parmer, Vernon Hills, IL), which introduced DO into the UASB. The pressure transducers for permeate suction, flow meters, water-level sensor, and temperature were automatically controlled by the software of ICONICS GENESIS 32, and resulting data were recorded throughout the study. The feed flow rate was regulated by the level sensor to be the same as the permeate flow rate. Feed Composition. The composition of the synthetic municipal wastewater contained the following nutrients: 180 mg/L glucose, 400 mg/L sodium acetate, 110 mg/L NH4Cl, 40 mg/L peptone, 30 mg/L meat extract, 9.2 mg/L KH2PO4, 7.5 mg/L FeSO4, 2.5 mg/L
MgSO4, 10 mg/L MnCl2 · 4H2O, 5 mg/L CoCl2 · 6H2O, 6 mg/L CaCl2 · 2H2O, 5 mg/L Cu(NO3)2 · 3H2O, 5 mg/L ZnSO4 · 7H2O, and 5 mg/L NiSO4 · 6H2O. The wastewater was stored in a refrigerator at 4 °C. Operating Conditions. The combined system was operated at different recirculation ratios for more than 8 months before these experiments started. An optimal recirculation ratio of 400% was applied in this study to investigate the effect of DO on system performance. The details of the operating parameters of the reactor are shown in Table 1. Each experimental run lasted more than 2 weeks to achieve steady-state conditions at the various DO concentrations. In this study, the steady-state condition was considered to be reached when the variation of the measurements was less than 10%. The gaseous and aqueous samples were collected daily. The data collected under steady-state conditions were used to calculate the average value. Analytical Methods. Total organic carbon (TOC) and total nitrogen (TN) were evaluated using a TOC/TN analyzer (Shimadzu, TOC/TN VCSH, Japan). The overall flow of biogas produced was measured using a water replacement method. The methane concentration was determined by a gas chromatography (Agilent, 6890N, Santa Clara, CA) using a flame ionization detector. The anaerobic sludge concentration was analyzed on the basis of the standard methods.9 The dissolved oxygen concentration was measured by a DO meter (YSI 200, Yellow Springs, OH), and pH was measured by a pH meter (YSI 100, Yellow Springs, OH). NH4+–N, NO2-–N, and NO3-–N were analyzed by a spectrophotometer (HACH, DR/2400, Germany). Soluble TOC was prepared by centrifugation (4000 rpm) at 4 °C for 10 min (Universal 32R, Hettich-Zentrifugen, Germany). The UASB efficiency for TN removal was calculated on the basis of the influent and effluent TN of the UASB reactor. The total efficiency for TN removal was calculated on the basis of the influent TN concentration of the UASB and the effluent TN concentration of the aerobic MBR. The nitrification efficiency was calculated on the basis of the influent NH4+–N of the UASB and the effluent NH4+–N of the aerobic MBR. The denitrification rate was calculated on the basis of the influent TN concentration of the UASB and the effluent NOx-–N concentration of the aerobic MBR.
Results and Discussion DO and Sludge Characteristics at Different Aeration Rates in the Aerobic MBR. The oxygen-limited environment in the UASB was created by recycling the mixed liquor of the aerobic MBR into the UASB as described above. The different DO concentrations were determined by the different aeration rates in the aerobic MBR at a constant recirculation ratio (recirculated liquid volume/influent volume ) 400%) in this study. The measurement of the DO concentration was done at the end of each operating condition. The DO values at the different aeration rates are shown in Figure 2. The DO concentration in the UASB increased linearly from 0.11 to 0.76 (9) Clesceri, L. S.; Greenberg, A. E.; Eaton, A. D. Standard Methods for the Examination of Water and Wastewater; American Public Health Association: Washington, D.C., 1998.
Combined UASB and Aerobic MBR
Figure 3. Sludge concentrations at different aeration rates in the aerobic MBR.
Figure 4. Biogas production in the UASB at different aeration rates in the aerobic MBR.
mg/L, while the DO concentration in the aerobic MBR increased from 1.02 to 4.94 mg/L with the increase in the aeration rate from 1.0 to 8.0 L/min. Figure 3 shows the sludge concentrations in the UASB and aerobic MBR as well as the sludge height in the UASB at different aeration rates in the aerobic MBR. In comparison to a strictly anaerobic UASB (no aeration and recirculation were employed), the sludge in the UASB was distributed more evenly. A decrease in the sludge concentration at 10 cm and an increase of the sludge concentration at 30 cm were observed. In addition, the height of the sludge in the UASB increased gradually with the increase in the aeration rate. The reason was possibly due to the increased biogas production (Figure 4), which resulted in better mixing. Another possible reason may be due to the lower sludge settling capability under the oxygen-limited condition. This corresponded to a higher amount of loose sludge being present in the supernatant of the UASB, especially at the highest aeration rate of 8.0 L/min. More sludge with a low settling capability was washed out from the UASB into the aerobic MBR,10 inducing the sludge concentration in the aerobic MBR to increase. Hence, more flocs were observed in the aerobic MBR and recirculated back into the UASB. Finally, the system balance was destroyed when the high DO was introduced into the UASB. Effect of DO on the Biogas Production and Composition in the UASB. The biogas produced consisted of methane, carbon dioxide, and nitrogen in this study. Figure 4 presents the total biogas production, which increased when aeration was employed as compared to a strictly anaerobic system (no aeration and recirculation were employed). This can be explained by the improvement in the removal of organic compounds (Figure 6) as well as an improvement in the distribution of the influent. The results were similar to those presented by other researchers.1 However, under the same recirculation ratio (400%), the total biogas and methane production decreased slightly with the increase in the aeration rate in the aerobic MBR, (10) Kato, M. T.; Field, J. A.; Versteeg, P.; Lettinga, G. Biotechnol. Bioeng. 1994, 44, 469–479.
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Figure 5. Biogas ratio at different aeration rates in the aerobic MBR.
while the nitrogen production remained constant. CO2 production decreased initially until the aeration rate of 2.5 L/min and then increased slightly. At the high aeration rate (see run 4 in Table 1), more organic substances were degraded through the anoxic or aerobic process, which resulted in more organic carbon being transferred to CO2 instead of methane according to the degradation theories, which also confirmed the hypothesis above (Figure 4). At the low aeration rate (see run 1 in Table 1), the methane production and ratio were high, which were due to the co-impact of the high organic carbon removal efficiency and low nitrogen removal efficiency. When the system was operated at steady state (1.0 L/min < aeration rate < 5.0 L/min), the biogas production and composition were maintained at stable values. Composition analysis results showed that the biogas ratios of CH4, CO2, and N2 were different because denitrification was introduced and the organic degradation process was affected under an oxygen-limited environment, which are shown in Figure 5. CH4 and CO2 ratios were always lower compared to that of a strictly anaerobic system because of an increase in the nitrogen production by the denitrification process when the recirculation was employed. The CO2 production and ratio were lower in this system opposite to the fact that more organic compounds were degraded through the anoxic and aerobic processes under the oxygen-limited system. Consequently, more tests were carried out to explain this phenomenon. It was found that higher pH values occurred in the UASB than in a strictly anaerobic UASB. Normally, the pH value of the UASB effluent would be approximately 6.5–7.0 (see blank in Table 1). After the recycling water was introduced, the effluent pH values varied between the range of 7.8–8.1. It was thought that the higher pH range benefited the growth of the methanogens and denitrifiers.11 The higher pH was due to the denitrification process. Theoretically, 3.0–3.6 mg of alkalinity as CaCO3 is produced per milligram of NO3––N being reduced to nitrogen gas during the denitrification process. This denitrification process was considered to be beneficial in improving the stability of anaerobic reactors by counteracting the decrease in pH as a result of acidogenesis. Hence, the denitrification reduced the pH decrease during the anaerobic treatment of low-strength wastewater. At a high pH, volatile fatty acids (VFAs) are potentially oxidized and carbon dioxide and hydrogen are more easily absorbed or stripped out.12 This gave rise to the lower CO2 production and ratio. Under this condition, a stable methane ratio of up to 50% could be achieved (1.0 L/min < aeration rate < 8.0 L/min). In comparison, Bernet et al.13 reported only (11) Baloch, M. I.; Akunna, J. C.; Collier, P. J. Bioresour. Technol. 2006, in press. (12) Zitomer, D. H.; Shrout, J. D. Waste Manage. 1998, 18, 107–116. (13) Bernet, N.; Delgenes, N.; Akunna, J. C.; Delgenes, J. P.; Moletta, R. Water Res. 2000, 34, 611–619.
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Figure 6. TOC removal at different aeration rates in the aerobic MBR.
approximately 30% CH4 in the biogas using a combined aerobic and anaerobic system treating piggery wastewater. In addition, some researchers have found that the noxious intermediate production, such as VFA and H2S, restrained the activity of the methanogens. However, in this oxygen-limited system, VFA and H2S could not be detected throughout all of the experimental runs. The results indicated that the oxygenlimited environment enhanced VFA and H2S removal, thus reducing the negative impact on methanogenesis, and maintained the high activity of the methanogens and denitrifiers.10,14,15 Therefore, the change in the DO concentration and the degradation of NOx-–N through the denitrification process did not totally inhibit the methane production, even at the high DO concentration of 0.76 mg/L when the aeration rate was 8.0 L/min in the aerobic MBR. This was possibly due to the aerobic bacteria being recirculated into the UASB, which exhausted the limited DO. It was suggested that the simultaneous denitrification and methanogenesis process under the oxygen-limited condition could produce biogas as an energy source when the aeration rate was below 5 L/min in the combined system. TOC Removal at Different Aeration Rates in the Aerobic MBR. Figure 6 shows the variation in the TOC removal performance at the different aeration rates. The overall TOC removal efficiencies for the different DO concentrations in the UASB were compared. The effluent TOC values were lower for a methanogenesis and denitrification system at lower DO concentrations as compared to a strictly anaerobic system. This may be possibly associated with anoxic biodegradation and aerobic processes.13 More even water distribution was also achieved with recirculation, which favored the contact reaction between wastewater and sludge; thus, the TOC removal performance increased from 83 to 91%. However, at the high aeration rate of 8.0 L/min in the aerobic MBR (DO concentration of UASB of 0.76 mg/L), the TOC removal efficiency in the UASB decreased. Normally, in well-operated anaerobic systems, the majority of the residual organic carbon (TOC) present in the effluent are due to the soluble microbial products (SMPs) generated by microorganisms during treatment. In this study, from the observation of the sludge characteristics, more flocs were present in the UASB supernatant at the high DO concentration of 0.76 mg/L, contributing to the higher TOC in the UASB effluent. Another possible reason may be due to the lower activity of the anaerobic bacteria at the oxygen-limited DO level. However, the overall TOC in the permeate from the aerobic MBR was always below 3 mg/L, thus achieving a total removal efficiency above 98%. It was thought that, by employing membrane separation to retain part of the particles in the reactor, a more reliable effluent quality could be obtained for (14) Zitomer, D. H. Water Res. 1998, 32, 669–676. (15) Stephenson, R. J.; Patoine, A.; Guiot, S. R. Water Res. 1999, 33, 2855–2863.
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Figure 7. TN removal at different aeration rates in the aerobic MBR.
Figure 8. NI and DN at different aeration rates in the aerobic MBR.
the combined UASB/aerobic MBR system than in a conventional strictly anaerobic process. Nitrogen Removal at Different Aeration Rates in the Aerobic MBR. The NOx-–N was produced in the aerobic MBR by the process of nitrification, and the process of denitrification was then carried out in the UASB to achieve TN removal in the combined system. Figure 7 shows the variation of the TN removal performance at the different aeration rates. At the lowest aeration rate of 1.0 L/min, the total TN removal efficiency was only 35.2%. A stable high level of total TN removal efficiency of approximately 80.0% was achieved when the aeration rate was higher than 2.5 L/min in the combined system. In Figure 7, it can be observed that the TN removal occurred in both the UASB and aerobic MBR, in which the aerobic MBR contributed approximately 10% of the TN removal efficiency. This phenomenon of “aerobic denitrification” has also been reported by other researchers.16,17 Figure 8 shows the nitrification efficiency (NI) and denitrification efficiency (DN) at the different aeration rates. The NI was restrained by the low aeration rate, which was supported by the detection of NH4+–N in the permeate of the aerobic MBR (Figure 8). The DN was stable throughout all of the experimental runs even though the DO concentration increased up to 0.76 mg/L in the UASB. NOx-–N in the UASB supernatant was always below the detection limit, which indicated that denitrification was complete in the UASB. Normally, the NI increases and DN decreases with an increase in DO. Hence, the TN removal performance dropped with the decreased NI at the lowest aeration rate of 1.0 L/min. However, when the aeration rate was above 2.5 L/min in the aerobic MBR, the TN removal performance was not affected by the NI and DN. Conclusions A combined bioenergy production and nitrogen removal system consisting of an oxygen-limited UASB and aerobic MBR (16) Ahn, Y.-H. Process Biochem. 2006, 41, 1709–1721. (17) Robertson, L.; Kuenen, J. Arch. Microbiol. 1984, 139, 351–354.
Combined UASB and Aerobic MBR
was developed to treat synthetic municipal wastewater. The following conclusions were drawn on the basis of the results of this paper: At the lowest aeration rate of 1.0 L/min, high methane production and ratio were achieved but the TN removal performance was restrained by the nitrification efficiency. When the aeration rate was above 2.5 L/min, the TN removal performance increased to approximately 80% and was not affected by the NI and DN. However, too high of an aeration rate destroyed the system balance, which resulted in reduced methane production and ratio by degradation of more organic compounds through the anoxic or aerobic processes.
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The optimal aeration range of ∼2.5–5.0 L/min was suggested on the basis of the methane production and ratio (>50%) as well as stable TOC (>98%) and TN (>80%) removal efficiencies. In the integrated oxygen-limited UASB and aerobic MBR system, simultaneous methane production and denitrification was technically feasible as an energy efficient way to achieve high effluent quality, minimal biosolid generation, and nitrogen removal. Acknowledgment. Financial support was provided by the Public Utilities Board of Singapore (project number R 05-044-3). EF7002816
