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Effect of pH on nutrient dynamics and electricity production using microbial fuel cells
Bioresource Technology 101 (2010) 9594–9599
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Effect of pH on nutrient dynamics and electricity production using microbial fuel cells Sebastià Puig a,b,*, Marc Serra a, Marta Coma a, Marina Cabré a, M. Dolors Balaguer a, Jesús Colprim a a Laboratory of Chemical and Environmental Engineering (LEQUIA-UdG), Institute of the Environment, University of Girona, Campus Montilivi s/n, Facultat de Ciències, E-17071 Girona, Spain b Catalan Institute for Water Research (ICRA), Carrer Emili Grahit 101, Edifici H2O, E-17003 Girona, Spain
a r t i c l e
i n f o
Article history: Received 27 May 2010 Received in revised form 18 July 2010 Accepted 20 July 2010 Available online 24 July 2010 Keywords: Domestic wastewater Microbial fuel cells Nutrients Organic matter pH
a b s t r a c t The aim of this work was to study the effect of pH on electricity production and contaminant dynamics using microbial fuel cells (MFCs). To investigate these effects, an air-cathode MFC was used to treat urban wastewater by adjusting the pH between 6 and 10. The short-term tests showed that the highest power production (0.66 Wm3) was at pH 9.5. The MFC operation in continuous control mode for 30 days and at the optimal pH improved the performance of the cell relative to power generation to 1.8 Wm3. Organic matter removal (77% of influent COD) and physical ammonium loss were directly influenced by pH and followed the same behavior as the power generation. At a pH higher than the optimal one, anodic bacteria were affected, and power generation ceased. However, biological nitrogen processes and phosphorus dynamics were independent of the exoelectrogenic bacteria. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Biological domestic wastewater treatment is an energydemanding and sludge-producing process. Energy use for aeration in wastewater treatment plants (WWTPs) can account for up to 50% of operational costs. Aerated treatment processes also produce large amounts of excess sludge. The treatment and disposal of this sludge currently presents an increasing challenge due to economic, environmental and regulatory factors (Wei et al., 2003). Therefore, the community is looking for new technologies that require a lower energy demand or that produce energy and lower sludge production. One of the newest and most promising approaches is the use of microbial fuel cells (MFCs) to simultaneously treat wastewater while also generating electricity (Liu et al., 2004; Logan et al., 2006). MFCs offer the possibility of harvesting electricity from organic waste and renewable biomass (Lee et al., 2008; Rabaey and Verstraete, 2005). These are attractive sources of energy because they are ‘carbon-neutral’; the oxidation of the organic matter only releases recently fixed carbon back into the atmosphere (Lovley, 2006).
* Corresponding author at: Laboratory of Chemical and Environmental Engineering (LEQUIA-UdG), Institute of the Environment, University of Girona, Campus Montilivi s/n, Facultat de Ciències, E-17071 Girona, Spain. Tel.: +34 972418281; fax: +34 972418150. E-mail address: [email protected] (S. Puig). 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.07.082
In a MFC, organic substrates are oxidized by exoelectrogenic bacteria (Logan, 2009). This oxidation produces electrons that are transferred to an anode electrode and then flow to a cathode. The anode and cathode are linked by a conductive material containing a resistor. Protons produced at the anode migrate through the solution across a cation exchange membrane (CEM) to the cathode chamber, where they combine with a reducible compound and electrons. A fuel cell converts chemical energy into electrical energy without the inefficiencies that arise from combusting fuel (Lovley, 2006). A wide range of substrates can be oxidized (Pant et al., 2010). However, domestic wastewater treatment using MFCs is still under investigation. Aelterman et al. (2006) demonstrated that MFCs might occupy a market niche in terms of a stand-alone source of electricity for use in wastewater treatment. Cha et al. (2010) studied the direct application of MFCs in aeration tanks. Rodrigo et al. (2007) reached a maximum power density of 25 mWm2 by oxidizing the organic matter from urban wastewater. Ahn and Logan (2010) treated domestic wastewater at an organic loading rate of 54 kg COD m3 d1 (25.8% COD removal) and a highest power generation of 0.42 Wm2 (12.8 Wm3). In terms of suitable MFC operational conditions, Liu et al. (2005) examined the effect of ionic strength, temperature and reactor configuration on power generation. They found that the power densities can be increased by changing operation parameters. In this way, Ahn and Logan (2010) demonstrated that using temperature-phased conditions in MFCs for domestic wastewater
S. Puig et al. / Bioresource Technology 101 (2010) 9594–9599
treatment produced fewer solids, higher efficiency and power savings. The pH is another operational parameter that affects MFC performance (2007). The pH is crucial to the MFCs’ power output. Generally, bacteria require a pH close to neutral for optimal growth; however, oxygen reduction on the cathode electrode results in an alkaline pH (Rozendal et al., 2006). Ren et al. (2007) found that in a double-chamber MFC, the power production decreased significantly when the final pH dropped to 5.2 due to the acidic products of fermentation; the power resumed quickly when the pH was recovered to 7.0. However, in air-cathode or singlechamber MFCs, the pH affects both the anodic and cathodic reactions in which a high pH (8–10) inhibits the anodic bacterial activities to some extent, but might be favorable to the cathodic reaction, thus improving the overall performance (He et al., 2008). Jadhav and Ghangrekar (2009) studied the performance of MFCs subjected to a variation in pH while treating synthetic wastewater in two-chamber MFCs. They found that the highest current was generated when the difference in pH between the chambers was higher. Fan et al. (2007) demonstrated the excellent performance of MFCs using bicarbonate as pH buffer and proton carrier. Torres et al. (2008) found that decreasing the pH gradient resulted in an increase in voltage efficiency by simply adding carbon dioxide to the cathode air. However, with respect to domestic wastewater treatment, questions remain on how to deal with the effects of pH on (i) power production and especially (ii) the dynamics of organic matter, nitrogen and phosphorus compounds. Additionally, pH strongly affects the biological processes that usually take place in WWTPs, such as nitrification, denitrification and biological phosphorus removal (Filipe et al., 2001; Metcalf and Eddy, 2003). The objective of this study was to investigate the shortand long-term optimal pH conditions for an air-cathode MFC to maximize the electricity generation and its effect on the dynamics of organic matter, nitrogen and phosphorus compounds for treating domestic wastewater.
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phosphorus removal difficult in a classical treatment scenario (Puig et al., 2007). 2.2. MFC configuration and operation The air-cathode MFC consisted of an anode and cathode placed on opposite sides of a single methacrylate rectangular chamber with dimensions of 32 26 400 mm (empty bed volume of 395 mL). The anode was filled with 6 35 mm rod graphite (Alfa Aesar, Germany), which reduced the compartment volume to 242 mL (net anodic compartment). The electrodes were washed in 1 N HCl and 1 N NaOH to remove possible metal and biomass contamination (Bond and Lovley, 2003). A thinner graphite electrode (28 35 mm, Sofacel, Spain) was connected to the external electricity circuit. C-cloth 0.35 mg cm2 of Pt catalyst 30% wetproofing (Clean Fuel Cell Energy LLC, USA) was used as the cathode electrode. A cation exchange membrane (CEM, NafionÒ 117, Dupont) was treated according to Liu and Logan (2004) and put between the anode and cathode frames. The anode and the cathode were connected through an external resistor to close the circuit. Fig. 1 shows a scheme of the air-cathode MFC used. Wastewater was continuously fed to the recirculation loop to (i) obtain the desired volumetric loading rate, (ii) maintain well-mixed conditions, (iii) avoid concentration gradients and (iv) avoid clogging of the granular matrix. The system was thermostated at 23 ± 2 °C, and the pH was monitored during the study. Prior to starting the study, the MFC was inoculated with effluent from a parent MFC treating synthetic wastewater (mainly composed of sodium acetate and a buffer solution). The system operated for 90 days at a mean organic loading rate of 1.86 kg COD m3 d1 (COD removal efficiency of 81%) for treating urban wastewater. The maximum power density achieved was 0.44 Wm3. Once the MFC achieved steady state, a series of experiments were carried out to study the effect of pH. 2.3. Experimental performance
2. Methods 2.1. Domestic wastewater The urban sewage came from the Quart WWTP (N.E. Spain). Raw wastewater without any primary treatment was taken. The concentration range and mean values of the principal chemical compounds during the experimental period are summarized in Table 1. The mean COD concentration in the domestic wastewater was 503 ± 148 mg COD L1 (BOD5/CODT ratio = 0.77). This value was in the range of typical wastewater concentrations. However, the nitrogen concentration was higher than expected (109.7 ± 2.5 mg N L1; 67% as ammonium) maybe for industrial influences. Consequently, the wastewater presented a low C/N ratio (5 mg COD mg1 N) that, a priori, would make biological nitrogen and
Table 1 Wastewater characteristics throughout the study. Units
Total chemical oxygen demand, CODT Soluble chemical oxygen demand, CODS BOD5/CODT ratio Total Kjeldahl nitrogen, TKN Ammonium, NHþ 4 Nitrite, NO 2 Nitrate, NO3
mg COD L1
753–378
503 ± 148
mg COD L1
418–188
328 ± 93
– mg N L1
– 112.1–106.1
0.77 109.7 ± 2.5
Phosphate, PO3 4
1 mg P-PO3 4 L
1 mg N-NHþ 4 L 1 mg N-NO 2 L 1 mg N-NO 3 L
Range
Mean ± r
Compound
75.1–72.5 0.22–0.00 0.17–0.03 8.58–5.11
73.8 ± 1.2 0.12 ± 0.09 0.10 ± 0.06 6.99 ± 1.26
A series of short- (hours) and long-term (30 days) experiments were performed in the air-cathode MFC to evaluate the pH effect on energy production and organic matter, nitrogen and phosphorus compound dynamics. The pH was controlled at different values ranging from 6 to 10 with a peristaltic pump (Fig. 1) and dosing the system with base (0.2 M NaOH) or acid (0.2 M HCl). In the shortterm experiments, we controlled the pH at the desired value and, after at least twice the hydraulic retention time (HRT; 0.14 days), we analyzed the effluent concentration and the power produced. The long-term experiment had an optimal pH fixed for 30 days. During the long-term experiments, polarization curves were observed, and influent and effluent samples were taken periodically. 2.4. Analyses and calculations Standard wastewater measurements for organic matter (total and soluble chemical oxygen demand (CODT and CODS) and 5day biochemical oxygen demand (BOD5)), nitrogen (total Kjeldahl nitrogen (TKN), ammonium NHþ NO and nitrates 2 4 , nitrites 3 NO3 and phosphate PO4 were taken regularly and analyzed according to APHA (2005). Cell potential (V) in the MFC circuit was monitored at 30-min intervals using an on-line multimeter (Alpha-P, Ditel) with a data acquisition system (MemographÒ M RSG40, Endress + Hauser). Current (I) and power (P = IV) were calculated according to Ohm’s law. Power density was calculated by dividing the power by net anode volume (Wm3). Polarization curves were obtained by varying the external resistance in the circuit and measuring the voltage according to Logan et al. (2006).
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V pH
CONTROL PANEL
ACID/BASE DOSAGE
URBAN WASTEWATER
EFFLUENT
Fig. 1. Scheme of the air-cathode MFC used to treat urban wastewater.
3. Results and discussion 3.1. Short-term power production and organic matter removal as a function of pH It is known that power generation is affected by the operational pH. In a double-chamber MFC, the highest current was observed at neutral pH (between 6.5 and 8) (Jadhav and Ghangrekar, 2009; Gil et al., 2003). However, a basic pH (around 9) was optimal in an aircathode system treating synthetic wastewater (He et al., 2008). In this study, the pH effect was studied by treating urban wastewater in the air-cathode MFC. Fig. 2 shows the influence of pH on the power density and organic matter removal. The power increased 80% from 0.36 Wm3 (at pH 6) to 0.66 Wm3 (at pH 9.5). However, at pH 10, the power density dropped to 0.5 Wm3. Fig. 2 shows the organic matter removal efficiency as a function of pH. COD removal efficiencies remained stable at around 77% ± 6% at a pH between 6 and 9.5. This efficiency coincided with the BOD5/COD influent ratio (Table 1). The mean COD concentration in the effluent was 108 ± 28 mg COD L1 and was related to the non-biodegradable organic matter. However, 404 mg COD L1 was measured in the effluent at pH 10 (removal efficiency 46%). This drop in efficiency was in relation to the power density and indicated that exoelectrogenic bacteria were affected by a pH higher than 9.5. 3.2. Effect of pH on nitrogen transformations The urban wastewater used in this study contained 109.7 ± 2.5 mg L1 of nitrogen (67% as ammonium) on average (Table 1). This nitrogen underwent some transformations in the MFC. Between
1 46.37 and 73.95 mg N-NHþ was removed over the pH range 4 L (Fig. 3). This ammonium loss could be a result of physical–chemical and/or biological mechanisms. The presence of nitrate (15.31 ± 1 5.27 mg N-NO ; 14% of influent TKN) suggested that biological 3 L nitrogen processes (mainly nitrification) took place. This production was a result of oxygen diffusion into the anode chamber (experimental oxygen mass transfer coefficient of 1.76 103 cm s1) and remained stable over the pH range, except at pH 6, where 1 only 1 mg N-NO was produced, which indicated that the bac3 L teria were being affected. Another way that ammonium was removed was via physical–chemical mechanisms. Kim et al. (2008) determined that these ammonia losses occurred in single-chamber MFCs due to volatilization of ammonium at the cathode. They found that ammonium present in the wastewater disappeared because of an elevated pH near the cathode and was accelerated by electricity generation. In our case, ammonium removal increased at higher pH and power density in correlation with the literature. Moreover, batch studies were performed to determine the rate of ammonium diffusion into the cathode chamber (ammonium mass transfer coefficient of 3.11 104 cm s1). Once power generation ceased at pH 10 (Fig. 2), the concentration gradient was reduced due to passive diffusion of ammonium across the membrane and was accompanied by transfer of the other charged species (Kim et al., 2008). In addition, to determine the pH of the cathode in the MFC, the carbon cloth was washed with distilled wastewater. The cathodic pH was 11.0 because of proton consumption in the cathode reaction. Consequently, transport of cationic species (in our case, ammonium) other than protons resulted in an increased pH in the cathode chamber (Rozendal et al., 2006). In addition to these two processes, heterotrophic denitrification (reduction of nitrate to nitrogen gas using organic matter from
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pH 5.5
6.0
6.5
7.0
7.5
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
10.5
8.0
8.5
9.0
9.5
10.0
10.5
COD removal (%)
Power density (W·m-3)
0.8
0.6
0.4
0.2
0.0 90 80 70 60 50 40 30 20 10 0
pH Fig. 2. Comparison between the power density and the COD removal efficiencies as a function of pH for treating urban wastewater.
pH
Power
density (W·m-3)
55
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
10.5
7.5
8.0
8.5
9.0
9.5
10.0
10.5
0.8 0.6 0.4 0.2 0.0 Ammonium removed Nitrate observed Nitrogen difference
70 60
(mg N·L-1)
Nitrogen concentration
6.0
50 40 30 20 10 0 5.5
6.0
6.5
7.0
pH Fig. 3. Comparison between the power density and the nitrogen compounds concentrations as a function of the pH for treating urban wastewater.
wastewater) and nutrient assimilation should be taken into account. Unfortunately, the percentage of each process could not be determined in this study. 3.3. Effect of pH on phosphate evolution The air-cathode MFC was used to treat urban wastewater with a phosphate content of 6.99 ± 1.26 mg P-PO3 4 (Table 1). Fig. 4 shows the difference between the phosphate concentrations in the effluent with respect to the influent at different pH values. Phosphate
was removed at pH values higher than 8.5 and was released at lower pH values, thereby increasing the concentration in the effluent. As stated earlier, the nitrogen and phosphate transformations could be due to two mechanisms: biological and/or physical– chemical. Biological phosphorus bioaccumulation was ruled out because fluorescence in situ hybridization (FISH) analysis showed a low population of phosphate-accumulating organisms (1% of total bacteria). Thus, the phosphate trends should be due to the precipitation of phosphate salts (usually as calcium or potassium phosphate). This chemical precipitation is strongly influenced by
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1.0
10
6 4
0.6
2
0.4
-2
0
-4 -6
0.2 Power density Phosphate increment
-8
0.0 55
6.0
6.5
7.0
concentration (mg P·L-1)
0.8
Increment of phosphorus
Power density (W·m-3)
8
7.5
8.0
8.5
9.0
9.5
10.0
-10 10.5
pH Fig. 4. Increment of phosphorus concentration (effluent minus influent) and power density at different pHs for treating urban wastewater.
800
2.0
700
Power production (W·m-3)
pH 7.5 (day -10) pH 9.5 (day 2) pH 9.5 (day 15) pH 9.5 (day 30)
Voltage (mV)
600 500 400 300 200
1.5
1.0
0.5
100 0
0.0 0
2
4
6
8
10
12
14
16
0
2
Current density (A·m-3)
4
6
8
10
12
14
16
Current density (A·m-3)
Fig. 5. Polarization curves of the MFC operated for treating urban wastewater. Cell voltage (left) and power production (right) as a function of the current density.
pH (Carlsson et al., 1997). The solubility equilibrium of these phosphate salts decrease at a pH higher than 8.5, resulting in a decrease in soluble phosphate. On the other hand, at acidic pH levels, the salts were soluble and phosphate increased. This chemical mechanism was independent of the exoelectrogenic bacteria because it did not follow the power generation trend. 3.4. Effect of the optimal pH on energy production and removal efficiency A pH of 9.5 was considered optimal for treating urban wastewater and generating electricity based on the short-term experiments. To be confident in the results and to remove any inhibitory effect, the pH was maintained at 9.52 ± 0.01 in the aircathode MFC for 30 days. Fig. 5 compares the polarization curves of the MFC operating at pH 7.5, 10 days before fixing the pH setpoint and then 2, 15 and 30 days after the set-point implementation at pH 9.5. Ten days before starting the pH control, the polarization curve reached the maximum current and power densities of 4.89 A m3 and 0.42 Wm3, respectively. After 2 days at pH 9.5, the power density was greater than the density previously obtained. The current and power densities increased 26% and 75%, respectively. The MFC continued evolving and reached 9.37 A m3 and 1.18 Wm3 on day 15. Moreover, after 30 days of continuous
pH control, the maximum current and power densities achieved were 13.74 A m3 and 1.8 Wm3, respectively. The maximum power increased, but the internal resistance decreased from 1454 (before pH control) to 216.8 O (after 30 days of pH control). The increase in the electricity production corresponded with the organic matter removal efficiency (83% of the influent) and ammonium losses (72% of influent ammonium), but it did not affect the biological processes associated with nitrogen and phosphorus and followed the same behavior in both the short- and long-term experiments. 4. Conclusions The pH effect on electricity production treating wastewater was studied using an air-cathode MFC. The optimal pH for the shortterm study was 9.5 (power density of 0.66 Wm3). In the longterm study, the bacteria grew at this pH increasing the power density to 1.8 Wm3. Electricity production was based on the amount of organic matter; however, the removal efficiencies remained stable around 77 ± 6%. Ammonium loss via physical–chemical mechanisms was directly influenced by the power production and pH, but biological nitrogen processes remained stable. Finally, phosphate equilibrium was independent of the power production and influenced by pH changing the solubility constant.
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Acknowledgements This research was financially supported by the Spanish Government (CONSOLIDER-CSD2007-00055 and BES-2006-12277) and the AGAUR-Catalan Government (Post-Doctoral Fellowship BPB1-00193-2007). References Aelterman, P., Rabaey, K., Clauwaert, P., Verstraete, W., 2006. Microbial fuel cells for wastewater treatment. Water Sci. Technol. 54 (8), 9–15. Ahn, Y., Logan, B.E., 2010. Effectiveness of domestic wastewater treatment using microbial fuel cells and electrical energy production. Bioresour. Technol. 101 (2), 469–475. APHA, 2005. Standard Methods for the Examination of Water and Wastewater, 19th ed. American Public Health Association, Washington D.C., USA. Bond, D.R., Lovley, D.R., 2003. Electricity production by Geobacter sulfurreducens attached to electrodes. Appl. Environ. Microbiol. 69 (3), 1548–1555. Carlsson, H., Aspegren, H., Lee, N., Hilmer, A., 1997. Calcium phosphate precipitation in biological phosphorus removal systems. Water Res. 31 (5), 1047–1055. Cha, J., Choi, S., Yu, H., Kim, H., Kim, C., 2010. Directly applicable microbial fuel cells in aeration tank for wastewater treatment. Bioelectrochemistry 78 (1), 72–79. Fan, Y., Hu, H., Liu, H., 2007. Sustainable power generation in microbial fuel cells using bicarbonate buffer and proton transfer mechanisms. Environ. Sci. Technol. 41, 8154–8158. Filipe, C.D.M., Daigger, G.T., Grady Jr., C.P.L., 2001. pH as a key factor in the competition between glycogen-accumulating organisms and phosphorus accumulating organisms. Water Environ. Res. 73 (2), 223–232. Gil, G.C., Chang, I.S., Kim, B.H., Kim, M., Jang, J.K., Park, H.S., Kim, H.J., 2003. Operational parameters affecting the performance of a mediator-less microbial fuel cell. Biosens. Bioelectron. 18 (4), 327–334. He, Z., Huang, Y., Manohar, A.K., Mansfeld, F., 2008. Effect of electrolyte pH on the rate of the anodic and cathodic reactions in an air-cathode microbial fuel cell. Bioelectrochem. 74 (1), 78–82. Jadhav, G.S., Ghangrekar, M.M., 2009. Performance of microbial fuel cell subjected to variation in pH, temperature, external load and substrate concentration. Bioresour. Technol. 100 (2), 717–723. Kim, J.R., Zuo, Y., Regan, J.M., Logan, B.E., 2008. Analysis of ammonia loss mechanisms in microbial fuel cells treating animal wastewater. Biotechnol. Bioeng. 99 (5), 1120–1127. Lee, H.S., Parameswaran, P., Kato-Marcus, A., Torres, C.I., Rittmann, B.E., 2008. Evaluation of energy-conversion efficiencies in microbial fuel cells (MFCs)
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utilizing fermentable and non-fermentable substrates. Water Res. 42 (6–7), 1501–1510. Liu, H., Ramnarayanan, R., Logan, B.E., 2004. Production of electricity during wastewater treatment using a single chamber microbial fuel cell. Environ. Sci. Technol. 38 (7), 2281–2285. Liu, H., Logan, B.E., 2004. Electricity generation using an air-cathode single chamber microbial fuel cell in the presence and absence of a proton exchange membrane. Environ. Sci. Technol. 38 (14), 4040–4046. Liu, H., Cheng, S., Logan, B.E., 2005. Power generation in fed-batch microbial fuel cells as a function of ionic strength, temperature and reactor configuration. Environ. Sci. Technol. 39 (19), 5488–5493. Logan, B.E., Aelterman, P., Hamelers, B., Rozendal, R., Schröder, U., Keller, J., Freguia, S., Verstraete, W., Rabaey, K., 2006. Microbial fuel cells: methodology and technology. Environ. Sci. Technol. 40 (17), 5181–5192. Logan, B.E., 2009. Exoelectrogenic bacteria that power microbial fuel cells. Nat. Rev. Microbiol. 7 (5), 375–381. Lovley, D.E., 2006. Bug juice. harvesting electricity with microorganisms. Nat. Rev. Microbiol. 4, 497–508. Metcalf and Eddy (Eds.), 2003. Wastewater engineering: treatment and reuse, 4th ed. McGraw-Hill Higher Education, New York. Pant, D., Van Bogaert, G., Diels, L., Vanbroekhoven, K., 2010. A review of the substrates used in microbial fuel cells (MFCs) for sustainable energy production. Bioresour. Technol. 101 (6), 1533–1543. Puig, S., Corominas, Ll., Balaguer, M.D., Colprim, J., 2007. Biological nutrient removal by applying SBR technology in small wastewater treatment plants: carbon source and C/N/P ratio effects. Water Sci. Technol. 55 (7), 135–141. Rabaey, K., Verstraete, W., 2005. Microbial fuel cells: novel biotechnology for energy generation. Trends Biotechnol. 23 (6), 291–298. Ren, Z.Y., Ward, T.E., Regan, J.M., 2007. Electricity production from cellulose in a microbial fuel cell using a defined binary culture. Environ. Sci. Technol. 41 (13), 4781–4786. Rodrigo, M.A., Cañizares, P., Lobato, J., Paz, R., Sáez, C., Linares, J.J., 2007. Production of electricity from the treatment of urban wastewater using a microbial fuel cell. J. Power Sources 169 (1), 198–204. Rozendal, R.A., Hamelers, H.V.M., Buisman, C.J.N., 2006. Effects of membrane cation transport on pH and microbial fuel cell performance. Environ. Sci. Technol. 40 (17), 5206–5211. Torres, C.I., Lee, H.S., Rittmann, B.E., 2008. Carbonate species as OH carriers for decreasing the pH gradient between cathode and anode in biological fuel cells. Environ. Sci. Technol. 42 (23), 8773–8777. Wei, Y., Van Houten, R.T., Borger, A.R., Eikelboom, D.H., Fan, Y., 2003. Minimization of excess sludge production for biological wastewater treatment. Water Res. 37 (18), 4453–4467.
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Effect of pH on nutrient dynamics and electricity production using microbial fuel cells Sebastià Puig a,b,*, Marc Serra a, Marta Coma a, Marina Cabré a, M. Dolors Balaguer a, Jesús Colprim a a Laboratory of Chemical and Environmental Engineering (LEQUIA-UdG), Institute of the Environment, University of Girona, Campus Montilivi s/n, Facultat de Ciències, E-17071 Girona, Spain b Catalan Institute for Water Research (ICRA), Carrer Emili Grahit 101, Edifici H2O, E-17003 Girona, Spain
a r t i c l e
i n f o
Article history: Received 27 May 2010 Received in revised form 18 July 2010 Accepted 20 July 2010 Available online 24 July 2010 Keywords: Domestic wastewater Microbial fuel cells Nutrients Organic matter pH
a b s t r a c t The aim of this work was to study the effect of pH on electricity production and contaminant dynamics using microbial fuel cells (MFCs). To investigate these effects, an air-cathode MFC was used to treat urban wastewater by adjusting the pH between 6 and 10. The short-term tests showed that the highest power production (0.66 Wm3) was at pH 9.5. The MFC operation in continuous control mode for 30 days and at the optimal pH improved the performance of the cell relative to power generation to 1.8 Wm3. Organic matter removal (77% of influent COD) and physical ammonium loss were directly influenced by pH and followed the same behavior as the power generation. At a pH higher than the optimal one, anodic bacteria were affected, and power generation ceased. However, biological nitrogen processes and phosphorus dynamics were independent of the exoelectrogenic bacteria. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Biological domestic wastewater treatment is an energydemanding and sludge-producing process. Energy use for aeration in wastewater treatment plants (WWTPs) can account for up to 50% of operational costs. Aerated treatment processes also produce large amounts of excess sludge. The treatment and disposal of this sludge currently presents an increasing challenge due to economic, environmental and regulatory factors (Wei et al., 2003). Therefore, the community is looking for new technologies that require a lower energy demand or that produce energy and lower sludge production. One of the newest and most promising approaches is the use of microbial fuel cells (MFCs) to simultaneously treat wastewater while also generating electricity (Liu et al., 2004; Logan et al., 2006). MFCs offer the possibility of harvesting electricity from organic waste and renewable biomass (Lee et al., 2008; Rabaey and Verstraete, 2005). These are attractive sources of energy because they are ‘carbon-neutral’; the oxidation of the organic matter only releases recently fixed carbon back into the atmosphere (Lovley, 2006).
* Corresponding author at: Laboratory of Chemical and Environmental Engineering (LEQUIA-UdG), Institute of the Environment, University of Girona, Campus Montilivi s/n, Facultat de Ciències, E-17071 Girona, Spain. Tel.: +34 972418281; fax: +34 972418150. E-mail address: [email protected] (S. Puig). 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.07.082
In a MFC, organic substrates are oxidized by exoelectrogenic bacteria (Logan, 2009). This oxidation produces electrons that are transferred to an anode electrode and then flow to a cathode. The anode and cathode are linked by a conductive material containing a resistor. Protons produced at the anode migrate through the solution across a cation exchange membrane (CEM) to the cathode chamber, where they combine with a reducible compound and electrons. A fuel cell converts chemical energy into electrical energy without the inefficiencies that arise from combusting fuel (Lovley, 2006). A wide range of substrates can be oxidized (Pant et al., 2010). However, domestic wastewater treatment using MFCs is still under investigation. Aelterman et al. (2006) demonstrated that MFCs might occupy a market niche in terms of a stand-alone source of electricity for use in wastewater treatment. Cha et al. (2010) studied the direct application of MFCs in aeration tanks. Rodrigo et al. (2007) reached a maximum power density of 25 mWm2 by oxidizing the organic matter from urban wastewater. Ahn and Logan (2010) treated domestic wastewater at an organic loading rate of 54 kg COD m3 d1 (25.8% COD removal) and a highest power generation of 0.42 Wm2 (12.8 Wm3). In terms of suitable MFC operational conditions, Liu et al. (2005) examined the effect of ionic strength, temperature and reactor configuration on power generation. They found that the power densities can be increased by changing operation parameters. In this way, Ahn and Logan (2010) demonstrated that using temperature-phased conditions in MFCs for domestic wastewater
S. Puig et al. / Bioresource Technology 101 (2010) 9594–9599
treatment produced fewer solids, higher efficiency and power savings. The pH is another operational parameter that affects MFC performance (2007). The pH is crucial to the MFCs’ power output. Generally, bacteria require a pH close to neutral for optimal growth; however, oxygen reduction on the cathode electrode results in an alkaline pH (Rozendal et al., 2006). Ren et al. (2007) found that in a double-chamber MFC, the power production decreased significantly when the final pH dropped to 5.2 due to the acidic products of fermentation; the power resumed quickly when the pH was recovered to 7.0. However, in air-cathode or singlechamber MFCs, the pH affects both the anodic and cathodic reactions in which a high pH (8–10) inhibits the anodic bacterial activities to some extent, but might be favorable to the cathodic reaction, thus improving the overall performance (He et al., 2008). Jadhav and Ghangrekar (2009) studied the performance of MFCs subjected to a variation in pH while treating synthetic wastewater in two-chamber MFCs. They found that the highest current was generated when the difference in pH between the chambers was higher. Fan et al. (2007) demonstrated the excellent performance of MFCs using bicarbonate as pH buffer and proton carrier. Torres et al. (2008) found that decreasing the pH gradient resulted in an increase in voltage efficiency by simply adding carbon dioxide to the cathode air. However, with respect to domestic wastewater treatment, questions remain on how to deal with the effects of pH on (i) power production and especially (ii) the dynamics of organic matter, nitrogen and phosphorus compounds. Additionally, pH strongly affects the biological processes that usually take place in WWTPs, such as nitrification, denitrification and biological phosphorus removal (Filipe et al., 2001; Metcalf and Eddy, 2003). The objective of this study was to investigate the shortand long-term optimal pH conditions for an air-cathode MFC to maximize the electricity generation and its effect on the dynamics of organic matter, nitrogen and phosphorus compounds for treating domestic wastewater.
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phosphorus removal difficult in a classical treatment scenario (Puig et al., 2007). 2.2. MFC configuration and operation The air-cathode MFC consisted of an anode and cathode placed on opposite sides of a single methacrylate rectangular chamber with dimensions of 32 26 400 mm (empty bed volume of 395 mL). The anode was filled with 6 35 mm rod graphite (Alfa Aesar, Germany), which reduced the compartment volume to 242 mL (net anodic compartment). The electrodes were washed in 1 N HCl and 1 N NaOH to remove possible metal and biomass contamination (Bond and Lovley, 2003). A thinner graphite electrode (28 35 mm, Sofacel, Spain) was connected to the external electricity circuit. C-cloth 0.35 mg cm2 of Pt catalyst 30% wetproofing (Clean Fuel Cell Energy LLC, USA) was used as the cathode electrode. A cation exchange membrane (CEM, NafionÒ 117, Dupont) was treated according to Liu and Logan (2004) and put between the anode and cathode frames. The anode and the cathode were connected through an external resistor to close the circuit. Fig. 1 shows a scheme of the air-cathode MFC used. Wastewater was continuously fed to the recirculation loop to (i) obtain the desired volumetric loading rate, (ii) maintain well-mixed conditions, (iii) avoid concentration gradients and (iv) avoid clogging of the granular matrix. The system was thermostated at 23 ± 2 °C, and the pH was monitored during the study. Prior to starting the study, the MFC was inoculated with effluent from a parent MFC treating synthetic wastewater (mainly composed of sodium acetate and a buffer solution). The system operated for 90 days at a mean organic loading rate of 1.86 kg COD m3 d1 (COD removal efficiency of 81%) for treating urban wastewater. The maximum power density achieved was 0.44 Wm3. Once the MFC achieved steady state, a series of experiments were carried out to study the effect of pH. 2.3. Experimental performance
2. Methods 2.1. Domestic wastewater The urban sewage came from the Quart WWTP (N.E. Spain). Raw wastewater without any primary treatment was taken. The concentration range and mean values of the principal chemical compounds during the experimental period are summarized in Table 1. The mean COD concentration in the domestic wastewater was 503 ± 148 mg COD L1 (BOD5/CODT ratio = 0.77). This value was in the range of typical wastewater concentrations. However, the nitrogen concentration was higher than expected (109.7 ± 2.5 mg N L1; 67% as ammonium) maybe for industrial influences. Consequently, the wastewater presented a low C/N ratio (5 mg COD mg1 N) that, a priori, would make biological nitrogen and
Table 1 Wastewater characteristics throughout the study. Units
Total chemical oxygen demand, CODT Soluble chemical oxygen demand, CODS BOD5/CODT ratio Total Kjeldahl nitrogen, TKN Ammonium, NHþ 4 Nitrite, NO 2 Nitrate, NO3
mg COD L1
753–378
503 ± 148
mg COD L1
418–188
328 ± 93
– mg N L1
– 112.1–106.1
0.77 109.7 ± 2.5
Phosphate, PO3 4
1 mg P-PO3 4 L
1 mg N-NHþ 4 L 1 mg N-NO 2 L 1 mg N-NO 3 L
Range
Mean ± r
Compound
75.1–72.5 0.22–0.00 0.17–0.03 8.58–5.11
73.8 ± 1.2 0.12 ± 0.09 0.10 ± 0.06 6.99 ± 1.26
A series of short- (hours) and long-term (30 days) experiments were performed in the air-cathode MFC to evaluate the pH effect on energy production and organic matter, nitrogen and phosphorus compound dynamics. The pH was controlled at different values ranging from 6 to 10 with a peristaltic pump (Fig. 1) and dosing the system with base (0.2 M NaOH) or acid (0.2 M HCl). In the shortterm experiments, we controlled the pH at the desired value and, after at least twice the hydraulic retention time (HRT; 0.14 days), we analyzed the effluent concentration and the power produced. The long-term experiment had an optimal pH fixed for 30 days. During the long-term experiments, polarization curves were observed, and influent and effluent samples were taken periodically. 2.4. Analyses and calculations Standard wastewater measurements for organic matter (total and soluble chemical oxygen demand (CODT and CODS) and 5day biochemical oxygen demand (BOD5)), nitrogen (total Kjeldahl nitrogen (TKN), ammonium NHþ NO and nitrates 2 4 , nitrites 3 NO3 and phosphate PO4 were taken regularly and analyzed according to APHA (2005). Cell potential (V) in the MFC circuit was monitored at 30-min intervals using an on-line multimeter (Alpha-P, Ditel) with a data acquisition system (MemographÒ M RSG40, Endress + Hauser). Current (I) and power (P = IV) were calculated according to Ohm’s law. Power density was calculated by dividing the power by net anode volume (Wm3). Polarization curves were obtained by varying the external resistance in the circuit and measuring the voltage according to Logan et al. (2006).
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V pH
CONTROL PANEL
ACID/BASE DOSAGE
URBAN WASTEWATER
EFFLUENT
Fig. 1. Scheme of the air-cathode MFC used to treat urban wastewater.
3. Results and discussion 3.1. Short-term power production and organic matter removal as a function of pH It is known that power generation is affected by the operational pH. In a double-chamber MFC, the highest current was observed at neutral pH (between 6.5 and 8) (Jadhav and Ghangrekar, 2009; Gil et al., 2003). However, a basic pH (around 9) was optimal in an aircathode system treating synthetic wastewater (He et al., 2008). In this study, the pH effect was studied by treating urban wastewater in the air-cathode MFC. Fig. 2 shows the influence of pH on the power density and organic matter removal. The power increased 80% from 0.36 Wm3 (at pH 6) to 0.66 Wm3 (at pH 9.5). However, at pH 10, the power density dropped to 0.5 Wm3. Fig. 2 shows the organic matter removal efficiency as a function of pH. COD removal efficiencies remained stable at around 77% ± 6% at a pH between 6 and 9.5. This efficiency coincided with the BOD5/COD influent ratio (Table 1). The mean COD concentration in the effluent was 108 ± 28 mg COD L1 and was related to the non-biodegradable organic matter. However, 404 mg COD L1 was measured in the effluent at pH 10 (removal efficiency 46%). This drop in efficiency was in relation to the power density and indicated that exoelectrogenic bacteria were affected by a pH higher than 9.5. 3.2. Effect of pH on nitrogen transformations The urban wastewater used in this study contained 109.7 ± 2.5 mg L1 of nitrogen (67% as ammonium) on average (Table 1). This nitrogen underwent some transformations in the MFC. Between
1 46.37 and 73.95 mg N-NHþ was removed over the pH range 4 L (Fig. 3). This ammonium loss could be a result of physical–chemical and/or biological mechanisms. The presence of nitrate (15.31 ± 1 5.27 mg N-NO ; 14% of influent TKN) suggested that biological 3 L nitrogen processes (mainly nitrification) took place. This production was a result of oxygen diffusion into the anode chamber (experimental oxygen mass transfer coefficient of 1.76 103 cm s1) and remained stable over the pH range, except at pH 6, where 1 only 1 mg N-NO was produced, which indicated that the bac3 L teria were being affected. Another way that ammonium was removed was via physical–chemical mechanisms. Kim et al. (2008) determined that these ammonia losses occurred in single-chamber MFCs due to volatilization of ammonium at the cathode. They found that ammonium present in the wastewater disappeared because of an elevated pH near the cathode and was accelerated by electricity generation. In our case, ammonium removal increased at higher pH and power density in correlation with the literature. Moreover, batch studies were performed to determine the rate of ammonium diffusion into the cathode chamber (ammonium mass transfer coefficient of 3.11 104 cm s1). Once power generation ceased at pH 10 (Fig. 2), the concentration gradient was reduced due to passive diffusion of ammonium across the membrane and was accompanied by transfer of the other charged species (Kim et al., 2008). In addition, to determine the pH of the cathode in the MFC, the carbon cloth was washed with distilled wastewater. The cathodic pH was 11.0 because of proton consumption in the cathode reaction. Consequently, transport of cationic species (in our case, ammonium) other than protons resulted in an increased pH in the cathode chamber (Rozendal et al., 2006). In addition to these two processes, heterotrophic denitrification (reduction of nitrate to nitrogen gas using organic matter from
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pH 5.5
6.0
6.5
7.0
7.5
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
10.5
8.0
8.5
9.0
9.5
10.0
10.5
COD removal (%)
Power density (W·m-3)
0.8
0.6
0.4
0.2
0.0 90 80 70 60 50 40 30 20 10 0
pH Fig. 2. Comparison between the power density and the COD removal efficiencies as a function of pH for treating urban wastewater.
pH
Power
density (W·m-3)
55
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
10.5
7.5
8.0
8.5
9.0
9.5
10.0
10.5
0.8 0.6 0.4 0.2 0.0 Ammonium removed Nitrate observed Nitrogen difference
70 60
(mg N·L-1)
Nitrogen concentration
6.0
50 40 30 20 10 0 5.5
6.0
6.5
7.0
pH Fig. 3. Comparison between the power density and the nitrogen compounds concentrations as a function of the pH for treating urban wastewater.
wastewater) and nutrient assimilation should be taken into account. Unfortunately, the percentage of each process could not be determined in this study. 3.3. Effect of pH on phosphate evolution The air-cathode MFC was used to treat urban wastewater with a phosphate content of 6.99 ± 1.26 mg P-PO3 4 (Table 1). Fig. 4 shows the difference between the phosphate concentrations in the effluent with respect to the influent at different pH values. Phosphate
was removed at pH values higher than 8.5 and was released at lower pH values, thereby increasing the concentration in the effluent. As stated earlier, the nitrogen and phosphate transformations could be due to two mechanisms: biological and/or physical– chemical. Biological phosphorus bioaccumulation was ruled out because fluorescence in situ hybridization (FISH) analysis showed a low population of phosphate-accumulating organisms (1% of total bacteria). Thus, the phosphate trends should be due to the precipitation of phosphate salts (usually as calcium or potassium phosphate). This chemical precipitation is strongly influenced by
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1.0
10
6 4
0.6
2
0.4
-2
0
-4 -6
0.2 Power density Phosphate increment
-8
0.0 55
6.0
6.5
7.0
concentration (mg P·L-1)
0.8
Increment of phosphorus
Power density (W·m-3)
8
7.5
8.0
8.5
9.0
9.5
10.0
-10 10.5
pH Fig. 4. Increment of phosphorus concentration (effluent minus influent) and power density at different pHs for treating urban wastewater.
800
2.0
700
Power production (W·m-3)
pH 7.5 (day -10) pH 9.5 (day 2) pH 9.5 (day 15) pH 9.5 (day 30)
Voltage (mV)
600 500 400 300 200
1.5
1.0
0.5
100 0
0.0 0
2
4
6
8
10
12
14
16
0
2
Current density (A·m-3)
4
6
8
10
12
14
16
Current density (A·m-3)
Fig. 5. Polarization curves of the MFC operated for treating urban wastewater. Cell voltage (left) and power production (right) as a function of the current density.
pH (Carlsson et al., 1997). The solubility equilibrium of these phosphate salts decrease at a pH higher than 8.5, resulting in a decrease in soluble phosphate. On the other hand, at acidic pH levels, the salts were soluble and phosphate increased. This chemical mechanism was independent of the exoelectrogenic bacteria because it did not follow the power generation trend. 3.4. Effect of the optimal pH on energy production and removal efficiency A pH of 9.5 was considered optimal for treating urban wastewater and generating electricity based on the short-term experiments. To be confident in the results and to remove any inhibitory effect, the pH was maintained at 9.52 ± 0.01 in the aircathode MFC for 30 days. Fig. 5 compares the polarization curves of the MFC operating at pH 7.5, 10 days before fixing the pH setpoint and then 2, 15 and 30 days after the set-point implementation at pH 9.5. Ten days before starting the pH control, the polarization curve reached the maximum current and power densities of 4.89 A m3 and 0.42 Wm3, respectively. After 2 days at pH 9.5, the power density was greater than the density previously obtained. The current and power densities increased 26% and 75%, respectively. The MFC continued evolving and reached 9.37 A m3 and 1.18 Wm3 on day 15. Moreover, after 30 days of continuous
pH control, the maximum current and power densities achieved were 13.74 A m3 and 1.8 Wm3, respectively. The maximum power increased, but the internal resistance decreased from 1454 (before pH control) to 216.8 O (after 30 days of pH control). The increase in the electricity production corresponded with the organic matter removal efficiency (83% of the influent) and ammonium losses (72% of influent ammonium), but it did not affect the biological processes associated with nitrogen and phosphorus and followed the same behavior in both the short- and long-term experiments. 4. Conclusions The pH effect on electricity production treating wastewater was studied using an air-cathode MFC. The optimal pH for the shortterm study was 9.5 (power density of 0.66 Wm3). In the longterm study, the bacteria grew at this pH increasing the power density to 1.8 Wm3. Electricity production was based on the amount of organic matter; however, the removal efficiencies remained stable around 77 ± 6%. Ammonium loss via physical–chemical mechanisms was directly influenced by the power production and pH, but biological nitrogen processes remained stable. Finally, phosphate equilibrium was independent of the power production and influenced by pH changing the solubility constant.
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