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Effect of dissolved oxygen on nitrogen and phosphorus removal and electricity production in microbial fuel cell
Accepted Manuscript Effect of dissolved oxygen on nitrogen and phosphorus removal and electricity production using microbial fuel cell Qinqin Tao, Jingjing Luo, Juan Zhou, Shaoqi Zhou, Guangli Liu, Renduo Zhang PII: DOI: Reference:
S0960-8524(14)00661-0 http://dx.doi.org/10.1016/j.biortech.2014.05.002 BITE 13410
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Bioresource Technology
Received Date: Revised Date: Accepted Date:
8 March 2014 27 April 2014 2 May 2014
Please cite this article as: Tao, Q., Luo, J., Zhou, J., Zhou, S., Liu, G., Zhang, R., Effect of dissolved oxygen on nitrogen and phosphorus removal and electricity production using microbial fuel cell, Bioresource Technology (2014), doi: http://dx.doi.org/10.1016/j.biortech.2014.05.002
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Effect of dissolved oxygen on nitrogen and phosphorus removal and electricity production using microbial fuel cell Qinqin Taoa, Jingjing Luoa, Juan Zhou a, Shaoqi Zhou a,b,c,d* Guangli Liu e, Renduo Zhange a
College of Environmental Science and Energy, South China University of Technology,
Guangzhou Higher Education Mega Center, 510006, PR China b
Guizhou Academy of Sciences, Shanxi Road 1,Guiyang, 550001, PR China
c
State Key Laboratory of Subtropical Building Sciences, South China University of
Technology, Guangzhou, 510641, PR China d
Key Laboratory of Environmental Protection and Eco-remediation of Guangdong
Regular Higher Education Institutions, South China University of Technology, Guangzhou Higher Education Mega Center, 510006, PR China e
School of Environmental Science and Engineering, Sun Yat-Sen University,
Guangzhou, 510275, PR China Abstract Performance of a two-chamber microbial fuel cell (MFC) was evaluated with the influence of cathodic dissolved oxygen (DO). The maximum voltage, coulombic efficiency and maximum power density outputs of MFC decreased from 521 mV to 303 mV, 52.48 % to 23.09 % and 530 mW/ m2 to 178 mW/ m2 with cathodic DO declining. Furthermore, a great deal of total phosphorus (TP) was removed owing to chemical precipitation (about 80 %) and microbial absorption (around 4 % ~ 17 %). COD was *Corresponding author. Tel.: +86 13535279381; fax: +86 20 85511266. E-mail address: [email protected]
first removed in anode chamber (> 70 %) then in cathode chamber (< 5 %). Most of nitrogen was removed when the cathodic DO was at low levels. Chemical precipitates formed in cathode chamber were verified as phosphate, carbonate and hydroxyl compound with the aid of scanning electron microscope capable of energy dispersive spectroscopy (SEM–EDS), X-ray diffractometer (XRD) and fourier transform infrared spectroscopy (FTIR). Keywords: Microbial fuel cell; Phosphorus; Nitrogen; Dissolved oxygen; Precipitate 1. Introduction Nitrogen and phosphorus were two main contaminants of wastewater. Excess discharge of nitrogen and phosphorus has not only caused vast waste of nitrogen and phosphorus resources, but has also led to eutrophication of lakes (Chang et al., 2000; Domagalski et al., 2007), which leads to taste and odor problems and even contribute to human health problem and can therefore limit the direct use of lake for drinking water purposes. To protect lakes and other natural water from eutrophication, stringent nutrient level is set for the effluent from the wastewater treatment plants (WWTP). Recently, microbial fuel cells (MFCs) have drawn much attention (Logan et al., 2006; Liu et al., 2004; Jiang et al., 2011) as a new technology for simultaneous wastewater treatment and electricity generation. A typical MFC consists of biological anode and abiotic cathode chambers that are separated by a proton exchange membrane (PEM) (Liu et al., 2005). In the anode compartment, the microorganisms oxidize organic compounds and generate electrons and protons in the process. The electrons are absorbed by the anode and are transported to the cathode through an external circuit.
The protons migrate through the solution across a PEM to the cathode chamber. In the cathode compartment, free electrons are delivered to terminal electron acceptor (such as oxygen), then combined with protons to form water (Clauwaert et al., 2007; Logan, 2009). The particular advantages of MFC technology for wastewater treatment include high efficiency, ambient operating conditions, small equipment sizes, minimal sludge generation and rapid start-up as compared to conventional biological wastewater treatment methods such as activated sludge process and bio-film process. A number of MFCs were used for simultaneous nitrate removal and electricity generation, in which denitrification was accomplished by microorganisms in the cathode chamber (Clauwaert et al., 2007; Lefebvre et al., 2008). Nitrate could be reduced to nitrite and finally to nitrogen gas on cathode. The eight electrons involved in the reaction are balanced by protons or electrons provided through anodic reactions. Sodium bicarbonate is required to maintain the pH during electrochemical reduction of nitrate since the electrolyte gradually becomes alkaline. High alkaline environments prompt the generation of precipitates of magnesium hydroxide and calcium carbonate around the cathode when soluble calcium and magnesium salts are present in the water. Virdis et al. (Virdis et al., 2008) investigated simultaneous power generation, carbon and nitrogen removal by using a combined denitrification MFC and nitrifying bioreactor. In this configuration, the ammonium containing effluent from the anode chamber was pumped into an external aerobic reactor for nitrification, and the nitrified liquid subsequently flowed into the cathode chamber for denitrification. This system obtained a maximum power output and a maximum nitrogen removal efficiency of 34.6 W/m3
and 0.41 kg NO3--N/(m3 ·d), respectively. However, the set-up of an external nitrifying reactor makes it difficult to use in the field. Some different nitrogen removal methods with MFCs were reported in published papers. High levels of ammonium in swine wastewater was removed by MFCs (Min et al., 2005), further investigation concluded that ammonium removal was due to either ammonium ion diffusion from anode chamber to cathode chamber in a two-chambered MFC or ammonium volatilization in an single-chamber MFC (Kim et al., 2008). Ammonium used as the fuel for electricity generation in a rotating-cathode MFC was first reported by He et al. (He et al.,2009), high ammonium removal efficiency and electricity generation were achieved. The study solely emphasizes the feasibility of electricity generation from synthetic wastewater using ammonium as the fuel. It can’t exclude the possibility that during ammonium oxidation, autotrophs produced organic compounds for heterotrophs to generate electricity. As for phosphorus removal by MFCs, only a few studies have been published to date. Struvite crystallization is a promising method for phosphorus recovery from waste water (Nelson et al., 2003). The flux of alkali cations in combination with the proton consumption during the oxygen reduction leads to the accumulation of hydroxide, and thus to a pH increase (Zhao et al., 2006). Struvite can be recovered near/on the cathode (Ichihashi et al., 2012), for the solubility of it decreases with increasing pH (Doyle and Parsons, 2002). Fischer et al. (Fischer et al., 2011) investigated phosphate recovery from digested sewage sludge through MFC technology. In this system, MFC was a place for orthophosphate release from iron phosphate contained digested sewage sludge.
Orthophosphate containing effluent from MFC with a subsequent addition of magnesium and ammonium, as well as pH adjustment, was needed for phosphorus recovery as struvite. When air-cathode MFCs were operated with swine wastewater (Ichihashi et al., 2012), a great amount of crystals were observed accumulated on the surface of the liquid side of the cathode. The crystals were proved to be struvite after a series of chemical characterization. Based on those results illustrated above, we hypothesized the possibility that nitrogen and phosphorus can be removed from wastewater by using MFC technology. In this study, a two-chamber MFC was used for simultaneous nitrogen and phosphorus removal from synthetic wastewater. The effects of cathodic DO on chemical oxygen demand (COD), nitrogen and phosphorus removal along power generation were evaluated. Furthermore, the proportion of phosphorus and nitrogen removed by microbial absorption or chemical precipitation were estimated. Sediments deposited on the inner wall of anode chamber, cathode chamber, anode surface and cathode surface were characterized by scanning electron microscopy (SEM). Chemical precipitates were also characterized by energy dispersive spectroscopy (EDS), X-ray diffractometer (XRD) and fourier transform infrared spectroscopy (FTIR). 2. Methods 2.1. MFC configuration and operation The configuration of the two-chamber MFC is shown in Fig. 1. The reactor was based on the tubular-reactor design of Liu and Logan (Liu and Logan, 2004). This reactor was converted into a two-chamber structure by assembling two 4-cm-long
cylindrical chambers (3-cm diameter, formed of Plexiglas plastic) together. The carbon paper (3-cm diameter, the anode), proton exchange membrane (PEM, NAFION 117, 3-cm diameter) and carbon cloth (3-cm diameter, the cathode, containing 0.5 mg Pt/cm2) were sandwiched with PEM in the middle. The sandwiched structure was placed between anode chamber and cathode chamber. Sealing was ensured by silicone sheets inserted between each frame. The cathode chamber was connected to a blown glass bottle with a volume of 100 mL. Oxygenation of the solution in blown glass bottle was supplied by an air pump, and the airflow was adjusted with an air flow rotameter. The cathode electrolyte was recirculated between cathode chamber and glass bottle at a flow rate of approximately 20 mL/min, as previously described by Ichihashi and Hirooka (Ichihashi and Hirooka, 2012). The anode and cathode were connected through an external resistor with resistance 1000 Ω. All experiments were performed at room temperature (25 ℃~31 ℃). The anode and cathode chambers were inoculated with mixed anaerobic sludge and aerobic sludge (v/v = 1:1), which were collected from Shijing municipal wastewater treatment plant, Guangzhou, China. A synthetic wastewater was used throughout the study. The synthetic wastewater mainly containing NaHCO3 5.96 g/L, NaC2H3O2 1.00 g/L, KH2PO4 0.54 g/L, NH4Cl 0.21 g/L, metals, trace minerals and vitamins. The initial pH of synthetic wastewater was adjusted to 7.0±0.2. Synthetic wastewater was fed into anode chamber when every time the voltage decreased to less than 50 mV. The effluent from the anode chamber was subsequently directed into the cathode chamber. To investigate the effects of cathodic dissolved oxygen (DO) on MFC performance
for electricity production and nitrogen and phosphorus removal, experiments were conducted under the following conditions: The two-chamber MFC was operated in fed–batch mode. After MFC achieved 3 cycles of stable voltage outputs, experiments for each treatment were carried out in duplicate. To make certain the relationship between cathodic DO and nitrogen/phosphorus removal and electricity production, four DOs ( 3.5 mg/L, 2.8 mg/L, 2.5 mg/L, 2.0 mg/L) were successively applied to the cathode liquor ( effluent from the anode chamber). The DO of anode liquor was controlled at 0.05 ~ 0.10 mg/L throughout the study. The effluent from anode chamber was aerated in another brown glass bottle which is independent of the two-chamber MFC at four DOs (3.5 mg/L, 2.8 mg/L, 2.5 mg/L, 2.0 mg/L) to investigate the effect of microbes in cathode chamber for nitrogen and phosphorus removal. Moreover, in order to investigate the effect of catholyte pH increase on nitrogen and phosphorus removal, effluents from anode chamber were centrifuged at 8000 rpm for 5 min and the supernatants (solution I) were filtered (0.45 µm). The pH of filtrate (solution II) was adjusted to 9.1 (approximately the pH of effluent from cathode chamber) with 2-3 droplets of strong sodium hydroxide solution (precipitation occurs with increasing pH), then they were centrifuged at 8000 rpm for 5 min, the supernatants (solution III) were used for measurement. 2.2. Analytical techniques Samples from the anode chamber and the cathode chamber were taken at the end of an electricity production cycle. Samples were centrifuged at 8000 rpm for 5 min and the supernatants were used for measurement. Chemical oxygen demand (COD), total
nitrogen (TN, the sum of ammonium (N-NH4+), nitrates (N-NO3-) and nitrites (N-NO2-)) and total phosphorus (TP) were measured according to the Standard Methods (APHA) (APHA, 2005). Values of pH were measured using a pH meter (PHS-25, Leici, China). Solution DO values were measured using a dissolved oxygen meter (JPB-607, Leici, China). Voltage across the external resistance was monitored at one minute intervals using a data acquisition system (M2700, Keithley, USA) connected with a computer. Current (I) and power (P=IV) were calculated according to Ohm’s law. The areal current density and power density were calculated by dividing current and power by the net anodic or cathodic area (7.065 cm2). Coulombic efficiency (CE) was calculated as previously described (Chen et al., 2012). During the test, the polarization curves were detected by varying external resistances from 30 Ω to 50000 Ω with an interval of 1 min to gain stable voltages. The internal resistance and maximum power density were obtained by analyzing the polarization curves (Watanabe, 2008). The precipitate samples were analyzed by scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS, Car Zeiss EVO LS10, UK) to examine the morphology of the samples as well as elemental composition. The precipitates obtained when pH of solution II adjusted to 9.1 were also identified by an 18 kW rotating anode X-ray diffractometer (D8 ADVANCE, Bruker. Co., Germany) and a fourier transform infrared spectroscopy (Vector 33, Bruker. Co., Germany). Precipitates that were used for measurement were washed and re-suspended in milli-Q water to remove soluble chemicals, and then they were centrifuged again at 8000 rpm for 5 min. These re-suspension and centrifugation steps were repeated twice, and finally the
precipitates were dried in dried pot. 3. Results and discussion 3.1. Nitrogen and phosphorus removal from wastewater Nitrogen and phosphorus in the synthetic wastewater were in the form of N-NH4+ and P-PO43- in the beginning. The concentration of N-NH4+ and P-PO43- were 55.0 and 122.9 mg/L, respectively. In order to find out the relationship between cathodic DO and nitrogen/phosphorus removal, the change in the TP, N-NH4+, N-NO3- and N-NO2- were measured during the electricity generation cycle. The TP, N-NH4+, N-NO3- and N-NO2concentrations in the electricity generation process at different cathodic DO are list in Table 1. Since anodic DO was low, there was not enough oxygen for bacteria in anode chamber to aerobic accumulate phosphorus and partial nitrification or nitrification of ammonia. Therefore, the concentration of TP, N-NO3- and N-NO2- in the effluent of anode chamber was nearly the same as synthetic wastewater. However, the concentration of N-NH4+ in the effluent of anode chamber decreased about 10 mg/L compared with synthetic wastewater. The decrease of N-NH4+ may due to the consumption for microorganism's growth and reproduction, ammonium ion diffusion from the anode chamber to the cathode chamber or ammonium volatilization. When the cathodic DO was at 3.5, 2.5 and 2.0 mg/L, TP removal efficiencies were more than 90 % and have no significant difference with each other. However, when the cathodic DO was at 2.8 mg/L, TP removal efficiency decreased a little, but still more than 85%. The rate of N-NH4+ removed and converted to N-NO3- or N-NO2- are very different.
Oxygen in the cathode chamber may be consumed in the following ways: 1) used as the electron acceptors for electricity generation; 2) used by polyphosphate accumulating microorganisms (PAOs) to aerobic accumulate phosphorus; 3) used by nitrifiers to partial nitrification or nitrification of ammonia; 4) used by some electrochemically inactive microorganisms in the cathode chamber. When the cathodic DO was at high levels (3.5 and 2.8 mg/L), most of N-NH4+ in the catholyte was converted to N-NO2-, but the TN didn’t decline markedly. There are two possible reasons: 1) oxygen in the catholyte was too much for denitrifying bacteria; 2) N-NO2wouldn’t be used as the electron acceptor for electricity generation when there was enough oxygen in the catholyte. A slight drop in TN was most likely owning to ammonium volatilization (because the cathode chamber was an open system, and was aerated with air) and microorganism’s growth and reproduction. When the cathodic DO was at low levels (2.5 and 2.0 mg/L), the removal efficiency of TN was more than 85%. Nitrogen may be removed by the following three ways: 1) nitrogen was removed by the anaerobic ammonia oxidation (ANAMMOX) process; 2) N-NH4+ was first oxidized to N-NO2- by oxygen, then N-NO2- was reduced to N2 under the action of denitrifying phosphate accumulating organisms (DNPAOs) for there was not enough oxygen in the catholyte. DNPAOs have the ability of simultaneous nitrogen removal by denitrification and phosphorus removal (Liu et al., 2013). 3) N-NH4 + was first oxidized to N-NO2- by oxygen, then N-NO2- was reduced to N2 by using as the electron acceptor for electricity generation. The optimum pH for nitration and denitrification were 6~9 and 7.0~7.5. The pH of the effluent from anode chamber and cathode chamber were 7.0±0.2 and 9.2±
0.2, this observation is equivalent to saying that the pH of the catholyte was increased from 7.0±0.2 to 9.2±0.2 during the electricity generation process. High pH will make negative effect on denitrification reaction. The pH of the catholyte was increased gradually, so the nitration and denitrification will occur at the beginning when the pH was not too high. When the pH rise to a certain value, denitrification reaction will be seriously inhibited, then the nitrogen was removed for using N-NO2- as the electron acceptor for electricity generation. 3.2. Mechanism of nitrogen and phosphorus removal and precipitates formation Effluent from anode chamber was aerated in a brown glass bottle which is independent of the two-chamber MFC to investigate the role of the cathode chamber for nitrogen and phosphorus removal.The optimal aeration time for TP removal is 6 h (data not show). The concentration of TP declined more than 80 % under each DO, however, N-NH4+ in the effluent almost have no change whether in form or quantity (Table 2). No obvious change of N-NH4+ in form or quantity indicating that there was no nitrifying bacteria or anaerobic ammonia oxidation bacteria in the effluent for the long time anaerobic environment in anode chamber. Microbial community of nitrifying bacteria or anaerobic ammonia oxidation bacteria could not be detected in anode chamber (Zang et al., 2012). Nitrogen removal was due to the nitrifying bacteria or anaerobic ammonia oxidation bacteria in cathode chamber, but not in the effluent from anode chamber. Some sediment deposited on the inner wall of anode chamber, cathode chamber, anode surface and cathode surface after operating for a period of time. Sediments were
characterized by SEM, results show that sediments in cathode chamber was the mixture of microbe cells and chemical precipitates, however, no chemical precipitates could be found on anode surface, cathode surface or in anode chamber, they were all microbe cells (Supplementary Fig S1). No needle shaped prismatic morphology typical of struvite described by literature (Cusick and Logan, 2012) was found in all SEM images. Chemical precipitates formed in cathode chamber may due to the progressively alkaline cathode liquor, and they were believed to be a possible pathway for phosphorus and nitrogen removal. No obvious concentration decrease of TP, N-NH4+, N-NO3- and N-NO2- in solution II proved no nitrogen or phosphorus containing precipitates were formed in anode chamber (Table 3). That’s why no chemical precipitate was observed in anode chamber. Concentration of TP decreased from 121.8±3.6 to 24.0±2.1 mg/L when solution pH was adjusted to 9.1. This result suggests that most of TP (about 80 %) was removed by chemical precipitation, only a small portion of TP (around 4 %~17 %) was removed by microbe absorption. Also, a slight drop in concentration of N-NH4+ was observed, that may be because ammonium volatilization with the progressively alkaline cathode liquor and continuous aeration. 3.3. Precipitates analysis In order to learn more about the structure and composition of the precipitates, EDS, FTIR and XRD were recorded (Supplementary Fig S2, Fig S3 and Fig S4). The EDS patterns from the precipitate deposit in cathode chamber and obtained when pH of solution II adjusted to 9.1 are exactly similar. This means that the composition of two chemical precipitates were extremely similar. They were both mainly composed of O,
Mg, P, C, Na. Mn and Zn absorption peaks were also detected in the precipitate obtained when pH of solution II adjusted to 9.1, but not in the precipitate from cathode chamber. That’s because Mn and Zn were trace elements for microbes, they will be consumed with microorganism existence. There are lots of microbes in MFC, but nearly no microbes in solution II (they were filtered out). Characteristic absorption peaks of PO43-, CO32- and OH- were found in FTIR spectrum of the precipitate obtained when pH of solution II adjusted to 9.1. This suggests that phosphate, carbonate and hydroxyl compound precipitates were formed with increasing pH. A rather broad peak without any obvious sharp peak was detected in the XRD pattern. This might be because particles in the precipitate were not in crystal but amorphous, or the precipitate is a mix of various chemical substances. Each chemical substance has specific XRD characteristic peaks. There were numerous character peaks in the XRD pattern and were close to each other. As a result, we see a rather broad peak in the XRD pattern. 3.4. MFC performance After acclimation for approximately twenty days, stable electricity production was achieved in the two-chamber MFC. Fig. 2 shows the voltage vs. time for two-chamber MFC with cathodic DO over three operation cycles. The output voltage of two-chamber MFC under each cathodic DO began to increase noticeably after substrate dosing and peaked soon afterwards. The maximum output voltage of two-chamber MFC under four cathode DOs were 521 mV, 476 mV, 453 mV and 303 mV, respectively. The maximum output voltage of two-chamber MFC was decreased with the declining cathodic DO, especially when cathodic DO declined from 2.5 to 2.0 mg/L, 150 mV maximum output
voltage was decreased. This demonstrates that cathodic DO is a key factor for electricity generation and 2.0 mg/L DO in cathode liquor is far from enough for electron acceptor but sufficient for microbes in cathode chamber for nitrogen and phosphorus removal (good removal efficiency of N and P were obtained when cathodic DO was at 2.0 mg/L, the datum were list in Table 1). The average COD removal efficiency by two-chamber MFC under four cathodic DOs were range from 77.3 % to 81.0 %, in which more than 70 % was removed by microbes in anode chamber and less than 5 % by microbes in cathode chamber (Fig.3). There was no significant COD removal efficiency difference when the cathodic DO was among four DOs. The vast majority of COD was consumed by microbes in anode chamber. It can be hypothesized that a lot of organics was assimilated by phosphorus-accumulating bacteria in anaerobic anode chamber and stored in cells in polyhydroxybutyrate (PHB) form. However, when the effluent of anode chamber was directed to aerobic or oxygen-poor cathode chamber, PHB was used as the carbon source by phosphorus-accumulating bacteria (Comeau et al., 1986). Some electrochemically inactive microorganisms consumed a small amount of organics in cathode liquor. The average coulombic efficiency of two-chamber MFC was decreased from 52.48 % to 23.09 % with declining cathodic DO (Fig. 3). This is thought to be a result of the increase of the ratio of electrochemically inactive microorganisms in MFC or increase of internal resistance with decreasing cathodic DO. It was estimated that up to 28 % of the glucose added to a MFC could be lost through aerobic respiration by
bacteria sustained by oxygen fluxes through the membrane (Liu and Logan, 2004). The polarization curves of the MFC with cathodic DO are illustrated in Fig. 4 The maximum power densities obtained in MFC under four cathdic DOs were 530, 459, 428 and 178 mW/ m2, respectively. The internal resistances were 238, 284, 298 and 366 Ω， respectively. The reduction of cathodic DO resulted in an increased internal resistance that leads to maximum power density reduction. Compared with other MFCs reported in the literatures (Freguia et al., 2007; Xia et al., 2013), these values are not very high. This may be attributed to the MFC configuration and operating mode used in our work. 4. The advantages of the two-chamber MFC system The two-chamber MFC configuration proposed in the study shows many advantages. Firstly, the MFCs system is an economical technology not only for energy production but also for the reduction of aeration costs. In the two-chamber MFC system, nitrification, denitrification, aerobic accumulate phosphorus processes and precipitation take place in the cathode chamber. Oxygen in the aerated air was effectively used for electricity production, nitrogen and phosphorus removal, which reduces the aeration costs. Secondly, the two-chamber MFC is a biocathode MFC, which uses bacteria as a biocatalyst to accept electrons from the electrode, provides a different path that avoids the use of chemical catalysts. Compared with an abiotic cathode MFC, bacterial biocathode MFC not only lowers the cost of construction and operation of MFCs, but also increases the environmental sustainability of MFC systems. Thirdly, most of phosphorus in the wastewater could be recovered by the way of sedimentation for the increasing pH in cathode chamber.
5. Conclusions Cathodic DO makes important effects on MFC performance and phosphorus/nitrogen removal. When cathodic DO decreased, the electricity generation performance declined sharply for the less and less electron acceptor in cathode chamber. However, changes on the removal of TP and COD are small with declining cathodic DO. Nitrogen could be removed when the cathodic DO was at low levels. Phosphate, carbonate and hydroxyl compound were formed in the cathode chamber during the reaction process. The TP was removed by chemical precipitation and microbe absorption.
Acknowledgements Financial supports from the National Natural Science Foundation (21277052), the Department of Guangdong Education and the Science and Technology Bureau (2010), the State Key Laboratory of Subtropical Building Science (2013ZC03, 2014ZB04) and the Environmental Protection Bureau (201203) are gratefully acknowledged.
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Figure Captions Fig.1. Generalized schematic of the two-chamber MFC, showing two chambers separated by carbon paper-proton exchange membrane (PEM)-carbon cloth. The cathode liquor was constantly recirculated between cathode chamber and brown glass bottle. Fig. 2. Electricity production of two-chamber MFC with cathodic DO. Fig.3. COD removal efficiency and coulombic efficiency of two-chamber MFC with cathodic DOs. Fig. 4. Voltage and power density vs. current density of the MFC with cathodic DO
Tables and Figures Table.1. TP, N-NH4+, N-NO3- and N-NO2- concentrations with cathodic DO Synthetic water
TP (mg/L) N-NH4+ (mg/L) N-NO3(mg/L) N-NO2(mg/L)
Effluent of anode chamber
122.9
Effluent of cathode chamber (DO=3.5 mg/L) 121.8±3.6 7.5±2.2
Effluent of cathode chamber (DO=2.8 mg/L) 13.7±2.3
Effluent of cathode chamber (DO=2.5 mg/L) 8.6±1.3
Effluent of cathode chamber (DO=2.0 mg/L) 8.5±1.6
55.0
45.2±1.0
6.1±1.4
14.5±1.7
2.1±0.3
8.0±0.5
0
≈0
0.1±0.1
0.4±0.2
4.4±1.5
0.2±0.1
0
≈0
32.3±2.8
25.9±3.3
0.5+0.2
1.5±0.2
Table.2. TP, N-NH4+, N-NO3- and N-NO2- concentrations after the effluents from anode chamber were aerated in a separate brown glass bottle at various DOs Effluent of anode chamber TP (mg/L) +
N-NH4 (mg/L) N-NO3(mg/L) N-NO2(mg/L)
121.8±3.6
Aerated with DO at 3.5 mg/L 16.5±1.4
Aerated with DO at 2.8 mg/L 16.6±2.1
Aerated with DO at 2.5 mg/L 18.2±2.2
Aerated with DO at 2.0 mg/L 18.9±1.4
45.2±1.0
38.8±4.3
41.3±2.8
42.0±1.3
44.1±1.5
≈0
≈0
≈0
≈0
≈0
≈0
≈0
≈0
≈0
≈0
Table.3. TP, N-NH4+, N-NO3- and N-NO2- concentrations in the effluent of anode chamber before and after pH adjustment Solution I
Solution II
Solution III
TP (mg/L)
121.8±3.6
120.4±3.6
24.0±2.1
N-NH4+ (mg/L)
45.2±1.0
43.8±1.6
39.5±2.8
N-NO3- (mg/L)
≈0
≈0
≈0
N-NO2- (mg/L)
≈0
≈0
≈0
Fig.1. Generalized schematic of the two-chamber MFC, showing two chambers separated by carbon paper-proton exchange membrane (PEM)-carbon cloth. The cathode liquor was constantly recirculated between cathode chamber and brown glass bottle.
DO=3.5 mg/L DO=2.5 mg/L
0.6
DO=2.8mg/L DO=2.0 mg/L
Voltage (V)
0.5 0.4 0.3 0.2 0.1 0.0 0
2000
4000
6000
8000 10000 12000 14000
Time (min)
Fig. 2. Electricity production of two-chamber MFC with cathodic DO.
aCOD removal efficiency coulombic efficiency
80
80
70
70
60
60
50
50
40
40
30
30
20
20
10
10
0
Coulombic efficiency (%)
COD removal efficiency (%)
TCOD removal efficiency cCOD removal efficiency
0 3.5
2.8 2.5 cathodic DO(mg/L)
2.0
Fig.3. COD removal efficiency and coulombic efficiency of two-chamber MFC with cathodic DOs. (TCOD: total COD; aCOD: COD removed in anode chamber; cCOD: COD removed in cathode chamber.)
700
600
400
power density2.8 400
Cell voltage(mV)
600
voltage2.5 300 power density2.5
300
voltage2.0 200 power density2.0
200
100
2
100
Power density(mW/m )
500
voltage3.5 power density3.5 500 voltage2.8
0 0
1000
2000
3000
4000
0 5000
2
Current density(mA/m )
Fig. 4. Voltage and power density vs. current density of the MFC with cathodic DO
Highlights:
(1) Nitrogen and phosphorus were simultaneous removed without pH adjustment. (2) Phosphorus was removed by chemical precipitation and microbial absorption. (3)Results confirm that no nitrifying or anammox bacteria exist in anode chamber. (4) Mechanisms of nitrogen and phosphorus removal were analyzed.
S0960-8524(14)00661-0 http://dx.doi.org/10.1016/j.biortech.2014.05.002 BITE 13410
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
8 March 2014 27 April 2014 2 May 2014
Please cite this article as: Tao, Q., Luo, J., Zhou, J., Zhou, S., Liu, G., Zhang, R., Effect of dissolved oxygen on nitrogen and phosphorus removal and electricity production using microbial fuel cell, Bioresource Technology (2014), doi: http://dx.doi.org/10.1016/j.biortech.2014.05.002
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of dissolved oxygen on nitrogen and phosphorus removal and electricity production using microbial fuel cell Qinqin Taoa, Jingjing Luoa, Juan Zhou a, Shaoqi Zhou a,b,c,d* Guangli Liu e, Renduo Zhange a
College of Environmental Science and Energy, South China University of Technology,
Guangzhou Higher Education Mega Center, 510006, PR China b
Guizhou Academy of Sciences, Shanxi Road 1,Guiyang, 550001, PR China
c
State Key Laboratory of Subtropical Building Sciences, South China University of
Technology, Guangzhou, 510641, PR China d
Key Laboratory of Environmental Protection and Eco-remediation of Guangdong
Regular Higher Education Institutions, South China University of Technology, Guangzhou Higher Education Mega Center, 510006, PR China e
School of Environmental Science and Engineering, Sun Yat-Sen University,
Guangzhou, 510275, PR China Abstract Performance of a two-chamber microbial fuel cell (MFC) was evaluated with the influence of cathodic dissolved oxygen (DO). The maximum voltage, coulombic efficiency and maximum power density outputs of MFC decreased from 521 mV to 303 mV, 52.48 % to 23.09 % and 530 mW/ m2 to 178 mW/ m2 with cathodic DO declining. Furthermore, a great deal of total phosphorus (TP) was removed owing to chemical precipitation (about 80 %) and microbial absorption (around 4 % ~ 17 %). COD was *Corresponding author. Tel.: +86 13535279381; fax: +86 20 85511266. E-mail address: [email protected]
first removed in anode chamber (> 70 %) then in cathode chamber (< 5 %). Most of nitrogen was removed when the cathodic DO was at low levels. Chemical precipitates formed in cathode chamber were verified as phosphate, carbonate and hydroxyl compound with the aid of scanning electron microscope capable of energy dispersive spectroscopy (SEM–EDS), X-ray diffractometer (XRD) and fourier transform infrared spectroscopy (FTIR). Keywords: Microbial fuel cell; Phosphorus; Nitrogen; Dissolved oxygen; Precipitate 1. Introduction Nitrogen and phosphorus were two main contaminants of wastewater. Excess discharge of nitrogen and phosphorus has not only caused vast waste of nitrogen and phosphorus resources, but has also led to eutrophication of lakes (Chang et al., 2000; Domagalski et al., 2007), which leads to taste and odor problems and even contribute to human health problem and can therefore limit the direct use of lake for drinking water purposes. To protect lakes and other natural water from eutrophication, stringent nutrient level is set for the effluent from the wastewater treatment plants (WWTP). Recently, microbial fuel cells (MFCs) have drawn much attention (Logan et al., 2006; Liu et al., 2004; Jiang et al., 2011) as a new technology for simultaneous wastewater treatment and electricity generation. A typical MFC consists of biological anode and abiotic cathode chambers that are separated by a proton exchange membrane (PEM) (Liu et al., 2005). In the anode compartment, the microorganisms oxidize organic compounds and generate electrons and protons in the process. The electrons are absorbed by the anode and are transported to the cathode through an external circuit.
The protons migrate through the solution across a PEM to the cathode chamber. In the cathode compartment, free electrons are delivered to terminal electron acceptor (such as oxygen), then combined with protons to form water (Clauwaert et al., 2007; Logan, 2009). The particular advantages of MFC technology for wastewater treatment include high efficiency, ambient operating conditions, small equipment sizes, minimal sludge generation and rapid start-up as compared to conventional biological wastewater treatment methods such as activated sludge process and bio-film process. A number of MFCs were used for simultaneous nitrate removal and electricity generation, in which denitrification was accomplished by microorganisms in the cathode chamber (Clauwaert et al., 2007; Lefebvre et al., 2008). Nitrate could be reduced to nitrite and finally to nitrogen gas on cathode. The eight electrons involved in the reaction are balanced by protons or electrons provided through anodic reactions. Sodium bicarbonate is required to maintain the pH during electrochemical reduction of nitrate since the electrolyte gradually becomes alkaline. High alkaline environments prompt the generation of precipitates of magnesium hydroxide and calcium carbonate around the cathode when soluble calcium and magnesium salts are present in the water. Virdis et al. (Virdis et al., 2008) investigated simultaneous power generation, carbon and nitrogen removal by using a combined denitrification MFC and nitrifying bioreactor. In this configuration, the ammonium containing effluent from the anode chamber was pumped into an external aerobic reactor for nitrification, and the nitrified liquid subsequently flowed into the cathode chamber for denitrification. This system obtained a maximum power output and a maximum nitrogen removal efficiency of 34.6 W/m3
and 0.41 kg NO3--N/(m3 ·d), respectively. However, the set-up of an external nitrifying reactor makes it difficult to use in the field. Some different nitrogen removal methods with MFCs were reported in published papers. High levels of ammonium in swine wastewater was removed by MFCs (Min et al., 2005), further investigation concluded that ammonium removal was due to either ammonium ion diffusion from anode chamber to cathode chamber in a two-chambered MFC or ammonium volatilization in an single-chamber MFC (Kim et al., 2008). Ammonium used as the fuel for electricity generation in a rotating-cathode MFC was first reported by He et al. (He et al.,2009), high ammonium removal efficiency and electricity generation were achieved. The study solely emphasizes the feasibility of electricity generation from synthetic wastewater using ammonium as the fuel. It can’t exclude the possibility that during ammonium oxidation, autotrophs produced organic compounds for heterotrophs to generate electricity. As for phosphorus removal by MFCs, only a few studies have been published to date. Struvite crystallization is a promising method for phosphorus recovery from waste water (Nelson et al., 2003). The flux of alkali cations in combination with the proton consumption during the oxygen reduction leads to the accumulation of hydroxide, and thus to a pH increase (Zhao et al., 2006). Struvite can be recovered near/on the cathode (Ichihashi et al., 2012), for the solubility of it decreases with increasing pH (Doyle and Parsons, 2002). Fischer et al. (Fischer et al., 2011) investigated phosphate recovery from digested sewage sludge through MFC technology. In this system, MFC was a place for orthophosphate release from iron phosphate contained digested sewage sludge.
Orthophosphate containing effluent from MFC with a subsequent addition of magnesium and ammonium, as well as pH adjustment, was needed for phosphorus recovery as struvite. When air-cathode MFCs were operated with swine wastewater (Ichihashi et al., 2012), a great amount of crystals were observed accumulated on the surface of the liquid side of the cathode. The crystals were proved to be struvite after a series of chemical characterization. Based on those results illustrated above, we hypothesized the possibility that nitrogen and phosphorus can be removed from wastewater by using MFC technology. In this study, a two-chamber MFC was used for simultaneous nitrogen and phosphorus removal from synthetic wastewater. The effects of cathodic DO on chemical oxygen demand (COD), nitrogen and phosphorus removal along power generation were evaluated. Furthermore, the proportion of phosphorus and nitrogen removed by microbial absorption or chemical precipitation were estimated. Sediments deposited on the inner wall of anode chamber, cathode chamber, anode surface and cathode surface were characterized by scanning electron microscopy (SEM). Chemical precipitates were also characterized by energy dispersive spectroscopy (EDS), X-ray diffractometer (XRD) and fourier transform infrared spectroscopy (FTIR). 2. Methods 2.1. MFC configuration and operation The configuration of the two-chamber MFC is shown in Fig. 1. The reactor was based on the tubular-reactor design of Liu and Logan (Liu and Logan, 2004). This reactor was converted into a two-chamber structure by assembling two 4-cm-long
cylindrical chambers (3-cm diameter, formed of Plexiglas plastic) together. The carbon paper (3-cm diameter, the anode), proton exchange membrane (PEM, NAFION 117, 3-cm diameter) and carbon cloth (3-cm diameter, the cathode, containing 0.5 mg Pt/cm2) were sandwiched with PEM in the middle. The sandwiched structure was placed between anode chamber and cathode chamber. Sealing was ensured by silicone sheets inserted between each frame. The cathode chamber was connected to a blown glass bottle with a volume of 100 mL. Oxygenation of the solution in blown glass bottle was supplied by an air pump, and the airflow was adjusted with an air flow rotameter. The cathode electrolyte was recirculated between cathode chamber and glass bottle at a flow rate of approximately 20 mL/min, as previously described by Ichihashi and Hirooka (Ichihashi and Hirooka, 2012). The anode and cathode were connected through an external resistor with resistance 1000 Ω. All experiments were performed at room temperature (25 ℃~31 ℃). The anode and cathode chambers were inoculated with mixed anaerobic sludge and aerobic sludge (v/v = 1:1), which were collected from Shijing municipal wastewater treatment plant, Guangzhou, China. A synthetic wastewater was used throughout the study. The synthetic wastewater mainly containing NaHCO3 5.96 g/L, NaC2H3O2 1.00 g/L, KH2PO4 0.54 g/L, NH4Cl 0.21 g/L, metals, trace minerals and vitamins. The initial pH of synthetic wastewater was adjusted to 7.0±0.2. Synthetic wastewater was fed into anode chamber when every time the voltage decreased to less than 50 mV. The effluent from the anode chamber was subsequently directed into the cathode chamber. To investigate the effects of cathodic dissolved oxygen (DO) on MFC performance
for electricity production and nitrogen and phosphorus removal, experiments were conducted under the following conditions: The two-chamber MFC was operated in fed–batch mode. After MFC achieved 3 cycles of stable voltage outputs, experiments for each treatment were carried out in duplicate. To make certain the relationship between cathodic DO and nitrogen/phosphorus removal and electricity production, four DOs ( 3.5 mg/L, 2.8 mg/L, 2.5 mg/L, 2.0 mg/L) were successively applied to the cathode liquor ( effluent from the anode chamber). The DO of anode liquor was controlled at 0.05 ~ 0.10 mg/L throughout the study. The effluent from anode chamber was aerated in another brown glass bottle which is independent of the two-chamber MFC at four DOs (3.5 mg/L, 2.8 mg/L, 2.5 mg/L, 2.0 mg/L) to investigate the effect of microbes in cathode chamber for nitrogen and phosphorus removal. Moreover, in order to investigate the effect of catholyte pH increase on nitrogen and phosphorus removal, effluents from anode chamber were centrifuged at 8000 rpm for 5 min and the supernatants (solution I) were filtered (0.45 µm). The pH of filtrate (solution II) was adjusted to 9.1 (approximately the pH of effluent from cathode chamber) with 2-3 droplets of strong sodium hydroxide solution (precipitation occurs with increasing pH), then they were centrifuged at 8000 rpm for 5 min, the supernatants (solution III) were used for measurement. 2.2. Analytical techniques Samples from the anode chamber and the cathode chamber were taken at the end of an electricity production cycle. Samples were centrifuged at 8000 rpm for 5 min and the supernatants were used for measurement. Chemical oxygen demand (COD), total
nitrogen (TN, the sum of ammonium (N-NH4+), nitrates (N-NO3-) and nitrites (N-NO2-)) and total phosphorus (TP) were measured according to the Standard Methods (APHA) (APHA, 2005). Values of pH were measured using a pH meter (PHS-25, Leici, China). Solution DO values were measured using a dissolved oxygen meter (JPB-607, Leici, China). Voltage across the external resistance was monitored at one minute intervals using a data acquisition system (M2700, Keithley, USA) connected with a computer. Current (I) and power (P=IV) were calculated according to Ohm’s law. The areal current density and power density were calculated by dividing current and power by the net anodic or cathodic area (7.065 cm2). Coulombic efficiency (CE) was calculated as previously described (Chen et al., 2012). During the test, the polarization curves were detected by varying external resistances from 30 Ω to 50000 Ω with an interval of 1 min to gain stable voltages. The internal resistance and maximum power density were obtained by analyzing the polarization curves (Watanabe, 2008). The precipitate samples were analyzed by scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS, Car Zeiss EVO LS10, UK) to examine the morphology of the samples as well as elemental composition. The precipitates obtained when pH of solution II adjusted to 9.1 were also identified by an 18 kW rotating anode X-ray diffractometer (D8 ADVANCE, Bruker. Co., Germany) and a fourier transform infrared spectroscopy (Vector 33, Bruker. Co., Germany). Precipitates that were used for measurement were washed and re-suspended in milli-Q water to remove soluble chemicals, and then they were centrifuged again at 8000 rpm for 5 min. These re-suspension and centrifugation steps were repeated twice, and finally the
precipitates were dried in dried pot. 3. Results and discussion 3.1. Nitrogen and phosphorus removal from wastewater Nitrogen and phosphorus in the synthetic wastewater were in the form of N-NH4+ and P-PO43- in the beginning. The concentration of N-NH4+ and P-PO43- were 55.0 and 122.9 mg/L, respectively. In order to find out the relationship between cathodic DO and nitrogen/phosphorus removal, the change in the TP, N-NH4+, N-NO3- and N-NO2- were measured during the electricity generation cycle. The TP, N-NH4+, N-NO3- and N-NO2concentrations in the electricity generation process at different cathodic DO are list in Table 1. Since anodic DO was low, there was not enough oxygen for bacteria in anode chamber to aerobic accumulate phosphorus and partial nitrification or nitrification of ammonia. Therefore, the concentration of TP, N-NO3- and N-NO2- in the effluent of anode chamber was nearly the same as synthetic wastewater. However, the concentration of N-NH4+ in the effluent of anode chamber decreased about 10 mg/L compared with synthetic wastewater. The decrease of N-NH4+ may due to the consumption for microorganism's growth and reproduction, ammonium ion diffusion from the anode chamber to the cathode chamber or ammonium volatilization. When the cathodic DO was at 3.5, 2.5 and 2.0 mg/L, TP removal efficiencies were more than 90 % and have no significant difference with each other. However, when the cathodic DO was at 2.8 mg/L, TP removal efficiency decreased a little, but still more than 85%. The rate of N-NH4+ removed and converted to N-NO3- or N-NO2- are very different.
Oxygen in the cathode chamber may be consumed in the following ways: 1) used as the electron acceptors for electricity generation; 2) used by polyphosphate accumulating microorganisms (PAOs) to aerobic accumulate phosphorus; 3) used by nitrifiers to partial nitrification or nitrification of ammonia; 4) used by some electrochemically inactive microorganisms in the cathode chamber. When the cathodic DO was at high levels (3.5 and 2.8 mg/L), most of N-NH4+ in the catholyte was converted to N-NO2-, but the TN didn’t decline markedly. There are two possible reasons: 1) oxygen in the catholyte was too much for denitrifying bacteria; 2) N-NO2wouldn’t be used as the electron acceptor for electricity generation when there was enough oxygen in the catholyte. A slight drop in TN was most likely owning to ammonium volatilization (because the cathode chamber was an open system, and was aerated with air) and microorganism’s growth and reproduction. When the cathodic DO was at low levels (2.5 and 2.0 mg/L), the removal efficiency of TN was more than 85%. Nitrogen may be removed by the following three ways: 1) nitrogen was removed by the anaerobic ammonia oxidation (ANAMMOX) process; 2) N-NH4+ was first oxidized to N-NO2- by oxygen, then N-NO2- was reduced to N2 under the action of denitrifying phosphate accumulating organisms (DNPAOs) for there was not enough oxygen in the catholyte. DNPAOs have the ability of simultaneous nitrogen removal by denitrification and phosphorus removal (Liu et al., 2013). 3) N-NH4 + was first oxidized to N-NO2- by oxygen, then N-NO2- was reduced to N2 by using as the electron acceptor for electricity generation. The optimum pH for nitration and denitrification were 6~9 and 7.0~7.5. The pH of the effluent from anode chamber and cathode chamber were 7.0±0.2 and 9.2±
0.2, this observation is equivalent to saying that the pH of the catholyte was increased from 7.0±0.2 to 9.2±0.2 during the electricity generation process. High pH will make negative effect on denitrification reaction. The pH of the catholyte was increased gradually, so the nitration and denitrification will occur at the beginning when the pH was not too high. When the pH rise to a certain value, denitrification reaction will be seriously inhibited, then the nitrogen was removed for using N-NO2- as the electron acceptor for electricity generation. 3.2. Mechanism of nitrogen and phosphorus removal and precipitates formation Effluent from anode chamber was aerated in a brown glass bottle which is independent of the two-chamber MFC to investigate the role of the cathode chamber for nitrogen and phosphorus removal.The optimal aeration time for TP removal is 6 h (data not show). The concentration of TP declined more than 80 % under each DO, however, N-NH4+ in the effluent almost have no change whether in form or quantity (Table 2). No obvious change of N-NH4+ in form or quantity indicating that there was no nitrifying bacteria or anaerobic ammonia oxidation bacteria in the effluent for the long time anaerobic environment in anode chamber. Microbial community of nitrifying bacteria or anaerobic ammonia oxidation bacteria could not be detected in anode chamber (Zang et al., 2012). Nitrogen removal was due to the nitrifying bacteria or anaerobic ammonia oxidation bacteria in cathode chamber, but not in the effluent from anode chamber. Some sediment deposited on the inner wall of anode chamber, cathode chamber, anode surface and cathode surface after operating for a period of time. Sediments were
characterized by SEM, results show that sediments in cathode chamber was the mixture of microbe cells and chemical precipitates, however, no chemical precipitates could be found on anode surface, cathode surface or in anode chamber, they were all microbe cells (Supplementary Fig S1). No needle shaped prismatic morphology typical of struvite described by literature (Cusick and Logan, 2012) was found in all SEM images. Chemical precipitates formed in cathode chamber may due to the progressively alkaline cathode liquor, and they were believed to be a possible pathway for phosphorus and nitrogen removal. No obvious concentration decrease of TP, N-NH4+, N-NO3- and N-NO2- in solution II proved no nitrogen or phosphorus containing precipitates were formed in anode chamber (Table 3). That’s why no chemical precipitate was observed in anode chamber. Concentration of TP decreased from 121.8±3.6 to 24.0±2.1 mg/L when solution pH was adjusted to 9.1. This result suggests that most of TP (about 80 %) was removed by chemical precipitation, only a small portion of TP (around 4 %~17 %) was removed by microbe absorption. Also, a slight drop in concentration of N-NH4+ was observed, that may be because ammonium volatilization with the progressively alkaline cathode liquor and continuous aeration. 3.3. Precipitates analysis In order to learn more about the structure and composition of the precipitates, EDS, FTIR and XRD were recorded (Supplementary Fig S2, Fig S3 and Fig S4). The EDS patterns from the precipitate deposit in cathode chamber and obtained when pH of solution II adjusted to 9.1 are exactly similar. This means that the composition of two chemical precipitates were extremely similar. They were both mainly composed of O,
Mg, P, C, Na. Mn and Zn absorption peaks were also detected in the precipitate obtained when pH of solution II adjusted to 9.1, but not in the precipitate from cathode chamber. That’s because Mn and Zn were trace elements for microbes, they will be consumed with microorganism existence. There are lots of microbes in MFC, but nearly no microbes in solution II (they were filtered out). Characteristic absorption peaks of PO43-, CO32- and OH- were found in FTIR spectrum of the precipitate obtained when pH of solution II adjusted to 9.1. This suggests that phosphate, carbonate and hydroxyl compound precipitates were formed with increasing pH. A rather broad peak without any obvious sharp peak was detected in the XRD pattern. This might be because particles in the precipitate were not in crystal but amorphous, or the precipitate is a mix of various chemical substances. Each chemical substance has specific XRD characteristic peaks. There were numerous character peaks in the XRD pattern and were close to each other. As a result, we see a rather broad peak in the XRD pattern. 3.4. MFC performance After acclimation for approximately twenty days, stable electricity production was achieved in the two-chamber MFC. Fig. 2 shows the voltage vs. time for two-chamber MFC with cathodic DO over three operation cycles. The output voltage of two-chamber MFC under each cathodic DO began to increase noticeably after substrate dosing and peaked soon afterwards. The maximum output voltage of two-chamber MFC under four cathode DOs were 521 mV, 476 mV, 453 mV and 303 mV, respectively. The maximum output voltage of two-chamber MFC was decreased with the declining cathodic DO, especially when cathodic DO declined from 2.5 to 2.0 mg/L, 150 mV maximum output
voltage was decreased. This demonstrates that cathodic DO is a key factor for electricity generation and 2.0 mg/L DO in cathode liquor is far from enough for electron acceptor but sufficient for microbes in cathode chamber for nitrogen and phosphorus removal (good removal efficiency of N and P were obtained when cathodic DO was at 2.0 mg/L, the datum were list in Table 1). The average COD removal efficiency by two-chamber MFC under four cathodic DOs were range from 77.3 % to 81.0 %, in which more than 70 % was removed by microbes in anode chamber and less than 5 % by microbes in cathode chamber (Fig.3). There was no significant COD removal efficiency difference when the cathodic DO was among four DOs. The vast majority of COD was consumed by microbes in anode chamber. It can be hypothesized that a lot of organics was assimilated by phosphorus-accumulating bacteria in anaerobic anode chamber and stored in cells in polyhydroxybutyrate (PHB) form. However, when the effluent of anode chamber was directed to aerobic or oxygen-poor cathode chamber, PHB was used as the carbon source by phosphorus-accumulating bacteria (Comeau et al., 1986). Some electrochemically inactive microorganisms consumed a small amount of organics in cathode liquor. The average coulombic efficiency of two-chamber MFC was decreased from 52.48 % to 23.09 % with declining cathodic DO (Fig. 3). This is thought to be a result of the increase of the ratio of electrochemically inactive microorganisms in MFC or increase of internal resistance with decreasing cathodic DO. It was estimated that up to 28 % of the glucose added to a MFC could be lost through aerobic respiration by
bacteria sustained by oxygen fluxes through the membrane (Liu and Logan, 2004). The polarization curves of the MFC with cathodic DO are illustrated in Fig. 4 The maximum power densities obtained in MFC under four cathdic DOs were 530, 459, 428 and 178 mW/ m2, respectively. The internal resistances were 238, 284, 298 and 366 Ω， respectively. The reduction of cathodic DO resulted in an increased internal resistance that leads to maximum power density reduction. Compared with other MFCs reported in the literatures (Freguia et al., 2007; Xia et al., 2013), these values are not very high. This may be attributed to the MFC configuration and operating mode used in our work. 4. The advantages of the two-chamber MFC system The two-chamber MFC configuration proposed in the study shows many advantages. Firstly, the MFCs system is an economical technology not only for energy production but also for the reduction of aeration costs. In the two-chamber MFC system, nitrification, denitrification, aerobic accumulate phosphorus processes and precipitation take place in the cathode chamber. Oxygen in the aerated air was effectively used for electricity production, nitrogen and phosphorus removal, which reduces the aeration costs. Secondly, the two-chamber MFC is a biocathode MFC, which uses bacteria as a biocatalyst to accept electrons from the electrode, provides a different path that avoids the use of chemical catalysts. Compared with an abiotic cathode MFC, bacterial biocathode MFC not only lowers the cost of construction and operation of MFCs, but also increases the environmental sustainability of MFC systems. Thirdly, most of phosphorus in the wastewater could be recovered by the way of sedimentation for the increasing pH in cathode chamber.
5. Conclusions Cathodic DO makes important effects on MFC performance and phosphorus/nitrogen removal. When cathodic DO decreased, the electricity generation performance declined sharply for the less and less electron acceptor in cathode chamber. However, changes on the removal of TP and COD are small with declining cathodic DO. Nitrogen could be removed when the cathodic DO was at low levels. Phosphate, carbonate and hydroxyl compound were formed in the cathode chamber during the reaction process. The TP was removed by chemical precipitation and microbe absorption.
Acknowledgements Financial supports from the National Natural Science Foundation (21277052), the Department of Guangdong Education and the Science and Technology Bureau (2010), the State Key Laboratory of Subtropical Building Science (2013ZC03, 2014ZB04) and the Environmental Protection Bureau (201203) are gratefully acknowledged.
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Figure Captions Fig.1. Generalized schematic of the two-chamber MFC, showing two chambers separated by carbon paper-proton exchange membrane (PEM)-carbon cloth. The cathode liquor was constantly recirculated between cathode chamber and brown glass bottle. Fig. 2. Electricity production of two-chamber MFC with cathodic DO. Fig.3. COD removal efficiency and coulombic efficiency of two-chamber MFC with cathodic DOs. Fig. 4. Voltage and power density vs. current density of the MFC with cathodic DO
Tables and Figures Table.1. TP, N-NH4+, N-NO3- and N-NO2- concentrations with cathodic DO Synthetic water
TP (mg/L) N-NH4+ (mg/L) N-NO3(mg/L) N-NO2(mg/L)
Effluent of anode chamber
122.9
Effluent of cathode chamber (DO=3.5 mg/L) 121.8±3.6 7.5±2.2
Effluent of cathode chamber (DO=2.8 mg/L) 13.7±2.3
Effluent of cathode chamber (DO=2.5 mg/L) 8.6±1.3
Effluent of cathode chamber (DO=2.0 mg/L) 8.5±1.6
55.0
45.2±1.0
6.1±1.4
14.5±1.7
2.1±0.3
8.0±0.5
0
≈0
0.1±0.1
0.4±0.2
4.4±1.5
0.2±0.1
0
≈0
32.3±2.8
25.9±3.3
0.5+0.2
1.5±0.2
Table.2. TP, N-NH4+, N-NO3- and N-NO2- concentrations after the effluents from anode chamber were aerated in a separate brown glass bottle at various DOs Effluent of anode chamber TP (mg/L) +
N-NH4 (mg/L) N-NO3(mg/L) N-NO2(mg/L)
121.8±3.6
Aerated with DO at 3.5 mg/L 16.5±1.4
Aerated with DO at 2.8 mg/L 16.6±2.1
Aerated with DO at 2.5 mg/L 18.2±2.2
Aerated with DO at 2.0 mg/L 18.9±1.4
45.2±1.0
38.8±4.3
41.3±2.8
42.0±1.3
44.1±1.5
≈0
≈0
≈0
≈0
≈0
≈0
≈0
≈0
≈0
≈0
Table.3. TP, N-NH4+, N-NO3- and N-NO2- concentrations in the effluent of anode chamber before and after pH adjustment Solution I
Solution II
Solution III
TP (mg/L)
121.8±3.6
120.4±3.6
24.0±2.1
N-NH4+ (mg/L)
45.2±1.0
43.8±1.6
39.5±2.8
N-NO3- (mg/L)
≈0
≈0
≈0
N-NO2- (mg/L)
≈0
≈0
≈0
Fig.1. Generalized schematic of the two-chamber MFC, showing two chambers separated by carbon paper-proton exchange membrane (PEM)-carbon cloth. The cathode liquor was constantly recirculated between cathode chamber and brown glass bottle.
DO=3.5 mg/L DO=2.5 mg/L
0.6
DO=2.8mg/L DO=2.0 mg/L
Voltage (V)
0.5 0.4 0.3 0.2 0.1 0.0 0
2000
4000
6000
8000 10000 12000 14000
Time (min)
Fig. 2. Electricity production of two-chamber MFC with cathodic DO.
aCOD removal efficiency coulombic efficiency
80
80
70
70
60
60
50
50
40
40
30
30
20
20
10
10
0
Coulombic efficiency (%)
COD removal efficiency (%)
TCOD removal efficiency cCOD removal efficiency
0 3.5
2.8 2.5 cathodic DO(mg/L)
2.0
Fig.3. COD removal efficiency and coulombic efficiency of two-chamber MFC with cathodic DOs. (TCOD: total COD; aCOD: COD removed in anode chamber; cCOD: COD removed in cathode chamber.)
700
600
400
power density2.8 400
Cell voltage(mV)
600
voltage2.5 300 power density2.5
300
voltage2.0 200 power density2.0
200
100
2
100
Power density(mW/m )
500
voltage3.5 power density3.5 500 voltage2.8
0 0
1000
2000
3000
4000
0 5000
2
Current density(mA/m )
Fig. 4. Voltage and power density vs. current density of the MFC with cathodic DO
Highlights:
(1) Nitrogen and phosphorus were simultaneous removed without pH adjustment. (2) Phosphorus was removed by chemical precipitation and microbial absorption. (3)Results confirm that no nitrifying or anammox bacteria exist in anode chamber. (4) Mechanisms of nitrogen and phosphorus removal were analyzed.