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Assessment of a novel overflow-type electrochemical membrane bioreactor (EMBR) for wastewater treatment, energy recovery and membrane fouling mitigation
Bioresource Technology 196 (2015) 648–655
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Assessment of a novel overflow-type electrochemical membrane bioreactor (EMBR) for wastewater treatment, energy recovery and membrane fouling mitigation Guowang Zhou, Yuhong Zhou, Guoqiang Zhou, Lian Lu, Xiankai Wan, Huixiang Shi ⇑ Department of Environmental Engineering, Zhejiang University, Hangzhou 310058, PR China
h i g h l i g h t s A novel overflow-type EMBR with no ion exchange membrane was developed. The performance of the EMBR was affected by the range of HRTs. +
92.6% COD, 96.5% NH4 -N and 73.9% TN removal efficiencies were achieved. Electrochemically active bacteria were abundant in the biofilm by sequencing. The sludge properties influenced by MFC integration might mitigate membrane fouling.
a r t i c l e
i n f o
Article history: Received 15 June 2015 Received in revised form 9 August 2015 Accepted 12 August 2015 Available online 18 August 2015 Keywords: Microbial fuel cell Membrane bioreactor Wastewater treatment Energy recovery Membrane fouling mitigation
a b s t r a c t A novel overflow-type electrochemical membrane bioreactor (EMBR) without ion exchange membrane, was developed for wastewater treatment and utilized electricity recovered by microbial fuel cell (MFC) for membrane fouling mitigation in membrane bioreactor (MBR). The maximum power density of 629 mW/m3 or 7.18 mW/m2 was obtained. The removal efficiencies of chemical oxygen demand, ammonia nitrogen and total nitrogen under appropriate ranges of hydraulic retention times (16.9–8.5 h) were 92.6 ± 5.4%, 96.5 ± 2.8% and 73.9 ± 9.7%, respectively. Sequencing showed electrochemically active bacteria Lactococcus, Bacillus and Saprospiraceae_uncultured were abundant in the biofilm. Compared with a conventional MBR, five significant effects of the MFC integration on the sludge properties, including particle zeta potential decrease, particle size distribution macroaggregation, soluble microbial products and extracellular polymeric substances reduction and SMPP/SMPC ratio increase, were achieved in this system, leading to membrane fouling mitigation. This system shows great promise for practical wastewater treatment application. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Water and energy shortage are two major global challenges (Gleeson et al., 2012). With an increasing demand for freshwater and a decreasing available water resource, reusing and recycling wastewater, as a viable water resource, is feasible (Grant et al., 2012). However, typical wastewater treatment systems, using high concentrations of bacteria supplied with oxygen to remove the organic matter, are energy intensive. The overall energy consumption on this conventional treatment is roughly 0.6 kW h m3 with
⇑ Corresponding author at: Department of Environmental Engineering, Zhejiang University, Yuhangtang Road 866#, Hangzhou 310058, PR China. Tel.: +86 0571 88982044; fax: +86 0571 81021989. E-mail address: [email protected] (H. Shi). http://dx.doi.org/10.1016/j.biortech.2015.08.032 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.
about half of that used for aeration. Based upon a theoretical 3.86 kW h energy production/kg COD oxidized to CO2 and H2O, it is meaningful to develop a new process that both captures the inherent energy from the dissolved organic matters and recovers clean water for reuse from domestic wastewater (McCarty et al., 2011). Microbial fuel cells (MFCs), which enable direct conversation of organic matters into useful electricity via microbial-catalyzed redox reaction, are promising approaches for capturing the energy in wastewater for diverse purposes (Logan et al., 2006). However, the poor effluent quality and a low treatment efficiency, due to limited biomass retention, limit the practical application of MFCs. To sort out this problem, several methods have been proposed to improve treatment, such as combining the MFC with an up-flow anaerobic sludge blanket reactor and a biological aerated filter
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(Zhang et al., 2009), submerging the MFC reactor in an anaerobic reactor or an aeration tank (Cha et al., 2010; Min and Angelidaki, 2008), integrating the MFC into a rotating biological contactor (Sayess et al., 2013), or integrating the MFC with a sequencing batch reactor (Liu et al., 2011) or a membrane aerated biofilm reactor (Yu et al., 2011). These combined systems are not effective at degrading particulate biomass because of lacking of subsequent sedimentation or filtration technique. Membrane bioreactor (MBR), which is a high-efficient technology for treating wastewater, has gained worldwide popularity for good effluent quality and its ability to retain and degrade particulate biomass (Judd, 2008). More recently, several systems incorporating MBR into MFCs for improving effluent quality have been reported. For example, a novel bioelectrochemical membrane reactor, which used stainless steel (SS) mesh as the cathode and the membrane, was developed to obtain a maximum power density of 4.35 W/m3 and high-quality effluent attributed to the biofilm formation on mesh (Wang et al., 2011). Thereafter, a more practical MFC–MBR integrated process was developed (Wang et al., 2012). Additionally, a combined system integrating anaerobic MBR (AnMBR) with MFCs was proposed (Tian et al., 2014). Furthermore, a combined MBR–MFC system could mitigate the membrane fouling in the MBR due to an electric field formed at the vicinity of cathode to suppress sludge deposition and H2O2 generated to clean the already deposited foulants in situ (Wang et al., 2013b), or modification of sludge characteristics (Su et al., 2013; Tian et al., 2014, 2015). While in most of previous works, ion exchange membrane (IEM) was still required to separate the anodic chamber and the cathodic chamber. However, the use of an IEM increases the overall internal resistance and the overall cost of the systems (Wang et al., 2013a). In this study, an overflow-type electrochemical membrane bioreactor (EMBR) was designed and tested for wastewater treatment, energy recovery and membrane fouling mitigation. There was no need to use IEM while anodic and cathodic chambers separated by an overflow channel, which allowed the wastewater with protons and substrate overflow directly into the cathode from the anode and restricted oxygen to transfer from the cathode to the anode. A SS mesh was used as the cathode and the MBR module. The unidirectional flow of the wastewater from the anode to cathode allows further degradation of the organic pollutants in the cathodic chamber by aerobic MBR process. In this system, there was an electric filed formed by the different potential between the anode and cathode, would induce an electrostatic repulsion force that could make the negatively-charged sludge away from the SS mesh, leading to sludge deposition suppressed and membrane fouling mitigation. In addition, the overflow-type EMBR was more practical to scale up than other combined MFC-MBR systems. The objective of this research was undertaken to evaluate the performances of the overflow-type EMBR: (1) to assess the electricity generation and organic pollutants removal at different hydraulic retention times (HRT); (2) to elucidate the biofilm formation and community composition of the SS mesh bio-cathode; (3) to investigate the membrane fouling mitigation and the sludge properties.
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376.8 cm2 surface area (Beijing Carbon Co., China; 120 mm diameter; 100 mm height; 2 mm thickness). A stainless steel (SS) mesh with 38 lm pore size and 62 cm2 surface area (Ruitong Ironware Co., China) was used as cathode without pretreatment. The total volume of the anodic chamber was 430 mL, while the net effective volume was 360 mL. The working volume of the cathodic chamber was 850 mL. The electric connections between the electrodes and a 1000-X external resistor were established with the titanium wires. The overflow-type EMBR was inserted with silicone tubes on the bottom of the anodic chamber and the top of the SS mesh cathode. Air was bubbled into the cathode continuously at the cathodic chamber bottom to provide a dissolved oxygen (DO) concentration of 4–5 mg/L. The reactor was operated in a constant-temperature incubator (35 °C, Yiheng Co., China). 2.2. Inoculation, enrichment and operation The anodic chamber was inoculated with 100 mL of effluent from a laboratory-scale air–cathode MFC, and the cathodic chamber was inoculated with 200 mL of activated sludge from a local municipal wastewater treatment plant in Hangzhou, China. The mixed liquor suspended solids (MLSS) concentrations for anodic and cathodic chambers were adjusted to 3 and 2 g/L, respectively. The overflow-type EMBR was operated in three phases. The first phase, which lasted for 20 days, was start-up for system under batch mode with a synthetic wastewater while an anodic biofilm of electricity-producing microbes was gradually formed. After the initial feeding, the cell voltage increased gradually and decreased subsequently for lack of substrate. At that time, the synthetic wastewater in the system was replaced by a fresh one. Following the second phase, the synthetic wastewater was continuously fed into the anodic chamber using a peristaltic pump (Lange Co., China), and the effluent from the anodic chamber then overflew into the cathodic chamber through an overflow channel, passed through the SS mesh, and was finally discharged from the system. The performance of the overflow-type EMBR was evaluated at different HRTs for 28 days, and the experimental design during this phase is summarized in Table 1. During the third phase, shortterm filtration test was performed to identify the impacts of the combined system on the membrane fouling without connecting the circuit (reference phase, simulating the conventional MBR, CMBR). The cathodic mixed liquor was substituted by new activated sludge and diluted to 2 g/L with the synthetic wastewater. The synthetic wastewater was continuously fed into the system at a rate of 0.53 mL/min. The synthetic wastewater composition was: CH3COONa, 1 g/L; NH4Cl, 115 mg/L; K2HPO43H2O, 44 mg/L; CaCl2, 11.5 mg/L; MgSO4, 12 mg/L and 10 mL of trace element solution. The membrane fouling degree could be indicated in terms of the trans-membrane pressure (TMP) across the SS mesh. With the sludge cake deposited on the SS mesh becoming thick during the system operation, the TMP would increase sharply and an offline backwashing with running tap water was conducted to recover SS mesh filterability. 2.3. Analytical methods
2. Methods 2.1. Reactor design and construction The overflow-type EMBR is illustrated in Fig. 1. The reactor was constructed with anodic and cathodic chambers which were connected through an overflow channel with a width of 5 mm to allow the flow of anodic effluent to the cathodic chamber for further treatment. Anode was made of an ‘‘O ring” carbon felt with
2.3.1. Electrochemical analyses The voltage (U) across the resistance was automatically record every 1 min using a data acquisition system (Agilent 34970A, Agilent Co., USA). The current (I) was calculated from the cell voltage according to Ohm’s law (I = V/R), with the power (P) obtained as P = IV, where U was the measured voltage (V), and R the external resistance (X). Columbic efficiency (CE) was calculated as CE = Cp/Cth 100%, where Cp was the total coulombs calculated by integrating the current over time, and Cth was the
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Fig. 1. (a) Schematic diagram of the overflow-type EMBR. (1) Anodic chamber; (2) graphite felt anode; (3) cathodic chamber; (4) hydraulic head; (5) biofilm; (6) SS mesh; (7) anodic chamber inner tube; (8) cathodic chamber tube; (9) overflow channel. (b) The top view of the overflow-type EMBR.
Table 1 Operation parameters of the overflow-type EMBR system during the second phase. Run
Operation time (day)
HRT (h)
Influent COD (mg/L)
Influent NH+4N (mg/L)
Influent TN (mg/L)
1 2 3 4
21–27 28–34 35–41 42–48
16.9 11.2 8.5 4.2
662.6 ± 4.6 654.7 ± 5.5 666.7 ± 12.4 672.1 ± 6.9
30.6 ± 1.3 31.0 ± 1.2 31.6 ± 0.9 31.9 ± 0.4
35.3 ± 1.0 35.3 ± 0.8 35.6 ± 0.5 35.6 ± 0.7
theoretical amount of coulombs available based on the COD removed in the anodic chamber over the same amount of time. Linear sweep voltammetry (1 mV/s) was performed to obtain polarization curves and power density data by use of a CHI660D electrochemical workstation (Chenhua Instrument Co., China) (Logan et al., 2006). First, the circuit of the overflow-type EMBR was opened for 12 h to measure the open circuit voltage (OCV). Then, voltammetry scanning was performed using the anode as the working electrode, the cathode as the counter electrode and reference electrode, respectively. The voltage range was set between zero and the OCV. The current density and power density were normalized to the total anodic chamber volume. 2.3.2. Chemical and microscopic analyses Duplicates measurements of COD, ammonium (NH+4-N), nitrate (NO 3 -N) and MLSS were performed according to the standard methods (Rice et al., 2012). Dissolved oxygen (DO) was monitored with a DO meter (HQ 30d, Hach Co., USA). The HRT of the anodic chamber was calculated according to the net effective volume of the anodic chamber and the influent flow rate. TMP was measured through the difference in water level, which was monitored every 1 min by a pressure transmitter (GB-3000G, Gangbei Ltd., China). Scanning electron microscope (SEM, Su8010, Hitachi, Japan) was employed to characterize the surface microstructure of the biofilm on the SS mesh. Prior to SEM imaging, the sample was dehydrated in a series of graded alcohol solutions and ovendried. After sputter-coating the sample with gold palladium for 30 s at 25 mA current in an argon atmosphere, SEM imaging was performed using an accelerating voltage of 25 kV and working distance of 10 mm. 2.3.3. Other analysis To compare the bacterial communities of the biofilm on the SS mesh in the overflow-type EMBR (A) and the C-MBR (B), two samples were collected when the systems reached a steady state. DNA extractions were employed using the E.Z.N.AÒ Soil DNA Kit (Omega Bio-tek, USA). The bacterial 16S rRNA PCR and sequencing was
performed using primers 338F (50 -ACTCCTACGGGAGGCAGCA-30 ) and 806R (50 -GGACTACHVGGGTWTCTAAT-30 ) to target the variable region V4–V5 for the samples (Muyzer et al., 1993). Then, the samples were sent to Shanghai Majorbio Technology (Shanghai, China) to perform amplicon sequencing on an Illumina MiSeq platform according to the standard protocols. The sequencing data was processed using QIIME (version 1.17). Samples of mixed liquor in cathodic chamber of the overflowtype EMBR and C-MBR were taken every 48 h for zeta potential, soluble microbial products (SMP), extracellular polymeric substances (EPS) and particle size distribution (PSD) analysis. The zeta potential of sludge flocs was measured using a zeta potential analyzer (Nano-ZS90, Malvern, UK). SMP was extracted by centrifugation at 5000 rpm for 5 min and filtration through a 0.45-mm filter. EPS was extracted using heating method at 80 °C for 30 min, and then prepared by the same method that SMP was extracted. SMP and EPS samples were further analyzed for the contents of protein and carbohydrate (Li and Yang, 2007). The concentrations of protein were obtained using the Bradford method, and the concentrations of carbohydrates were quantified by the phenol–sulfuric acid method. PSD of sludge flocs was measured using a particle size analyzer (Mastersizer 2000, Malvern, UK).
3. Results and discussion 3.1. Electrochemical performance The electricity generation of the overflow-type EMBR using two different feeding regimes is illustrated in Fig. 2a. Under batch mode, the maximum voltage output reached 250.3 mV (1000 X) after 12 h startup period. Following the second phase, the overflow-type EMBR electrochemical performance at different HRTs was evaluated under continuous mode. The voltage output increased gradually and fluctuated slightly around 466 mV at HRTs of 16.9–8.5 h (Runs 1–3). Continuous feeding nutrients would maintain continuous solution transportation with the protons from anode to cathode (Wang et al., 2013a). However, the voltage output decreased significantly at a HRT of 4.2 h (Run 4). In order to investigate the electricity producing ability of the overflow-type EMBR at different HRTs, the polarization curves and power density were obtained at the end of each run. As illustrated in Fig. 2b, the open circuit voltage (OCV) fluctuated slightly between 861 mV and 840 mV at HRTs of 16.9 h-8.5 h (Runs 1–3), and the maximum power density (MPD) varied slightly between 557 mW/m3 and 629 mW/m3. However, as the HRT further decreased in Run 4, the OCV and the power density obviously
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Fig. 2. (a) Electricity generation of the overflow-type EMBR using different nutrient feeding operations (periodic and continuous nutrient loading). (b) Polarization curves and power density of the overflow-type EMBR at different HRTs.
decreased to 458 mV and 133 mW/m3, respectively. The MPD was higher than the MPD of 30 mW/m2 (or 123 mW/m3 normalized to the total anodic volume) and 37.2 mW/m3 obtained in IEM-less MFCs coupled with aerobic activated sludge systems reported by Wang et al. (2013a) and Zhu et al. (2013), respectively. As a constant resistance of 1000 X was applied, the Coulombic efficiency (CE) of the overflow-type EMBR fluctuated slightly between 0.99% and 1.03% at HRTs of 16.9–8.5 h (Runs 1–3), being similar with the value of the MFC–MBR system proposed by Tian et al. (2015) and Wang et al. (2011) and the IEM-less MFC proposed by Zhu et al. (2013), and decreased to 0.41% when the HRT was further decreased to 4.2 h in Run 4. Fermentation or methanogenesis of some other bacteria in the anodic chamber would be the main reason led to the low anode CE (Logan et al., 2006). Thus, the continuous flow from anodic chamber to cathodic chamber and an appropriate range of HRT could enable a high stability in electricity generation and a high voltage output in this system, leading to efficient electricity generation from wastewater.
3.2. Wastewater treatment performance For wastewater treatment systems, the COD and nutrient removal efficiencies are important indicators to evaluate their performance. As illustrated in Table 2 and Fig. 3, at different HRTs during Phase 2, the effluent concentration of the COD and the removal efficiencies were 5.4–122.9 mg/L (averaged at 67.6 mg/L) and 81.7–99.2% (averaged at 89.8%), respectively, while those of NH+4-N and TN were 0.16–3.86 mg/L (averaged at 1.77 mg/L) and 87.9–99.5% (averaged at 94.4%), 5.33–16.06 mg/L (averaged at 10.94 mg/L) and 54.9–84.9% (averaged at 69.2%), respectively, indicating effective wastewater treatment in the overflow-type EMBR. The COD, NH+4-N and TN removal efficiencies were affected by HRT significantly. Compared with the COD and nutrient removal in Runs 1 and 4, a longer HRT (e.g., 16.9 h in Run 1) led to excellent COD and nutrient removal efficiencies, while a shorter HRT (e.g., 4.2 h in Run 4) resulted in poor COD and nutrient removal efficiencies.
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Table 2 System performance in different runs during the second phase. Run
Coulombic efficiency (%)
1 2 3 4
0.99 1.03 1.02 0.41
NH+4-N removal efficiency (%)
COD removal efficiency (%)
TN removal efficiency (%)
Anode
Cathode
EMBR
Anode
Cathode
EMBR
Anode
Cathode
EMBR
84.1 ± 3.0 65.6 ± 6.1 47.7 ± 6.0 31.8 ± 4.5
15.1 ± 2.8 25.7 ± 4.6 39.6 ± 6.9 49.9 ± 5.2
99.2 ± 0.5 91.3 ± 2.6 87.3 ± 2.3 81.7 ± 2.6
30.2 ± 3.5 26.7 ± 1.8 24.3 ± 3.4 21.4 ± 2.6
69.2 ± 3.6 69.1 ± 2.7 70.1 ± 3.7 66.5 ± 3.1
99.5 ± 0.3 95.8 ± 3.2 94.4 ± 0.7 87.9 ± 1.4
27.9 ± 3.5 25.0 ± 2.4 22.9 ± 2.3 20.5 ± 2.7
57.0 ± 7.1 48.5 ± 4.1 40.5 ± 1.5 34.4 ± 7.1
84.9 ± 5.0 73.5 ± 2.3 63.4 ± 3.2 54.9 ± 4.9
Fig. 3. Removal of (a) COD, (b) NH+4-N and (c) TN of the overflow-type EMBR. (j) Influent concentration, (s) effluent concentration and (4) removal efficiency.
The COD removal efficiencies by the anodic chamber in the overflow-type EMBR were 31.8–84.1%, which mainly included the COD recovery in the form of electricity (0.41–1.03%), COD metabolized by the electroactive bacteria and probable COD metabolized by other bacteria (Su et al., 2013). Approximately 15.1–49.9% of COD had been oxidized attributed to the respiration of heterotrophic bacteria in the cathodic chamber. About 21.4–30.2% of the NH+4-N was removed in the anodic chamber due to microbial growth. Most of the NH+4-N was oxidized by the out layer of biofilm on the SS mesh cathode, which was occupied by putative nitrifying organisms. TN removal in the anode was mainly attributed to NH+4-N removal. Two mechanisms are possibly responsible for the TN removal in the cathode: (1) there were many denitrifying organisms living in the inner layer of the biofilm, which could use the residual organics in the wastewater as the electron donors for denitrification (Virdis et al., 2011) and (2) there were also some species of electroactive bacteria (e.g. Geobacteraceae) living in the inner layer of the biofilm, which could transfer the electrons from the cathode for denitrification (Clauwaert et al., 2007; Tian et al., 2015). A shorter HRT (e.g., 4.2 h in Run 4) would result in insufficient denitrification, thereafter, led to a poor TN removal. Thus, a longer HRT would increase the TN removal. This overflow-type EMBR construction provided a bigger net volume of the anodic chamber than other EMBRs (Wang et al., 2012, 2013b) so that more organics were removed in the anodic chamber with more COD conversion to electricity. In addition, a SS mesh was used as the cathode and the MBR module. For more practical application, carbon-based electrode materials could be used as cathode, and the hollow-fiber membrane could be used as the membrane module (Tian et al., 2014, 2015).
3.3. Biofilm formation and community composition Aeration in the cathodic chamber provided a favorable environment for the heterotrophic bacteria growth. A layer of sludge cake was formed on the SS mesh surface in the overflow-type EMBR, and bacteria were abundant in the biofilm. The SS mesh with the biofilm formed on it could be used as both biocathode and filter to produce electricity and high quality treated effluent in the overflow-type EMBR (Wang et al., 2011). To investigate the specific composition of the bacterial communities from the biofilms on the SS mesh in the two different systems, amplicon sequencings on an Illumina MiSeq platform were performed. The bacterial communities of sequences at the genus level for the two samples were illustrated in Fig. 4. Lactococcus was strongly enriched, accounting for 28.3% of the total sequences in the overflow-type EMBR, followed by Bacillus (12.3%), Pseudomonas (8.8%), Saprospiraceae_uncultured (8.4%) and Solibacillus (6.8%), while those of the C-MBR were Pseudomonas (12.5%), Rhodocyclaceae_unclassified (11.9%), Lactococcus (10.1%) and Comamonas (9.8%), Plasticicumulans (8.4%). Obviously, the bacterial community composition showed very difference between the overflow-type EMBR and the C-MBR. Lactococcus, which is an electrochemically active gram-positive bacteria and produces several kinds of membrane associated quinones to mediate electron transfer to extracellular electron acceptors (Freguia et al., 2009), increased significantly in the overflow-type EMBR due to the continuous solution transportation with the bacteria from anode to cathode and abundant in the biofilm. The abundances of Bacillus and Saprospiraceae_uncultured in the overflow-type EMBR were much higher than those in the C-MBR. According to the previous studies, Bacillus strains has appeared in a system where
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Fig. 4. Genus-level abundance of sequences from the biofilms on the SS mesh in the overflow-type EMBR (A) and the C-MBR (B).
simultaneous aerobic nitrification/denitrification occured (Kim et al., 2005), and Saprospiraceae played an important role in protein degradation (Xia et al., 2008), leading to excellent NH+4-N and TN removal efficiencies. Pseudomonas is the well-known aerobic bacterium, which was also found in the MBR reported by De Gusseme et al. (2011), and degrades organic pollutants. These results showed that, these denitrifying bacteria in the overflow-type EMBR might be the electrochemically active bacteria or stimulated by electricity, which were abundant in the biofilm and contributed to electron acceptance from the cathode electrode under the electron transfer mediation of Lactococcus. 3.4. Membrane fouling mitigation in the overflow-type EMBR The membrane fouling degree could be indicated in terms of the TMP across the SS mesh. With the sludge cake deposited on the SS mesh becoming thick during the system operation, the TMP would increase sharply. The TMP changes in the overflow-type EMBR and C-MBR are illustrated in Fig. 5. The TMP in the C-MBR reached 1.2 kPa in 4 days while it reached 1.2 kPa at the 15th day in the
overflow-type EMBR, which was nearly fourfold as long as that in the C-MBR. An electric filed, which was formed by the different potential between the anode and cathode, would induce an electrostatic repulsion force that could make the negatively-charged sludge away from the SS mesh, leading to sludge deposition suppressed and membrane fouling mitigation (Akamatsu et al., 2010). Furthermore, H2O2, generated at the SS cathode, might help to in situ remove the membrane foulants, enabling a self-cleaning of the membrane (Wang et al., 2013b). In addition, the electric filed and H2O2 might influence the properties of the sludge, and influence the membrane fouling performance further (Liu et al., 2012). Therefore, to better understand the mechanisms of membrane fouling mitigation in the overflow-type EMBR, an analysis from the perspective of mixed liquor properties influencing the membrane fouling performance needs to be conducted.
3.5. Mechanisms of membrane fouling mitigation As illustrated in Table 3, four major parameters (i.e., MLSS, zeta potential, EPS and SMP of the overflow-type EMBR and C-MBR) were investigated to clarify the effects of the overflow-type EMBR on the sludge. The difference of these parameters between the two systems demonstrated that the sludge might be modified in the overflow-type EMBR, leading to membrane fouling mitigation. The MLSS concentrations were similar in these two systems (2301 ± 149 mg/L and 2154 ± 102 mg/L, respectively), indicating
Table 3 Properties of the mixed liquor in the cathodic chambers of the overflow-type EMBR and C-MBR.
Fig. 5. Variation of TMP in the overflow-type EMBR and C-MBR.
Parameter
Overflow-type EMBR
C-MBR
MLSS (mg/L) Zeta potential (mV) EPS (mg/g MLSS) EPSP EPSC EPSP/EPSC SMP (mg/L) SMPP SMPC SMPP/SMPC
2301 ± 149 17.6 ± 0.5 17.01 ± 1.89 11.16 ± 1.62 5.84 ± 0.91 1.94 ± 0.35 19.26 ± 1.18 2.75 ± 0.54 16.51 ± 0.88 0.167 ± 0.032
2154 ± 102 25.0 ± 0.3 20.87 ± 2.64 13.91 ± 1.60 6.96 ± 1.06 2.01 ± 0.10 21.18 ± 1.17 2.47 ± 0.39 18.71 ± 1.42 0.133 ± 0.030
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that the MLSS concentration was not the main parameter influencing the membrane fouling behaviors. The zeta potential of the mixed liquors in the cathodic chambers of the overflow-type EMBR and C-MBR were 17.6 mV and 25.0 mV, respectively. The value of the zeta potential is negative because the sludge particles are negatively charged, and the bigger the absolute vale of the zeta potential is, the stronger the repulsion between the sludge particles is. The sludge in the cathodic chambers of the overflow-type EMBR presented lower negative charges, indicating a strong tendency of aggregation (Azeredo et al., 1999), which would influence PSD of the sludge particles. The effect of the overflow-type EMBR on the sludge particle size distribution was illustrated in Fig. 6. While the cathodic mixed liquor was substituted by new activated sludge at the beginning of Phase 3, the sludge of these two systems showed a different particle size distribution. A significant alteration from smaller size to bigger size (the average size from 84.6 lm to 292.3 lm) was observed in the overflow-type EMBR compared with the C-MBR. According to Bai and Leow (2002), the particle size under 50 lm would cause severer filtration resistance, leading to membrane fouling aggravation. The porosity of the particle under 50 lm in the C-MBR was 41.8%, which was more than 3.4-fold compared with the overflow-type EMBR. Therefore, the decrease of the sludge particles zeta potential in the overflow-type EMBR presented lower negative charges and the decrease in the the repulsion between the sludge particles, leading to a strong tendency of aggregation to form larger sludge particles, which was beneficial to membrane fouling mitigation. EPS are a complex high-molecular-weight mixture of polymers secreted from microorganisms, substance produced by cell lysis and organic matter adsorbed from wastewater (Sheng et al., 2010). Because its concentration is directly relevant to the formation and morphology of biofilm on the membrane surface, EPS have been identified as the controlling factor of membrane fouling. The average EPS concentration in the overflow-type EMBR was 17.01 mg/g MLSS, including 11.16 mg/g MLSS proteins (EPSP) and 5.84 mg/g MLSS carbohydrates (EPSC), which were both lower than those in the C-MBR (13.91 mg/g MLSS and 6.96 mg/g MLSS, respectively). And there was no obvious difference of the EPSP/EPSC ratio for the overflow-type EMBR compared with that for the C-MBR. The EPS concentration of the sludge might be reduced due to the changes of sludge properties caused by the presence of the electric filed and H2O2 in the overflow-type EMBR. According to Sheng
et al. (2010), EPS can be used as the carbon source for bacteria metabolic activity when there is a substrate shortage. Considering higher activity of the sludge in the overflow-type EMBR stimulated by the electricity, there is an obvious demand of organic matter for bacteria (Tian et al., 2014, 2015). In addition, a low concentration of EPS decreased the zeta potential of the sludge particles, leading to an decrease in the repulsive forces between sludge particles (Morgan et al., 1990), indicating a strong tendency of aggregation. Therefore, the reduction of the EPS concentration was helpful for membrane fouling mitigation. The average SMP concentration in the overflow-type EMBR was 19.26 mg/L with a slight increase (from 2.47 mg/L to 2.75 mg/L) of the proteins (SMPP) and obvious decrease (from 18.71 mg/L to 16.51 mg/L) of the carbohydrates (SMPC) compared with the CMBR, which brought a 25.6% increase of SMPP/SMPC ratio. The SMP concentration in the overflow-type EMBR was reduced due to the higher activity bacteria stimulated by the presence of the electric filed. In addition, the SMPP/SMPC ratio in the overflowtype EMBR increased for the easier degradation of carbohydrate by bacteria. According to Yao et al. (2011), the concentration of SMP was proportional to the irreversible fouling resistance, and the higher SMPP/SMPC ratio induced less irreversible fouling, which were consistent with this study. Therefore, lower concentration of SMP and higher SMPP/SMPC ratio were an effective strategy in membrane fouling mitigation. 4. Conclusions In this study, the overflow-type EMBR system showed efficient electricity generation with a maximum power density of 629 mW/ m3 and excellent COD (92.6%), NH+4-N (96.5%) and TN (73.9%) removal efficiencies under an appropriate range of HRT (16.9– 8.5 h). Genus-level abundance of sequences showed Lactococcus, Bacillus, Pseudomonas, Saprospiraceae_uncultured and Solibacillus were the dominant genus, indicating that the electrochemically active bacteria were abundant in the biofilm. Additionally, five significant effects of the MFC integration on the sludge properties, including particle zeta potential decrease, particle size distribution alteration, EPS and SMP reduction and SMPP/SMPC ratio increase, were achieved, leading to membrane fouling mitigation. Acknowledgement This study was financially supported by Major Science and Technology Projects Focus on Social Development Projects of Zhejiang Province (2014C03002-3). References
Fig. 6. Particle size distribution (PSD) of the mixed liquor in the cathodic chambers of the overflow-type EMBR and C-MBR.
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Assessment of a novel overflow-type electrochemical membrane bioreactor (EMBR) for wastewater treatment, energy recovery and membrane fouling mitigation Guowang Zhou, Yuhong Zhou, Guoqiang Zhou, Lian Lu, Xiankai Wan, Huixiang Shi ⇑ Department of Environmental Engineering, Zhejiang University, Hangzhou 310058, PR China
h i g h l i g h t s A novel overflow-type EMBR with no ion exchange membrane was developed. The performance of the EMBR was affected by the range of HRTs. +
92.6% COD, 96.5% NH4 -N and 73.9% TN removal efficiencies were achieved. Electrochemically active bacteria were abundant in the biofilm by sequencing. The sludge properties influenced by MFC integration might mitigate membrane fouling.
a r t i c l e
i n f o
Article history: Received 15 June 2015 Received in revised form 9 August 2015 Accepted 12 August 2015 Available online 18 August 2015 Keywords: Microbial fuel cell Membrane bioreactor Wastewater treatment Energy recovery Membrane fouling mitigation
a b s t r a c t A novel overflow-type electrochemical membrane bioreactor (EMBR) without ion exchange membrane, was developed for wastewater treatment and utilized electricity recovered by microbial fuel cell (MFC) for membrane fouling mitigation in membrane bioreactor (MBR). The maximum power density of 629 mW/m3 or 7.18 mW/m2 was obtained. The removal efficiencies of chemical oxygen demand, ammonia nitrogen and total nitrogen under appropriate ranges of hydraulic retention times (16.9–8.5 h) were 92.6 ± 5.4%, 96.5 ± 2.8% and 73.9 ± 9.7%, respectively. Sequencing showed electrochemically active bacteria Lactococcus, Bacillus and Saprospiraceae_uncultured were abundant in the biofilm. Compared with a conventional MBR, five significant effects of the MFC integration on the sludge properties, including particle zeta potential decrease, particle size distribution macroaggregation, soluble microbial products and extracellular polymeric substances reduction and SMPP/SMPC ratio increase, were achieved in this system, leading to membrane fouling mitigation. This system shows great promise for practical wastewater treatment application. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Water and energy shortage are two major global challenges (Gleeson et al., 2012). With an increasing demand for freshwater and a decreasing available water resource, reusing and recycling wastewater, as a viable water resource, is feasible (Grant et al., 2012). However, typical wastewater treatment systems, using high concentrations of bacteria supplied with oxygen to remove the organic matter, are energy intensive. The overall energy consumption on this conventional treatment is roughly 0.6 kW h m3 with
⇑ Corresponding author at: Department of Environmental Engineering, Zhejiang University, Yuhangtang Road 866#, Hangzhou 310058, PR China. Tel.: +86 0571 88982044; fax: +86 0571 81021989. E-mail address: [email protected] (H. Shi). http://dx.doi.org/10.1016/j.biortech.2015.08.032 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.
about half of that used for aeration. Based upon a theoretical 3.86 kW h energy production/kg COD oxidized to CO2 and H2O, it is meaningful to develop a new process that both captures the inherent energy from the dissolved organic matters and recovers clean water for reuse from domestic wastewater (McCarty et al., 2011). Microbial fuel cells (MFCs), which enable direct conversation of organic matters into useful electricity via microbial-catalyzed redox reaction, are promising approaches for capturing the energy in wastewater for diverse purposes (Logan et al., 2006). However, the poor effluent quality and a low treatment efficiency, due to limited biomass retention, limit the practical application of MFCs. To sort out this problem, several methods have been proposed to improve treatment, such as combining the MFC with an up-flow anaerobic sludge blanket reactor and a biological aerated filter
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(Zhang et al., 2009), submerging the MFC reactor in an anaerobic reactor or an aeration tank (Cha et al., 2010; Min and Angelidaki, 2008), integrating the MFC into a rotating biological contactor (Sayess et al., 2013), or integrating the MFC with a sequencing batch reactor (Liu et al., 2011) or a membrane aerated biofilm reactor (Yu et al., 2011). These combined systems are not effective at degrading particulate biomass because of lacking of subsequent sedimentation or filtration technique. Membrane bioreactor (MBR), which is a high-efficient technology for treating wastewater, has gained worldwide popularity for good effluent quality and its ability to retain and degrade particulate biomass (Judd, 2008). More recently, several systems incorporating MBR into MFCs for improving effluent quality have been reported. For example, a novel bioelectrochemical membrane reactor, which used stainless steel (SS) mesh as the cathode and the membrane, was developed to obtain a maximum power density of 4.35 W/m3 and high-quality effluent attributed to the biofilm formation on mesh (Wang et al., 2011). Thereafter, a more practical MFC–MBR integrated process was developed (Wang et al., 2012). Additionally, a combined system integrating anaerobic MBR (AnMBR) with MFCs was proposed (Tian et al., 2014). Furthermore, a combined MBR–MFC system could mitigate the membrane fouling in the MBR due to an electric field formed at the vicinity of cathode to suppress sludge deposition and H2O2 generated to clean the already deposited foulants in situ (Wang et al., 2013b), or modification of sludge characteristics (Su et al., 2013; Tian et al., 2014, 2015). While in most of previous works, ion exchange membrane (IEM) was still required to separate the anodic chamber and the cathodic chamber. However, the use of an IEM increases the overall internal resistance and the overall cost of the systems (Wang et al., 2013a). In this study, an overflow-type electrochemical membrane bioreactor (EMBR) was designed and tested for wastewater treatment, energy recovery and membrane fouling mitigation. There was no need to use IEM while anodic and cathodic chambers separated by an overflow channel, which allowed the wastewater with protons and substrate overflow directly into the cathode from the anode and restricted oxygen to transfer from the cathode to the anode. A SS mesh was used as the cathode and the MBR module. The unidirectional flow of the wastewater from the anode to cathode allows further degradation of the organic pollutants in the cathodic chamber by aerobic MBR process. In this system, there was an electric filed formed by the different potential between the anode and cathode, would induce an electrostatic repulsion force that could make the negatively-charged sludge away from the SS mesh, leading to sludge deposition suppressed and membrane fouling mitigation. In addition, the overflow-type EMBR was more practical to scale up than other combined MFC-MBR systems. The objective of this research was undertaken to evaluate the performances of the overflow-type EMBR: (1) to assess the electricity generation and organic pollutants removal at different hydraulic retention times (HRT); (2) to elucidate the biofilm formation and community composition of the SS mesh bio-cathode; (3) to investigate the membrane fouling mitigation and the sludge properties.
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376.8 cm2 surface area (Beijing Carbon Co., China; 120 mm diameter; 100 mm height; 2 mm thickness). A stainless steel (SS) mesh with 38 lm pore size and 62 cm2 surface area (Ruitong Ironware Co., China) was used as cathode without pretreatment. The total volume of the anodic chamber was 430 mL, while the net effective volume was 360 mL. The working volume of the cathodic chamber was 850 mL. The electric connections between the electrodes and a 1000-X external resistor were established with the titanium wires. The overflow-type EMBR was inserted with silicone tubes on the bottom of the anodic chamber and the top of the SS mesh cathode. Air was bubbled into the cathode continuously at the cathodic chamber bottom to provide a dissolved oxygen (DO) concentration of 4–5 mg/L. The reactor was operated in a constant-temperature incubator (35 °C, Yiheng Co., China). 2.2. Inoculation, enrichment and operation The anodic chamber was inoculated with 100 mL of effluent from a laboratory-scale air–cathode MFC, and the cathodic chamber was inoculated with 200 mL of activated sludge from a local municipal wastewater treatment plant in Hangzhou, China. The mixed liquor suspended solids (MLSS) concentrations for anodic and cathodic chambers were adjusted to 3 and 2 g/L, respectively. The overflow-type EMBR was operated in three phases. The first phase, which lasted for 20 days, was start-up for system under batch mode with a synthetic wastewater while an anodic biofilm of electricity-producing microbes was gradually formed. After the initial feeding, the cell voltage increased gradually and decreased subsequently for lack of substrate. At that time, the synthetic wastewater in the system was replaced by a fresh one. Following the second phase, the synthetic wastewater was continuously fed into the anodic chamber using a peristaltic pump (Lange Co., China), and the effluent from the anodic chamber then overflew into the cathodic chamber through an overflow channel, passed through the SS mesh, and was finally discharged from the system. The performance of the overflow-type EMBR was evaluated at different HRTs for 28 days, and the experimental design during this phase is summarized in Table 1. During the third phase, shortterm filtration test was performed to identify the impacts of the combined system on the membrane fouling without connecting the circuit (reference phase, simulating the conventional MBR, CMBR). The cathodic mixed liquor was substituted by new activated sludge and diluted to 2 g/L with the synthetic wastewater. The synthetic wastewater was continuously fed into the system at a rate of 0.53 mL/min. The synthetic wastewater composition was: CH3COONa, 1 g/L; NH4Cl, 115 mg/L; K2HPO43H2O, 44 mg/L; CaCl2, 11.5 mg/L; MgSO4, 12 mg/L and 10 mL of trace element solution. The membrane fouling degree could be indicated in terms of the trans-membrane pressure (TMP) across the SS mesh. With the sludge cake deposited on the SS mesh becoming thick during the system operation, the TMP would increase sharply and an offline backwashing with running tap water was conducted to recover SS mesh filterability. 2.3. Analytical methods
2. Methods 2.1. Reactor design and construction The overflow-type EMBR is illustrated in Fig. 1. The reactor was constructed with anodic and cathodic chambers which were connected through an overflow channel with a width of 5 mm to allow the flow of anodic effluent to the cathodic chamber for further treatment. Anode was made of an ‘‘O ring” carbon felt with
2.3.1. Electrochemical analyses The voltage (U) across the resistance was automatically record every 1 min using a data acquisition system (Agilent 34970A, Agilent Co., USA). The current (I) was calculated from the cell voltage according to Ohm’s law (I = V/R), with the power (P) obtained as P = IV, where U was the measured voltage (V), and R the external resistance (X). Columbic efficiency (CE) was calculated as CE = Cp/Cth 100%, where Cp was the total coulombs calculated by integrating the current over time, and Cth was the
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Fig. 1. (a) Schematic diagram of the overflow-type EMBR. (1) Anodic chamber; (2) graphite felt anode; (3) cathodic chamber; (4) hydraulic head; (5) biofilm; (6) SS mesh; (7) anodic chamber inner tube; (8) cathodic chamber tube; (9) overflow channel. (b) The top view of the overflow-type EMBR.
Table 1 Operation parameters of the overflow-type EMBR system during the second phase. Run
Operation time (day)
HRT (h)
Influent COD (mg/L)
Influent NH+4N (mg/L)
Influent TN (mg/L)
1 2 3 4
21–27 28–34 35–41 42–48
16.9 11.2 8.5 4.2
662.6 ± 4.6 654.7 ± 5.5 666.7 ± 12.4 672.1 ± 6.9
30.6 ± 1.3 31.0 ± 1.2 31.6 ± 0.9 31.9 ± 0.4
35.3 ± 1.0 35.3 ± 0.8 35.6 ± 0.5 35.6 ± 0.7
theoretical amount of coulombs available based on the COD removed in the anodic chamber over the same amount of time. Linear sweep voltammetry (1 mV/s) was performed to obtain polarization curves and power density data by use of a CHI660D electrochemical workstation (Chenhua Instrument Co., China) (Logan et al., 2006). First, the circuit of the overflow-type EMBR was opened for 12 h to measure the open circuit voltage (OCV). Then, voltammetry scanning was performed using the anode as the working electrode, the cathode as the counter electrode and reference electrode, respectively. The voltage range was set between zero and the OCV. The current density and power density were normalized to the total anodic chamber volume. 2.3.2. Chemical and microscopic analyses Duplicates measurements of COD, ammonium (NH+4-N), nitrate (NO 3 -N) and MLSS were performed according to the standard methods (Rice et al., 2012). Dissolved oxygen (DO) was monitored with a DO meter (HQ 30d, Hach Co., USA). The HRT of the anodic chamber was calculated according to the net effective volume of the anodic chamber and the influent flow rate. TMP was measured through the difference in water level, which was monitored every 1 min by a pressure transmitter (GB-3000G, Gangbei Ltd., China). Scanning electron microscope (SEM, Su8010, Hitachi, Japan) was employed to characterize the surface microstructure of the biofilm on the SS mesh. Prior to SEM imaging, the sample was dehydrated in a series of graded alcohol solutions and ovendried. After sputter-coating the sample with gold palladium for 30 s at 25 mA current in an argon atmosphere, SEM imaging was performed using an accelerating voltage of 25 kV and working distance of 10 mm. 2.3.3. Other analysis To compare the bacterial communities of the biofilm on the SS mesh in the overflow-type EMBR (A) and the C-MBR (B), two samples were collected when the systems reached a steady state. DNA extractions were employed using the E.Z.N.AÒ Soil DNA Kit (Omega Bio-tek, USA). The bacterial 16S rRNA PCR and sequencing was
performed using primers 338F (50 -ACTCCTACGGGAGGCAGCA-30 ) and 806R (50 -GGACTACHVGGGTWTCTAAT-30 ) to target the variable region V4–V5 for the samples (Muyzer et al., 1993). Then, the samples were sent to Shanghai Majorbio Technology (Shanghai, China) to perform amplicon sequencing on an Illumina MiSeq platform according to the standard protocols. The sequencing data was processed using QIIME (version 1.17). Samples of mixed liquor in cathodic chamber of the overflowtype EMBR and C-MBR were taken every 48 h for zeta potential, soluble microbial products (SMP), extracellular polymeric substances (EPS) and particle size distribution (PSD) analysis. The zeta potential of sludge flocs was measured using a zeta potential analyzer (Nano-ZS90, Malvern, UK). SMP was extracted by centrifugation at 5000 rpm for 5 min and filtration through a 0.45-mm filter. EPS was extracted using heating method at 80 °C for 30 min, and then prepared by the same method that SMP was extracted. SMP and EPS samples were further analyzed for the contents of protein and carbohydrate (Li and Yang, 2007). The concentrations of protein were obtained using the Bradford method, and the concentrations of carbohydrates were quantified by the phenol–sulfuric acid method. PSD of sludge flocs was measured using a particle size analyzer (Mastersizer 2000, Malvern, UK).
3. Results and discussion 3.1. Electrochemical performance The electricity generation of the overflow-type EMBR using two different feeding regimes is illustrated in Fig. 2a. Under batch mode, the maximum voltage output reached 250.3 mV (1000 X) after 12 h startup period. Following the second phase, the overflow-type EMBR electrochemical performance at different HRTs was evaluated under continuous mode. The voltage output increased gradually and fluctuated slightly around 466 mV at HRTs of 16.9–8.5 h (Runs 1–3). Continuous feeding nutrients would maintain continuous solution transportation with the protons from anode to cathode (Wang et al., 2013a). However, the voltage output decreased significantly at a HRT of 4.2 h (Run 4). In order to investigate the electricity producing ability of the overflow-type EMBR at different HRTs, the polarization curves and power density were obtained at the end of each run. As illustrated in Fig. 2b, the open circuit voltage (OCV) fluctuated slightly between 861 mV and 840 mV at HRTs of 16.9 h-8.5 h (Runs 1–3), and the maximum power density (MPD) varied slightly between 557 mW/m3 and 629 mW/m3. However, as the HRT further decreased in Run 4, the OCV and the power density obviously
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Fig. 2. (a) Electricity generation of the overflow-type EMBR using different nutrient feeding operations (periodic and continuous nutrient loading). (b) Polarization curves and power density of the overflow-type EMBR at different HRTs.
decreased to 458 mV and 133 mW/m3, respectively. The MPD was higher than the MPD of 30 mW/m2 (or 123 mW/m3 normalized to the total anodic volume) and 37.2 mW/m3 obtained in IEM-less MFCs coupled with aerobic activated sludge systems reported by Wang et al. (2013a) and Zhu et al. (2013), respectively. As a constant resistance of 1000 X was applied, the Coulombic efficiency (CE) of the overflow-type EMBR fluctuated slightly between 0.99% and 1.03% at HRTs of 16.9–8.5 h (Runs 1–3), being similar with the value of the MFC–MBR system proposed by Tian et al. (2015) and Wang et al. (2011) and the IEM-less MFC proposed by Zhu et al. (2013), and decreased to 0.41% when the HRT was further decreased to 4.2 h in Run 4. Fermentation or methanogenesis of some other bacteria in the anodic chamber would be the main reason led to the low anode CE (Logan et al., 2006). Thus, the continuous flow from anodic chamber to cathodic chamber and an appropriate range of HRT could enable a high stability in electricity generation and a high voltage output in this system, leading to efficient electricity generation from wastewater.
3.2. Wastewater treatment performance For wastewater treatment systems, the COD and nutrient removal efficiencies are important indicators to evaluate their performance. As illustrated in Table 2 and Fig. 3, at different HRTs during Phase 2, the effluent concentration of the COD and the removal efficiencies were 5.4–122.9 mg/L (averaged at 67.6 mg/L) and 81.7–99.2% (averaged at 89.8%), respectively, while those of NH+4-N and TN were 0.16–3.86 mg/L (averaged at 1.77 mg/L) and 87.9–99.5% (averaged at 94.4%), 5.33–16.06 mg/L (averaged at 10.94 mg/L) and 54.9–84.9% (averaged at 69.2%), respectively, indicating effective wastewater treatment in the overflow-type EMBR. The COD, NH+4-N and TN removal efficiencies were affected by HRT significantly. Compared with the COD and nutrient removal in Runs 1 and 4, a longer HRT (e.g., 16.9 h in Run 1) led to excellent COD and nutrient removal efficiencies, while a shorter HRT (e.g., 4.2 h in Run 4) resulted in poor COD and nutrient removal efficiencies.
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Table 2 System performance in different runs during the second phase. Run
Coulombic efficiency (%)
1 2 3 4
0.99 1.03 1.02 0.41
NH+4-N removal efficiency (%)
COD removal efficiency (%)
TN removal efficiency (%)
Anode
Cathode
EMBR
Anode
Cathode
EMBR
Anode
Cathode
EMBR
84.1 ± 3.0 65.6 ± 6.1 47.7 ± 6.0 31.8 ± 4.5
15.1 ± 2.8 25.7 ± 4.6 39.6 ± 6.9 49.9 ± 5.2
99.2 ± 0.5 91.3 ± 2.6 87.3 ± 2.3 81.7 ± 2.6
30.2 ± 3.5 26.7 ± 1.8 24.3 ± 3.4 21.4 ± 2.6
69.2 ± 3.6 69.1 ± 2.7 70.1 ± 3.7 66.5 ± 3.1
99.5 ± 0.3 95.8 ± 3.2 94.4 ± 0.7 87.9 ± 1.4
27.9 ± 3.5 25.0 ± 2.4 22.9 ± 2.3 20.5 ± 2.7
57.0 ± 7.1 48.5 ± 4.1 40.5 ± 1.5 34.4 ± 7.1
84.9 ± 5.0 73.5 ± 2.3 63.4 ± 3.2 54.9 ± 4.9
Fig. 3. Removal of (a) COD, (b) NH+4-N and (c) TN of the overflow-type EMBR. (j) Influent concentration, (s) effluent concentration and (4) removal efficiency.
The COD removal efficiencies by the anodic chamber in the overflow-type EMBR were 31.8–84.1%, which mainly included the COD recovery in the form of electricity (0.41–1.03%), COD metabolized by the electroactive bacteria and probable COD metabolized by other bacteria (Su et al., 2013). Approximately 15.1–49.9% of COD had been oxidized attributed to the respiration of heterotrophic bacteria in the cathodic chamber. About 21.4–30.2% of the NH+4-N was removed in the anodic chamber due to microbial growth. Most of the NH+4-N was oxidized by the out layer of biofilm on the SS mesh cathode, which was occupied by putative nitrifying organisms. TN removal in the anode was mainly attributed to NH+4-N removal. Two mechanisms are possibly responsible for the TN removal in the cathode: (1) there were many denitrifying organisms living in the inner layer of the biofilm, which could use the residual organics in the wastewater as the electron donors for denitrification (Virdis et al., 2011) and (2) there were also some species of electroactive bacteria (e.g. Geobacteraceae) living in the inner layer of the biofilm, which could transfer the electrons from the cathode for denitrification (Clauwaert et al., 2007; Tian et al., 2015). A shorter HRT (e.g., 4.2 h in Run 4) would result in insufficient denitrification, thereafter, led to a poor TN removal. Thus, a longer HRT would increase the TN removal. This overflow-type EMBR construction provided a bigger net volume of the anodic chamber than other EMBRs (Wang et al., 2012, 2013b) so that more organics were removed in the anodic chamber with more COD conversion to electricity. In addition, a SS mesh was used as the cathode and the MBR module. For more practical application, carbon-based electrode materials could be used as cathode, and the hollow-fiber membrane could be used as the membrane module (Tian et al., 2014, 2015).
3.3. Biofilm formation and community composition Aeration in the cathodic chamber provided a favorable environment for the heterotrophic bacteria growth. A layer of sludge cake was formed on the SS mesh surface in the overflow-type EMBR, and bacteria were abundant in the biofilm. The SS mesh with the biofilm formed on it could be used as both biocathode and filter to produce electricity and high quality treated effluent in the overflow-type EMBR (Wang et al., 2011). To investigate the specific composition of the bacterial communities from the biofilms on the SS mesh in the two different systems, amplicon sequencings on an Illumina MiSeq platform were performed. The bacterial communities of sequences at the genus level for the two samples were illustrated in Fig. 4. Lactococcus was strongly enriched, accounting for 28.3% of the total sequences in the overflow-type EMBR, followed by Bacillus (12.3%), Pseudomonas (8.8%), Saprospiraceae_uncultured (8.4%) and Solibacillus (6.8%), while those of the C-MBR were Pseudomonas (12.5%), Rhodocyclaceae_unclassified (11.9%), Lactococcus (10.1%) and Comamonas (9.8%), Plasticicumulans (8.4%). Obviously, the bacterial community composition showed very difference between the overflow-type EMBR and the C-MBR. Lactococcus, which is an electrochemically active gram-positive bacteria and produces several kinds of membrane associated quinones to mediate electron transfer to extracellular electron acceptors (Freguia et al., 2009), increased significantly in the overflow-type EMBR due to the continuous solution transportation with the bacteria from anode to cathode and abundant in the biofilm. The abundances of Bacillus and Saprospiraceae_uncultured in the overflow-type EMBR were much higher than those in the C-MBR. According to the previous studies, Bacillus strains has appeared in a system where
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Fig. 4. Genus-level abundance of sequences from the biofilms on the SS mesh in the overflow-type EMBR (A) and the C-MBR (B).
simultaneous aerobic nitrification/denitrification occured (Kim et al., 2005), and Saprospiraceae played an important role in protein degradation (Xia et al., 2008), leading to excellent NH+4-N and TN removal efficiencies. Pseudomonas is the well-known aerobic bacterium, which was also found in the MBR reported by De Gusseme et al. (2011), and degrades organic pollutants. These results showed that, these denitrifying bacteria in the overflow-type EMBR might be the electrochemically active bacteria or stimulated by electricity, which were abundant in the biofilm and contributed to electron acceptance from the cathode electrode under the electron transfer mediation of Lactococcus. 3.4. Membrane fouling mitigation in the overflow-type EMBR The membrane fouling degree could be indicated in terms of the TMP across the SS mesh. With the sludge cake deposited on the SS mesh becoming thick during the system operation, the TMP would increase sharply. The TMP changes in the overflow-type EMBR and C-MBR are illustrated in Fig. 5. The TMP in the C-MBR reached 1.2 kPa in 4 days while it reached 1.2 kPa at the 15th day in the
overflow-type EMBR, which was nearly fourfold as long as that in the C-MBR. An electric filed, which was formed by the different potential between the anode and cathode, would induce an electrostatic repulsion force that could make the negatively-charged sludge away from the SS mesh, leading to sludge deposition suppressed and membrane fouling mitigation (Akamatsu et al., 2010). Furthermore, H2O2, generated at the SS cathode, might help to in situ remove the membrane foulants, enabling a self-cleaning of the membrane (Wang et al., 2013b). In addition, the electric filed and H2O2 might influence the properties of the sludge, and influence the membrane fouling performance further (Liu et al., 2012). Therefore, to better understand the mechanisms of membrane fouling mitigation in the overflow-type EMBR, an analysis from the perspective of mixed liquor properties influencing the membrane fouling performance needs to be conducted.
3.5. Mechanisms of membrane fouling mitigation As illustrated in Table 3, four major parameters (i.e., MLSS, zeta potential, EPS and SMP of the overflow-type EMBR and C-MBR) were investigated to clarify the effects of the overflow-type EMBR on the sludge. The difference of these parameters between the two systems demonstrated that the sludge might be modified in the overflow-type EMBR, leading to membrane fouling mitigation. The MLSS concentrations were similar in these two systems (2301 ± 149 mg/L and 2154 ± 102 mg/L, respectively), indicating
Table 3 Properties of the mixed liquor in the cathodic chambers of the overflow-type EMBR and C-MBR.
Fig. 5. Variation of TMP in the overflow-type EMBR and C-MBR.
Parameter
Overflow-type EMBR
C-MBR
MLSS (mg/L) Zeta potential (mV) EPS (mg/g MLSS) EPSP EPSC EPSP/EPSC SMP (mg/L) SMPP SMPC SMPP/SMPC
2301 ± 149 17.6 ± 0.5 17.01 ± 1.89 11.16 ± 1.62 5.84 ± 0.91 1.94 ± 0.35 19.26 ± 1.18 2.75 ± 0.54 16.51 ± 0.88 0.167 ± 0.032
2154 ± 102 25.0 ± 0.3 20.87 ± 2.64 13.91 ± 1.60 6.96 ± 1.06 2.01 ± 0.10 21.18 ± 1.17 2.47 ± 0.39 18.71 ± 1.42 0.133 ± 0.030
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that the MLSS concentration was not the main parameter influencing the membrane fouling behaviors. The zeta potential of the mixed liquors in the cathodic chambers of the overflow-type EMBR and C-MBR were 17.6 mV and 25.0 mV, respectively. The value of the zeta potential is negative because the sludge particles are negatively charged, and the bigger the absolute vale of the zeta potential is, the stronger the repulsion between the sludge particles is. The sludge in the cathodic chambers of the overflow-type EMBR presented lower negative charges, indicating a strong tendency of aggregation (Azeredo et al., 1999), which would influence PSD of the sludge particles. The effect of the overflow-type EMBR on the sludge particle size distribution was illustrated in Fig. 6. While the cathodic mixed liquor was substituted by new activated sludge at the beginning of Phase 3, the sludge of these two systems showed a different particle size distribution. A significant alteration from smaller size to bigger size (the average size from 84.6 lm to 292.3 lm) was observed in the overflow-type EMBR compared with the C-MBR. According to Bai and Leow (2002), the particle size under 50 lm would cause severer filtration resistance, leading to membrane fouling aggravation. The porosity of the particle under 50 lm in the C-MBR was 41.8%, which was more than 3.4-fold compared with the overflow-type EMBR. Therefore, the decrease of the sludge particles zeta potential in the overflow-type EMBR presented lower negative charges and the decrease in the the repulsion between the sludge particles, leading to a strong tendency of aggregation to form larger sludge particles, which was beneficial to membrane fouling mitigation. EPS are a complex high-molecular-weight mixture of polymers secreted from microorganisms, substance produced by cell lysis and organic matter adsorbed from wastewater (Sheng et al., 2010). Because its concentration is directly relevant to the formation and morphology of biofilm on the membrane surface, EPS have been identified as the controlling factor of membrane fouling. The average EPS concentration in the overflow-type EMBR was 17.01 mg/g MLSS, including 11.16 mg/g MLSS proteins (EPSP) and 5.84 mg/g MLSS carbohydrates (EPSC), which were both lower than those in the C-MBR (13.91 mg/g MLSS and 6.96 mg/g MLSS, respectively). And there was no obvious difference of the EPSP/EPSC ratio for the overflow-type EMBR compared with that for the C-MBR. The EPS concentration of the sludge might be reduced due to the changes of sludge properties caused by the presence of the electric filed and H2O2 in the overflow-type EMBR. According to Sheng
et al. (2010), EPS can be used as the carbon source for bacteria metabolic activity when there is a substrate shortage. Considering higher activity of the sludge in the overflow-type EMBR stimulated by the electricity, there is an obvious demand of organic matter for bacteria (Tian et al., 2014, 2015). In addition, a low concentration of EPS decreased the zeta potential of the sludge particles, leading to an decrease in the repulsive forces between sludge particles (Morgan et al., 1990), indicating a strong tendency of aggregation. Therefore, the reduction of the EPS concentration was helpful for membrane fouling mitigation. The average SMP concentration in the overflow-type EMBR was 19.26 mg/L with a slight increase (from 2.47 mg/L to 2.75 mg/L) of the proteins (SMPP) and obvious decrease (from 18.71 mg/L to 16.51 mg/L) of the carbohydrates (SMPC) compared with the CMBR, which brought a 25.6% increase of SMPP/SMPC ratio. The SMP concentration in the overflow-type EMBR was reduced due to the higher activity bacteria stimulated by the presence of the electric filed. In addition, the SMPP/SMPC ratio in the overflowtype EMBR increased for the easier degradation of carbohydrate by bacteria. According to Yao et al. (2011), the concentration of SMP was proportional to the irreversible fouling resistance, and the higher SMPP/SMPC ratio induced less irreversible fouling, which were consistent with this study. Therefore, lower concentration of SMP and higher SMPP/SMPC ratio were an effective strategy in membrane fouling mitigation. 4. Conclusions In this study, the overflow-type EMBR system showed efficient electricity generation with a maximum power density of 629 mW/ m3 and excellent COD (92.6%), NH+4-N (96.5%) and TN (73.9%) removal efficiencies under an appropriate range of HRT (16.9– 8.5 h). Genus-level abundance of sequences showed Lactococcus, Bacillus, Pseudomonas, Saprospiraceae_uncultured and Solibacillus were the dominant genus, indicating that the electrochemically active bacteria were abundant in the biofilm. Additionally, five significant effects of the MFC integration on the sludge properties, including particle zeta potential decrease, particle size distribution alteration, EPS and SMP reduction and SMPP/SMPC ratio increase, were achieved, leading to membrane fouling mitigation. Acknowledgement This study was financially supported by Major Science and Technology Projects Focus on Social Development Projects of Zhejiang Province (2014C03002-3). References
Fig. 6. Particle size distribution (PSD) of the mixed liquor in the cathodic chambers of the overflow-type EMBR and C-MBR.
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