•Granular activated carbon (GAC) was added over the membrane cathode to expand its size.•GAC expanded cathode coupled MFC/MBR system was studied.•ORR pathways were 4e− on GAC and 2e− on FeOOH/TiO2/GAC.•Formation and decomposition of H2O2 was catalyzed by loaded FeOOH/TiO2 on GAC.•Oxidative tetracycline hydrochloride removal (90%) was realized by hydroxy radical.In this study, membrane bioreactor (MBR) and microbial fuel cell (MFC) was coupled for wastewater treatment using a polyvinylidene fluoride (PVDF) coated carbon fiber cloth as cathode membrane. To generate more power and mitigate membrane fouling, granular activated carbon (GAC) was added as a dynamic layer on cathode membrane. With or without FeOOH/TiO2 doping on GAC, 2e− or 4e− oxygen reduction reactions (ORRs) took place. The maximum power density reached 5.1 W m−3 via 4e− ORR, practically the highest compared to similar MBR/MFC coupled systems. The removal of COD and NH4+-N was 90% and 80%, respectively. With FeOOH/TiO2/GAC, hydrogen peroxide (H2O2) was formed via 2e− ORR, at 0.13 mg L−1 in effluent. Oxidative removal of a model pollutant tetracycline hydrochloride was 90% by reactive oxidizing species such as •OH. This is the first report of H2O2 synthesis using doped GAC as expanded cathode in coupled bio-electrochemical MBR/MFC system. Compared to other electrochemical systems, our bio-electrochemical system was more energy-saving and environmental-friendly in wastewater treatment.

•The integrated electric MBR had significant COD, NH4+-N and TP removals.•QSC can replace expensive PEM, achieving better electrochemical performance.•The embedded TF/ACOB saved aeration cost and improved effluent quality.•The tested process and materials significantly reduced wastewater treatment cost.•The generated bio-electricity effectively alleviated membrane fouling in MBR.A novel combined system integrating MFC and electric membrane bioreactor (EMBR) was developed, in which a quartz sand chamber (QSC) was used, replacing expensive proton exchange membrane (PEM). An air contact oxidation bed (ACOB) and embedded trickling filter (TF) with filled volcano rock, was designed to increase dissolved oxygen (DO) in cathodic EMBR to save aeration cost. Membrane fouling in EMBR was successful inhibited/reduced by the generated bioelectricity of the system. The combined system demonstrated superior effluent quality in removing chemical oxygen demand (>97%) and ammonia nitrogen (>93%) during the stable operation, and the phosphorus removal was about 50%. Dominant bacteria (Nitrosomonas sp.; Comamonas sp.; Candidatus Kuenenia) played important roles in the removal of organic matter and ammonia nitrogen. The system has good application prospects in the efficient use of water and the development of sustainable wastewater recycling technology.Download high-res image (110KB)Download full-size image

Other than their use as new energy sources, microbial fuel cells (MFCs) are promising for wastewater treatment as they allow for significant energy savings and a high treatment efficiency if they are integrated with a MBR (membrane bioreactor), where the electricity can be in situ used over the cathode membrane, in spite of the insignificant power generation, the small current and low voltage output. The performance and cost of MFCs are largely influenced by the electrode materials. Nanocarbon materials with superior physical and chemical properties that conventional materials cannot match are crucial for the development of MFCs. In this review, recent research progress and applications of carbon nanotubes (CNTs), graphene, graphitic carbon nitride (g-C3N4) and their composites as MFC anode/cathodes are highlighted, for insights into the characteristics, the modification/preparation methods and the performance of such MFCs. Different composite catalytic cathode membranes in integrated MFC–MBR systems are also reviewed. Integrating a MBR with a catalytic cathode membrane in MFCs improves the effluent quality and overcomes the deficiencies of MFC, while using the recovered bio-energy to offset the energy consumption for aeration and filtration.

In this study, a photocatalytic composite membrane (PCM) coated with CoFe2O4(–rGO) and polyvinylidene fluoride (PVDF) on a carbon fiber cloth was firstly prepared by inclusion of nanoparticles in a PVDF casting solution. The PCM with CoFe2O4(–rGO) functioned as the cathode membrane in a photocatalysis-assisted MFC-MBR system. Comparison tests with four types of MFe2O4 photo-catalysts (M = Ni, Fe, Co, Zn) in PCM indicated that CoFe2O4 had the highest oxygen reduction reaction (ORR) activity. Upon compositing with reduced graphene oxide (rGO), the CoFe2O4–rGO greatly improved the catalytic activity. Photocatalysis in the cathode greatly promoted both power generation and contaminant removal. The maximum power density of 942 mW m−3 (versus anode volume) was achieved using this PCM with CoFe2O4–rGO, under visible-light irradiation, and the removal of tetracycline hydrochloride antibiotics in a photocatalysis-assisted MFC-MBR system was higher than that without irradiation. By efficiently decomposing recalcitrant substances, the photocatalysis-assisted MFC-MBR system exhibits better and broader application potential in wastewater treatment than a conventional MFC-MBR system. The beneficial effects of nanoparticles on flux and conductivity in the PVDF casting solution were also evaluated.

Photocatalytic fuel cells (PFCs) are a newly developed technology that degrade pollutants and simultaneously generate electricity. Stainless steel electrodes loaded with anodic Ag/AgCl/GO and cathodic ZnIn2S4, formed a one-chambered PFC, in which rhodamine B (RhB) was degraded under visible light (2 W LED). After 1 h of irradiation, 87.4% of the RhB was degraded and a 0.52 mA cm−2 current density was generated when the external resistance was 1 Ω. Increasing this resistance lowered the current density and decreased degradation. The current and cell voltage are affected by the degradation efficiency over the electrodes, and the photocatalytic electrode with higher degradation activity functioned as the anode, because of its relatively richer supply of electrons compared to the other. Ag/AgCl/GO can function as a cathode in a two-chambered PFC reactor with Fe as anode, which also had high degradation efficiency and higher electricity generation performance. The characteristics of the photoelectrodes were investigated using scanning or transparent electronic microscopy (SEM, TEM), continuous cyclic voltammograms (CV) and Electrochemical Impedance Spectroscopy (EIS). Electron Spin Resonance (ESR) was used to detect the reactive oxygen species (ROS). The effect of pH and RhB concentration on the degradation performance of this PFC was investigated. The PFCs can work in a broad range of pH.

•Carbon fiber cloth was coated with catalyst and PVDF binder for cathode membrane functions.•The C–Mn–Fe–O catalyst on cathode membrane performed well in MFC/MBR system.•The maximum power density and inner resistance of MFC/MBR system was 1358 mW m−3 and 71 Ω, respectively.•2 cm electrode distance prevented anode from oxygen without using exchange membrane.•Ammonium nitrogen in the wastewater was removed via conversion to N2.An effective new cathode membrane (CM) with carbon fiber cloth, polyvinylidene fluoride (PVDF) and a catalyst containing C, Mn, Fe and O elements, was prepared. Firstly, the catalyst was in-situ formed on the fiber cloth via impregnation and high-temperature pyrolysis. Then for cathode membrane preparation, the fiber cloth after pyrolysis was coated with PVDF via casting and phase inversion. The membrane prepared performed well as electrodes and successfully integrated MBR with MFC. The surface morphology of the CM was studied using scanning electronic microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX) analysis confirmed the existence of PVDF and catalyst components. The carbon cathode membrane with C–Mn–Fe–O catalyst (CCMT) was compared with carbon fiber cloth cathode membrane without the catalyst (CCMO) in MFC/MBR, the coupled wastewater treatment system (without inoculating sludge biomass in cathode chamber). In the configured system, the 2 cm distance between the electrodes could effectively isolate the anodic environment (DO=0.01 mg L−1) from the aerobic cathodic condition (DO=4.81 mg L−1). In the system with CCMT, the removal rates of COD, NH4+–N, and TP were above 90%, 80% and 65%, respectively, and removal of NH4+–N was found converted to N2. Its maximum generated power density reached 1358 mW m−3, 4 times higher than using CCMO (337 mW m−3). The inner resistance in system with CCMT was 71 Ω, 2.7 times lower than using CCMO (194 Ω).

Cost effective air-cathodes are very important for application of Microbial Fuel Cells (MFCs). Carbon fiber cloth was used as the base for preparing air-cathodes with cheap polyvinylidene fluoride (PVDF) coating on the solution-side to replace expensive Nafion as a separator regardless of the ion exchange part. The separator on the solution-side was a vital part for stable electricity generation. Another PVDF layer was coated on the air-side via a thickness-controllable method as the diffusion layer. The comparisons between Nafion and PVDF on the solution-side of cathode in electricity generation and fouling/regeneration were tested in MFCs without any catalyst. The cell voltage of the Nafion air-cathode MFC (0.23 V) was slightly higher than the PVDF MFC (0.2 V). Biofouling and cation deposition on air-cathodes became more severe with time, negatively affecting electricity generation. Soaking the air-cathode with solution-side Nafion in HCl (maximum power density: 338.1 mW m−2) can regenerate the cathodes and enhance the electricity generation much more than that regenerated by ultraviolet light (UV) irradiation.

Using the flat module membrane electrodes based on polyester filter modified with polyaniline (PANI)–phytic acid (PA), a new type of membrane bio-reactor (MBR) system is developed for both bio-electrochemical electricity generation and effluent filtration. A specifically prepared carbon foam–Fe–Co catalyst was coated on the conductive, filterable cathode membrane which makes it catalytic for enhancing cathodic oxygen reduction reaction and electricity generation. The cell voltage (with carbon foam–Fe–Co, 0.5–0.4 V) is superior to control test 1 (without any catalyst, 0.2 V), moreover, it is even better than control test 2 (with Pt–C catalyst, 0.5–0.3 V). The corresponding maximum power density of carbon foam–Fe–Co is 38.5 and 2.4 times higher than control test 1 and 2, respectively. More importantly, the base electrode materials and catalyst were both low-cost. The overall removal efficiency of COD and NH4+–N are satisfactory, at 95% ± 2.5% and 85% ± 2.5%, respectively. This integrated system is easy to scale up for practical application in waste-water treatment and offers a better option in operating and coupling MBR with bio-electricity generation.

•Simple integration of microbial fuel cell in membrane bioreactor was realized.•The integrated BEMR included independent cathode.•Membrane fouling was mitigated by the internally bio-generated electric field.•The repulsive force between membrane and foulant was calculated.A microbial fuel cell (MFC) was integrated with flat sheet membrane bioreactor (MBR) and studied for electricity generation, membrane fouling mitigation and artificial wastewater treatment. The cell potential was ∼0.2 V with 100 Ω external load during closed circuit operation. Batch tests identified that the sludge properties and aeration in cathodic chamber were the main affecting factors on electricity generation. Integration of microbial fuel cell can significantly alleviate membrane fouling, under closed circuit condition, membrane filtration lasted 21 days – 27 days and under the open circuit condition it lasted only 13 days - 15 days, before the transmembrane pressure (TMP) reached 0.03 MPa. The calculated electrostatic repulsion force between membrane surface and membrane foulant was about 2.5 × 10−14 N in this integrated reactor. The chemical oxygen demand (COD), ammonia nitrogen, phosphorus and offensive smell could be effectively removed by the sequential anaerobic-aerobic treatment system. The effluent pH was neutral and turbidity was very low.

•TiO2 was modified by ATRP grafting with PMMA or co-grafting with SBMA.•The modified TiO2/PVDF membrane had higher flux and better antifouling property.•With SBMA, the flux was further increased as compared to TiO2 PMMA/PVDF.•Optimal grafting time was 5 h, optimal ratio of KH550 to TiO2 was 3:1.•Optimal molar ratio of PMMA to SBMA is 2:1.To improve PVDF (polyvinylidine fluoride) membrane property, TiO2 nano-particles can be blended in casting solutions to form composite membranes. In this study, TiO2 nano-particles were modified by coupling with γ-aminopropyl triethoxy silane (KH550), then grafted with PMMA (Polymethyl methacrylate) or co-grafted with PMMA and more hydrophilic PSBMA (polysulfobetaine methacrylate) via ATRP (atom transfer radical polymerization). ATRP grafting time and weight ratio of KH550 to TiO2 were two main identified factors affecting TiO2 modification and membrane hydrophilicity, flux and antifouling properties. These were shown by Scanning electron microscopy (SEM) characterization, contact angles measurements and filtration tests with yeast suspensions. The grafted TiO2–PMMA nano particles formed after coupling at 3:1 weight ratio of KH550 to TiO2 and 5 h grafting, once composited with PVDF, formed a membrane with higher flux (up to 50%), higher flux recovery rate (FRR, 93.5%) and lower irreversible fouling (5.37%) than PVDF membrane composited with unmodified TiO2. Blending TiO2 co-grafted with PMMA and zwitterionic polymer PSBMA resulted in even better antifouling membranes (even lower Rir < 2% and even higher FRR 99%). When the molar ratio of MMA to SBMA was 2 to 1, the highest steady flux and the highest FRR were obtained.

Membrane fouling can be greatly reduced by increasing the turbulence near membrane surfaces via enhancing aeration intensity or using helical baffles. A newly designed helical membrane served that purpose and achieved flux enhancement without increasing the aeration intensity or energy consumption. In this paper, the particle image velocimetry (PIV) technique was used to demonstrate the difference in intensity and distribution of flow field/velocity vectors between the helical and the flat sheet membrane modules. The results showed that the helical membrane produced rotational flows near the membrane surface and enhanced the shearing rate/flow velocity. To further optimize the membrane geometries (dimensions and angles) and investigate the influence of membrane spacing and membrane piece numbers in a helical membrane module on its performance in filtering sludge suspensions in membrane bioreactor (MBR), various membrane modules were studied. To the single piece membrane module of different dimensions, the average flux of a helical membrane was 17–37.5% higher than that of a flat membrane. The more slender membranes had better fouling reduction and higher flux enhancement. For multi-piece membrane module, 8 piece helical module enjoyed more favorable flow dynamics, great fouling reductions and an enhancement of the average flux over 50% than the 8 piece flat module. An appropriate spacing and a lower trans-membrane pressure (TMP) was important for achieving higher permeate flux, and the use of flocculants significantly increased the sludge permeability and the permeate flux using the helical membrane in MBR.Graphical abstractHelical membranes with 8 pieces enjoyed more favorable dynamic flow, great fouling reductions and an enhancement of the average flux over >50% than using the flat membrane.Highlights► The flow field near the helical/flat membrane was compared using the PIV technique. ► The optimal membrane dimensions/geometries, spacing and piece number in a module were determined. ► Slender membrane and multi-piece modules are advantageous in flux enhancement. ► Lower operating TMP and using flocculants increased flux enhancements by the helical modules.

•PPy/AQS, PPy/ARS and PPy successfully modified SSM were tested in MBR/MFC for the first time.•The modified SSMs function as conductive membranes and catalytic cathodes.•The modification of SSM improved MB degradation and antifouling properties.•The modification of SSM improved electricity generation by 20–30 times.•The MBR/MFC with modified SSM has higher effluent quality and treatment efficiency.To increase effluent quality and membrane flux, membrane bioreactor was integrated with microbial fuel cell (MBR/MFC), in which functional cathode membranes could enable fouling reduction and even electro-catalytic pollutant degradation using the bio-generated electricity. Modifying stainless steel mesh (SSM) with only polypyrrole (PPy) or cheap ARS (Alizarin Red's) or expensive AQS (9,10-anthraquinone-2-sulfonic acid) doped PPy film, helped obtain high ORR (oxygen reduction reaction) activity and higher power output in the integrated MBR/MFC system. The ARS and PPy modified cathode membranes, could not only increase the degradation of (MB) methylene blue (> 90%, 1 h), but also enable higher antifouling property in filtrations. Most importantly, replacing the blank SSM, the use of PPy/AQS, PPy/ARS and PPy modified SSM, increased power density 31.37, 27.06 and 23.7 times respectively in the integrated MBR/MFC system. The new system has great application potential and economic feasibility in effective removal of COD and NH4+-N nutrients, has better effluent qualities and has higher energy recovery.

In this study, a photocatalytic composite membrane (PCM) coated with CoFe2O4(–rGO) and polyvinylidene fluoride (PVDF) on a carbon fiber cloth was firstly prepared by inclusion of nanoparticles in a PVDF casting solution. The PCM with CoFe2O4(–rGO) functioned as the cathode membrane in a photocatalysis-assisted MFC-MBR system. Comparison tests with four types of MFe2O4 photo-catalysts (M = Ni, Fe, Co, Zn) in PCM indicated that CoFe2O4 had the highest oxygen reduction reaction (ORR) activity. Upon compositing with reduced graphene oxide (rGO), the CoFe2O4–rGO greatly improved the catalytic activity. Photocatalysis in the cathode greatly promoted both power generation and contaminant removal. The maximum power density of 942 mW m−3 (versus anode volume) was achieved using this PCM with CoFe2O4–rGO, under visible-light irradiation, and the removal of tetracycline hydrochloride antibiotics in a photocatalysis-assisted MFC-MBR system was higher than that without irradiation. By efficiently decomposing recalcitrant substances, the photocatalysis-assisted MFC-MBR system exhibits better and broader application potential in wastewater treatment than a conventional MFC-MBR system. The beneficial effects of nanoparticles on flux and conductivity in the PVDF casting solution were also evaluated.

Other than their use as new energy sources, microbial fuel cells (MFCs) are promising for wastewater treatment as they allow for significant energy savings and a high treatment efficiency if they are integrated with a MBR (membrane bioreactor), where the electricity can be in situ used over the cathode membrane, in spite of the insignificant power generation, the small current and low voltage output. The performance and cost of MFCs are largely influenced by the electrode materials. Nanocarbon materials with superior physical and chemical properties that conventional materials cannot match are crucial for the development of MFCs. In this review, recent research progress and applications of carbon nanotubes (CNTs), graphene, graphitic carbon nitride (g-C3N4) and their composites as MFC anode/cathodes are highlighted, for insights into the characteristics, the modification/preparation methods and the performance of such MFCs. Different composite catalytic cathode membranes in integrated MFC–MBR systems are also reviewed. Integrating a MBR with a catalytic cathode membrane in MFCs improves the effluent quality and overcomes the deficiencies of MFC, while using the recovered bio-energy to offset the energy consumption for aeration and filtration.