The Modular Design And Key Factors In Scaling-up Of Air Cathode Microbial Fuel Cell

Microbial fuel cell was regarding as new genaration of wasteater treatment technology for the advantages of low wastewater treatment cost,high operating sustainability and synchronously energy recovery.Scaling up of MFC reactors was one of the key procedures in the promotion of the real-world application of this technology.However,it is lack of the MFC architectures that were really applicable and scalable.The little information of the key problems faced during scaling up process was known.Therefore,in this study,scalable MFCs were designed,constructed and operated for the investigation of the key factors in scaling up and the analysis of the operation rules and features of pilot MFC in wastewater treatment process,which were essential for the commercialization of the future MFC technology.An application-oriented stackable horizontal plug-flow MFC?SHMFC?of multiple stackable 250 L modules was designed and operated with domestic wastewater,which extended the air cathode MFC from liters to hundreds liters.During the stable operation period,a maximum current 0.435 ± 0.010 A in each module was observed under the external resistance of 1 ? and the maximum power density was 116 m W.The effluent COD was 70 ± 17 mg L^-1 with a removal rate of 79 ± 7% and the effluent TN was 13 ± 3 mg L^-1 with a removal rate of 71 ± 8%.From the comparison between SHMFC module?250 L?and samll MFCs?28 m L?,the unideal reducing of internal resistance with the enlargement of the cross-section increased the area specific resistance of the pilot reactor,which caused the decline of power densities in scaling up process.Therefore,modular design should be the way for the enlargment of MFC syatem and the compact strcuture should be a necessary feature for pilot MFC syatem.Pilot MFC required the operation of air cathode under water.A three meters hydraulic pressure test system was set up.Three kinds of cathode,including Pt-CC?carbon cloth with Pt?,Pt-CM?carbon mesh with Pt?,and AC-MM?metal mesh with active carbon?,are investigated.The hydraulic pressure tolerance point of Pt-CC was 100 cm,while that of Pt-CM was 130 cm.Yet current density of AC-MM remained stable under the tested water pressures from 0 to 30 Kpa.The water pressure tolerance of cathodes depended on the maximum deformation amplitude of cathodes before dilacerations of diffusion layer and the stretch force provided by cathode material under this deformation amplitude.Elastic modulus of AC-MM is 4 ± 0.4 × 103 MPa,which is over 10 times larger than that of Pt-CM and over 62.5 times larger than Pt-CC.The best mechanical properties enabled AC-MM for high hydraulic pressure tolerance.Using pressurized air flow for the cathode chamber could enable compact design for large scale MFC modules.For air cathodes with stailess steel mesh and activated carbon catalyst layer,raising pressure in gas phase of air cathode could initially enhance the performance of cathode by 17 ± 2% in maximum under 10 k Pa.Further gas pressure increase resulted in current decline and charge transfer resistance increase under gas pressure ? 25 k Pa.Under hydraulic and gas pressures coexisted and balanced circumstance,the maximum power density of MFC increased.The oxygen partial pressure and the current densities of air cathode followed Monod equation.The oxygen partial pressure higher than 12.5 kPa in cathode compartment maintained 92 ± 1% of the oxygen reduction rate than that of cathode in open air.Supplement for the consumed oxygen required low air flow rate and affordable energy cost.A new type of scalable MFC was developed based on using alternating modules an array of graphite fiber brush anodes and dual cathode modules in order to simplify construction,operation,and maintenance of the electrodes.The modular MFC design was tested with a single?two-sided?cathode module with a specific surface area of 20 m² m–3 based on total volume?2 L?,and two brush anode modules.Three different types of spacers were used in the cathode module to provide structural stability,and enhance air flow relative to previous cassette?combined anode-cathode?designs: a low-profile wire spacer;a rigid polycarbonate column spacer;and a flexible plastic mesh spacer.The best performance was obtained using the wire spacer that produced a maximum power density of 1100 ± 10 mW m^-2 of cathode?32 ± 0.3 W m–3 based on liquid volume?with an acetate-amended wastewater,compared to 1010 ± 10 mW m^-2 for the column and 650 ± 20 m W m^-2 for the mesh spacers.The power density with wire spacer was comparable with the power density obtained in cube MFC using the same cathode(1086 + 8 mW m^-2).Raw domestic wastewater produced a maximum of 400 ± 8 m W m^-2?11.3 ± 0.1 W m–3 based on liquid volume?under fed batch conditions?wire-spacers?,which is one of the highest power densities for this fuel.Low COD concentration restricted the anode performance located in downstream in continous flow mode.In domestic wastewater operation condition,the restriction point of COD was 200 mg L^-1,while that of SCOD was 120 mg L^-1.The anode degradation with low COD was mainly caused by the increasing anode internal resistance and potential.The extracellular electron transfer?current generation?of anode bacteria greatly enhance the COD removal rate.With a current density of 1.61 ± 0.13 A m^-2,the overall COD removal kinetics constant of MFC reactor was 0.157 ± 0.010 h–1,which was 2.1 times as much as that of open circuit condition.The COD removal of exoelectrogens was enhanced by ccurrent density,while the COD removal of nonexoelectrogens was mainly stimulated by anode potential.The exoelectrogens and nonexoelectrogens competed for substrate.Multiple modules stack is necessary to build large MFC reactor.A larger?6.1 L?multi-module MFC stack made in a scalable configuration was constructed with four anode modules and three?two-sided?cathode modules,and tested at a wastewater treatment plant for performance in terms of chemical oxygen demand?COD?removal and power generation.With higher COD concentrations,anode modules connected to two cathodes produced 6.0 ± 0.4 W m–3,which was 1.9 ± 0.2 times as that obtained for anodes connected to a single cathode.The anode module connected to two cathodes were observed higher maximum current output capability and lower internal resistance.With COD concentration restricted condition,the positive effects of coupling more cathodes were eliminated.Domestic wastewater was fed either in parallel?raw wastewater to each individual anode module?or series?sequentially through the chambers?,with the flow direction either alternated every one or two days or kept fixed in a single direction over time.The various flow modes affected the performance of MFC stack by regulating the substrate distribution between modules.Operated in alternating flow parallel flow,the MFC stack could obtain higher overall power output and uniform performance between modules.Overall performance of the MFC in terms of COD removal was not affected by flow modes and influent COD concentration.