No abstract

Microbial fuel cells (MFCs) have long held the promise of being a cost-effective technology for the energy-neutral treatment of wastewater. However, successful pilotscale demonstrations for this technology are still limited to very few. Here, we present a large-scale MFC system, composed of 12 MFCs with a total volume of 110 L, successfully treating swine wastewater at a small educational farm. The system was operated for over 200 days in continuous mode with hydraulic residence time of 4 hr. Very stable electrochemical and waste treatment performance was observed with up to 65% of chemical oxygen demand (COD) removed and a maximum treatment rate of 5.0 kg COD/m 3 .day. Robust microbial enrichment was performed and adapted to metabolize and transform a diversity of compounds present. The Net Energy Recovery (NER = 0.11 kWhr/kg COD) is not only competitive with conventional cogeneration processes, but is in fact sufficient to sustain the operational energy requirements of the system. • Practitioner points• This study demonstrates the design and operation of a large-scale microbial fuel cells (MFC) system for continuous treatment of swine wastewater. • The system achieved a high chemical oxygen demand removal rate within a short hydraulic residence time. • This study moves one-step closer to applying MFC technology for real wastewater treatment.

Water and energy are becoming important priorities as increasing demands on the world's resources force nations to make sustainable choices. Microbial fuel cells (MFCs) have emerged as a promising technology to provide energy efficient wastewater treatment at significant cost savings compared to conventional aerobic treatment processes. These systems are especially beneficial in areas that are difficult to connect to municipal sewage treatment networks.Initial laboratory results from MFC tests using swine waste indicated that improvements to power output could be achieved with a carbon-fabric pleats design when compared to a previous packed bed design. An 88-liter pilot scale demonstration system was developed based on these results that allowed for full system operation in an energy neutral configuration. Initial results from the startup phase, showed an 80% decrease in chemical oxygen demand (COD) with a 7-day treatment time (fed-batch mode). During continuous flow operation, an average of 93% COD removal was observed.

Utilizing anode-respiring bacteria, microbial fuel cells (MFCs) are a technology that is able to remove organic materials from waste streams and generate electricity in the process.  MFCs have been considered as an alternative to conventional wastewater treatment using activated sludge, due to potentially lower cost of operation (no pumping of oxygen) and the ability to directly control the removal of organics. Our study is focused on designing modular and scalable MFC reactors that can be utilized in pilot-scale installations.  We tested several operational parameters and two different anode electrode configurations to study the effect on current production and chemical oxygen demand (COD) removal.  The operational parameters included running the systems in MFC-mode with a constant resistance, alternating open circuit/closed circuit operation (open circuit for 30 minutes, once a day), and applying a set voltage to the circuit.  The different anode electrode configurations included graphite-coated stainless steel bolts or carbon fiber brushes.  All of the reactors utilized the same gas diffusion oxygen reducing cathode design.  Samples were taken daily to evaluate the COD removal rates, monitor/control pH, and determine the concentration of dissolved oxygen in the reactors.  Current production was monitored in real-time and polarization measurements were executed periodically for the electrodes to evaluate the activities of the associated microbial communities at the anodes, as well as the efficiency of cathode operation. Reactors under alternating open circuit/closed circuit operation demonstrated improved performance longevity than those held under constant resistance (MFC-mode). This may be a result of decreased biofouling on the cathode per our previous work[1] or improving the charge storage capacity of the anode biofilms[2]. Our preliminary results showed that lowering the applied resistance to 22Ω yielded current production up to ~15 mA and COD removal rates of 55 mg/L/day, as compared to a resistance of 560Ω, which yielded ~1mA and 21 mg/L/day, respectively.  In regards to anode material performance, our studies showed that carbon fiber brushes allowed for COD removal rates over 10 times higher than the coated stainless steel bolts, and up to an additional ~8mA of current production. Future work will include an analysis of COD removal and current production normalized by electrode surface area and electrode-associated biomass to better compare results and elucidate the system parameters that are highly correlated to improved MFC performance. Figure 1

A microbial fuel cell (MFC) is a device that utilizes bacteria to oxidize organic and inorganic matter, and through the process of extracellular electron transfer, generate electricity. When the electron transfer at the bacteria-electrode interface is rapid, the efficiency of a MFC is dependent on the flow distribution and mass transport in the anode, which controls the delivery of reactants and removal of waste products. It has been predicted that diffusion of soluble substrates is fast over the length scale of bacterium, but gets significantly slower over millimeter-scale distances and at the scale of centimeters, advection is required. Electrolyte advection along with optimal flow distribution will bring substrates to anode respiring bacteria and enable substrate utilization at high rates. Therefore, in this study we explored the influence of the anode configuration on flow distribution and MFC performance. The efficiency of the system was determined on the basis of chemical oxygen demand (COD) removal and Columbic Efficiency (CE).  The MFC used for these studies was a membrane based tubular MFC with a submerged oxygen-reducing cathode. Three anode configurations were explored and included: 1) packed graphite granules (MFC 1); 2) bundles of stacked graphite granules (MFC 2); and 3) a small polypropylene mesh cylinder filled with packed graphite granules, wrapped with pleated carbon cloth (MFC 3). The volume of the anode compartment of each reactor was 1L. The reactors were operated under MFC, semi-batch mode with a downstream flow. The reactors were first inoculated with swine waste and lagoon sediment and subsequently fed with swine waste only 1,300 mg-COD/L. Key parameters were measured on a weekly basis including COD, pH, dissolved oxygen (DO) and turbidity. Samples for 16S rRNA analysis were also collected on a weekly basis from the effluents of each reactor for the classification and identification of bacterial species present. Voltage data was recorded continuously and polarization curves and cyclic voltammetry were periodically performed to monitor biofilm development and MFCs operation. The preliminary results indicate that due to the advanced flow distribution, MFC 3 demonstrated the highest performance. It showed a startup time of approximately 4 days and achieved a voltage of 0.40 V. MFC 2 also had a startup time of approximately 4 days and achieved a voltage of 0.15 V. MFC 1 had the slowest startup time of about 11 days with a voltage of ≈ 0.10 V. All MFCs were operated under 560 Ω resistors. Future work will include a comprehensive analysis of microbial dynamics and system performance to determine which anode structure was most correlated to improved wastewater treatment and energy recovery activities.