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.
The presence of nitrogen encourages the growth of organic matter and algae, which leads to eutrophication of marine and freshwater ecosystems. In conventional wastewater treatment facilities the removal of nitrogen is achieved through simultaneous nitrification and denitrification of the wastewater. Microbial fuel cells (MFC) present an alternative technology to conventional methods for nitrate removal. The MFC is a bioelectrochemical device capable of harvesting electrons from organic sources. Bacteria in the anode compartment oxidize organic compounds, and the electrons are transferred across an external load to a terminal electron acceptor at the cathode. A typical MFC has an abiotic cathode and biotic anode. Recently more attention has been given toward the utilization of biological cathodes for the reduction of sulfate, nitrate, carbon dioxide, fumarate, and other electron acceptors. Previous studies have demonstrated an MFC’s capacity for biological denitrification at the cathode1 where bacteria can reduce nitrate to nitrite to nitrogen gas. In this study we monitored the performance and taxonomic dynamics of two denitrifying MFCs with different cathode materials. A two-chamber reactor configuration (600mL total volume) was used with alternative cathode catalysts. Acetate was provided at the anode as the electron donor and sodium nitrate was introduced at the cathode as the electron acceptor. Denitrification efficiency of the MFC was evaluated as a function of the different cathode materials and the taxonomic communities. One of the reactors had carbon cloth cathode with activated carbon/carbon black mixture (AC-MFC); the other reactor had carbon cloth cathode with carbon black layer (CB-MFC). Nitrate was periodically added to cathode chamber and the concentration of nitrate and nitrite was periodically monitored. Initial results show a removal rate of 2.02 mg/L/d and 5.31 mg/d [NO3 -] for the AC-MFC and CB-MFC after 230 days of operation. Biofilm and planktonic samples were periodically extracted from the anode, cathode, solution, and membranes of each system and were used to estimate the phylogenetic composition of each community based on 16S rRNA gene sequences. The presence of Geobacteraceae at the anode supports the observed acetate consumption and current production of both systems. Additionally, denitrifying and nitrifying bacteria were identified in the cathode compartment and support the simultaneous nitrification and denitrification observed in the cathode chamber. Further studies will be conducted to evaluate functional roles of dominant species in the community. 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.
Each year, Americans consume over 6.3 billion gallons of beer. As a result, over 50 billion gallons of brewery wastewater is created annually. Wastewater pre-treatment is an important investment for craft breweries as the biological oxygen demand (BOD) and the total suspended solids (TSS) from the waste stream leaving the brewery have to be significantly reduced before it enters the municipal system. Therefore, the treatment of brewery wastewater is an enormous and expensive part of the environmentally conscious brewing business. Brewery wastewater is characterized with high amounts of sugar, starch, and proteins, which makes it perfect candidate for Microbial Fuel Cell (MFC) application. MFCs have been proven as a promising technology for sustainable wastewater treatment and also as a way for direct recovery of electric energy [1]. In this study, two 10 liter MFCs were used to treat brewery waste collected from Stone Brewery, Escondido, CA. The MFCs had tubular design with a nanofiltration membrane separator, submerged oxygen reducing cathode and carbon cloth anode (Fig. 1). The two reactors were connected in series and operated under continuous flow. The objective of the study was to evaluate the long-term treatment capacity of the MFCs and optimize the operational parameters for maximum volumetric treatment rates and energy recovery. Three main parameters were used to evaluate the MFCs performance. These were chemical oxygen demand (COD) removal, the magnitude of the generated current density and the columbic efficiency (CE) of the COD transformation into electricity. The MFCs had an anode volume of 10L and anode surface area of 65 m2. The first MFC in the series (MFC #1) was first had an inflow of diluted buffered brewery wastewater effluent. MFC #2 was operated with the outflow of MFC #1. The MFCs was continuously running for 250 days when various flow rates of 1ml/min, 2ml/min, 3ml/min and 5ml/min were explored during the study. Over the period of 250 days and loading rates between 1ml/min to 5 ml/min, MFC #1 demonstrated an average COD removal rate of 51.6 ± 19.4%, current density of 2.77 ± 0.99 A/m3 and CE of 8 ± 7%. MFC #2 had an average COD removal of 50.9 ± 20.5%, current density of 2.0 ± 0.6 A/m3 and CE of 14 ± 11% (Fig. 2). Thus the whole system showed an average COD removal of 74.7 ± 19.2%, COD treatment rate of 0.56 ± 0.37 kg-COD/anode-m3-day, CE of 9 ± 6%, and current density of 2.4± 0.6 A/m3. This study demonstrated that MFCs could be useful devices for long-term continuous treatment of brewery wastewater. The system tested is modular, scalable and constructed from cost-effective materials. Future work will include additional large-scale pilot testing and further design and operational optimization to accomplish higher volumetric treatment rates. References: [1] Liu H, Ramnarayanan R, Logan BE. Production of electricity during wastewater treatment using a single chamber microbial fuel cell. Environ Sci Technol 2004;28:2281–5. Figure 1
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