The two main wastewaters produced at forward operating bases (FOBs) are graywater, which contains organic matter at concentrations of 300-500 mg/L based on chemical oxygen demand (COD), and black water, which has 7,000–10,000 mg COD/L. The purpose of this project was to investigate methods to treat these wastewaters (individually, or combined) using an energy neutral or energy positive process, to avoid the need to consume fuel for wastewater treatment. A microbial fuel cell (MFC) was examined here to serve as a net energy-producing technology to recover electrical energy from the organics in the wastewater. The specific goals of the project were to: develop novel materials for the electrodes and other components; examine configurations that could be used to scale up the system; and produce plans for a prototype reactor that could be installed at a Department of Defense (DoD) facility or a FOB to treat the gray and black wastewaters.
Anode electrode materials are needed to be electrically conductive, inexpensive, and have high surface areas for the exoelectrogenic bacteria. Different designs, numbers, sizes, and orientations were investigated as a part of this task to understand how these properties influence the performance of the MFCs. The separator was an important component of the MFCs as it was used to insulate the electrodes and prevent electrode short circuiting. It mitigated contamination of the cathode surface, and it reduced oxygen diffusion from the air-cathode into the anode chamber. A new separator material, poly(vinyl alcohol) (PVA), was tested to see if it could work well as existing materials, with the PVA used as either a separate membrane or as a cathode coating. Spacers were needed to prevent the collapse of electrodes due to water pressure, but they must also allow oxygen in air to reach the cathode surface. Various types of spacers (wired, plastic, mesh frame) were tested to identify the material and configuration that enabled the highest power densities. Selected spacers were then used in larger-scale MFC stacks that were operated with domestic wastewater. The cathode can be the most expensive component of the system, especially if expensive precious metal catalysts such as platinum (Pt) are used for oxygen reduction, and expensive binders are used such as Nafion to hold the catalyst. Novel cathode electrode materials were developed that could be effective alternatives to more expensive Pt catalysts. The wastewater treatment in terms of COD removal by the MFCs which was developed through this project was evaluated on the basis of achieving 80% COD removal. To maximize the efficiencies for power generation and wastewater treatment, MFCs need to be operated under continuous flow conditions. All the optimized materials developed in Tasks 1 through 4 were tested under various operational conditions using actual wastewaters under continuous flow conditions.Process flow diagrams were developed and conducted material and energy balances for various treatment scenarios at different FOB scales. The scenarios included both disposal of treated water and treatment for non-potable reuse. The life cycle assessment (LCA) focused on conventional metrics (e.g., fossil fuel depletion, climate change, etc.) as well as potential reduction in casualties, which was of specific interest to the DoD.
The importance of the brush anodes to fully cover the cathode was shown by reducing the number of brushes (either by disconnecting or removing them) as this reduced power. Performance was improved by using smaller brushes (0.8 cm) placed close to the cathode with acetate solutions, but stable performance with wastewater required brushes that were more than a centimeter in diameter, and so further tests were conducted using brushes 2.5 cm in diameter. PVA separators increased the coulombic efficiency and produced minimal power losses due to little impact on the internal resistance. PVA separators prepared using a simple spray-on application method produced more power in a low conductivity solution, but power was eventually reduced due to PVA degradation. It was concluded that cloth separators provided the best material properties for use in MFCs. Several cathode spacers were examined, with the wire spacers producing the best performance as they minimized blockage of the cathode surface and allowed good air flow to the cathode, resulting in a reactor design that produced the highest power densities yet obtained in larger reactors using domestic wastewater. Blending carbon black (CB) into the activated carbon (AC) when making the cathode was found to be a simple and effective strategy to enhance AC cathode performance in MFCs. Cathodes made using a phase inversion process with a polyvinylidene fluoride (PVDF) binder, placed onto a stainless-steel mesh current collector, produced a structure that did not require a separate diffusion layer to prevent water leakage. However, adding an additional hydrophobic PVDF membrane to the air side of the cathode further improved resistance to water leakage and did not adversely affect performance. Under continuous flow conditions, MFCs with full brush anodes (three total) and two cathodes produced more power than MFCs with a half-sized brush anodes placed closer to a single cathode in tests using domestic wastewater. Slightly higher COD removals were obtained using the single cathode system due to the slightly lower measured hydraulic retention times (HRTs) anodes. Experiments with diluted swine wastewater, used to simulate FOB black water, showed low rates of treatment, and thus it was concluded that black water should be mixed with gray water for treatment rather than being treated separately in MFCs.
A modular MFC design that had multiple anodes in a module (two modules) (1.4 L, 29 m2/m3) produced 250±20 mW/m2 of maximum power density using domestic wastewater under continuous flow conditions, with an average COD removal of 57±5% (HRT of 8 h). A larger (6.1 L) MFC stack made in the same scalable configuration (constructed with four alternating anode and three two-sided cathode modules) produced the total volumetric power density of 4.8 W/m3. Design plans and drawings were prepared based on these experimental data for a pilot-scale MFC comprised of 44 removable cathode modules and 45 anode modules, set in a tank measuring 8.1 ft long by 3.8 feet wide by 3 feet high. Power harvesting from the MFC was based on use of commercially available off the shelf (COTS) boost converters. The life cycle assessment (LCA) developed for MFCs as a part of this project was conducted in collaboration with SERDP project ER-2239, and therefore the LCA results will be presented in the report for that project.
Transport of water, wastewater, fuel, and treatment chemicals to and from FOBs are major security risks. MFCs can minimize this risk by treating combined gray water and black water in a near energy-neutral configuration. Wastewater aeration, which is the primary major energy consumer for conventional wastewater treatment processes is avoided by using MFCs. While MFCs eliminate the need for active aeration and produce net energy, an additional process, such as an anaerobic fluidized membrane bioreactor or biofilters, which are being investigated as a part of the ESTCP program, must be used to produce water suitable for reuse. The combined effluent can be of sufficient quality for reuse applications that include vehicle coolants, aircraft washing, pest control, laundry, centralized hygiene (field showers), personnel decontamination, retrograde cargo washing, and heat casualty body cooling.