The military requires resilient systems for energy and water management that support mission assurance objectives while protecting health and the environment. Due to increasing water stress across the nation and the large amount of energy expended in water conveyance and wastewater treatment, new energy-efficient technologies for decentralized wastewater treatment and reuse are being investigated. The objective of this project was to demonstrate and validate a distributed treatment system for domestic wastewater that integrates newly developed and highly complementary microbial fuel cell (MFC) and biofilter (BF) technologies. Specific objectives included the following:
This project investigated the combined use of MFC and BF technologies for decentralized wastewater treatment applications. The two technologies were selected due to their potential for low energy consumption. In the MFC, bacteria oxidize organic matter in wastewater (treating the wastewater) and release electrons to the anode (graphite fiber brushes) where they flow through a circuit to the cathode (activated carbon on a stainless steel mesh current collector) producing partially treated water. The electrons are released from the cathode to the terminal electron acceptor oxygen in air. No aeration of the wastewater is needed, and some electricity can be generated, making it a potentially attractive alternative to conventional aeration-based wastewater treatment processes. MFC technology operates most efficiently when organic contaminants are present at high levels relative to effluent discharge or reuse criteria. As such, the MFC was followed in this study by a BF process to further treat the water. The BF is composed of granular activated carbon media with high surface area that facilitates adsorption of organic contaminants as well as growth of microbes that degrade the organic contaminants. Rather than pumping air into the system, the filters are alternately drained and passively aerated during a bioregeneration phase. The resulting biomass is periodically removed through a brief backwash and air scour process.
During this project, the MFC and BF technologies were designed, assembled, and integrated into an automated pilot-scale wastewater treatment skid that could treat up to 1 gallon-per-minute of wastewater. The skid included ultrafiltration and ultraviolet light emitting diode units downstream of the BF unit to provide additional pathogen removal. The pilot skid was installed in a temporary test bed at the Tobyhanna Army Depot wastewater treatment plant and assessed over a six-month period from September 2020 through May 2021. Over the course of the pilot study, the system performance was measured in terms of water quality improvement for various relevant modes of operation.
The combined system provided a robust level of treatment for most contaminants, even as influent water quality varied widely during the study period. The removal of organic matter, as measured by chemical oxygen demand (COD) and five-day biochemical oxygen demand (BOD-5), indicated that the combined system removed 91% of the organics from the water, achieving effluent BOD-5 levels of 13 ± 13 mg/L. The effluent COD levels were 36 ± 13 mg/L throughout the study period. The system was also effective in clarifying the water, reducing turbidity levels by 98%. It removed ammonia by > 95%, but it was not effective in removing phosphorus, so an additional process would be required for design cases in which phosphorus removal was required. The system also removed pathogens from the water, providing up to 6-log reductions of indicator bacteria. The pathogen removal occurred primarily within the MFC and ultrafiltration steps.
In addition to water quality improvement measurements, the practical factors of throughput, energy consumption and production, and cost were considered. A detailed analysis was performed for the theoretical case of a 5000 gallons per day system installed in a building for reuse applications. While the projected costs for the system were relatively high compared to existing technologies such as membrane bioreactors, it is expected that future MFC technology developments and market maturation could reduce costs.
Based on the results of this study, further improvements of MFC technology are needed prior to its transition into use in military installations. Future research should focus on improving the throughput and energy production of the technology, with the goal of reducing capital costs and footprint. The BF technology was determined to be potentially economical for building scale applications, and independent studies using BF followed by membrane filtration for wastewater treatment should be considered. Despite these limitations, several advancements were made during this demonstration study. Prior to this demonstration, MFC technology has been limited to mostly pilot scale demonstrations using less efficient cathodes than those demonstrated in the present study. The patented BF technology had only been tested for gray water treatment, and the present study was the first demonstration of its use for wastewater treatment at pilot scale. (Project Completion - 2022)
Kim, K.Y., R. Rossi, J. Regan, and B.E. Logan. 2020. Enumeration of Exoelectrogens in Microbial Fuel Cell Effluents Fed Acetate or Wastewater Substrates. Biochemical Engineering Journal, 165:107816. doi.org/10.1016/j.bej.2020.107816.