Novel Anaerobic Wastewater Treatment System for Energy Generation at Forward Operating Bases
Dr. Kathryn Guy | U.S. Army ERDC-CERL
Forward Operating Bases (FOBs) currently lack sustainable wastewater treatment options, creating operational inefficiency, personnel vulnerability, and environmental degradation. The objective of this research was to develop a sustainable wastewater treatment system for FOBs that converts wastewater contaminants, including organics and ammonia, into harvestable products for energy production. The system aimed to combine sustainable materials with recent technological advances to treat wastewater with minimal material input and reduced disposal issues, while producing a net return on energy.
The novel approach pursued in this research utilizes three sustainable technologies in concert, to achieve not only feasibility within the limitations of a FOB operating environment but also the generation of valuable fuels for renewable energy production. The three enabling technologies were anaerobic membrane bioreactors (AnMBRs), clinoptilolite ion exchange (IX), and GreenBoxTM ammonia electrolysis. Design goals for water treated using the approach included meeting the following metrics: BOD < 30 mg/L (COD < 90 mg/L), TSS < 30 mg/L, and EC < 2 cfu/mL. The synergistic combination of these technologies results in an anaerobic treatment system that is capable of removing ammonia, a critical pollutant in municipal waste streams that is conventionally degraded using energy-intensive aeration processes. The component technologies were initially at an embryonic stage of development. Therefore, research efforts focused initially on the optimization of each in the context of the FOB operating environment, followed by system integration and pilot scale evaluation. The complete wastewater treatment system generates two forms of useful fuel—methane and hydrogen—that can be easily converted into electrical and thermal energy. The system was designed to require minimal chemical inputs, reduce sludge production, be simple to operate, and be scalable.
AnMBR technology is applied to degrade organics in wastewater and generate methane, which can be harvested for electrical and thermal energy using microturbine cogeneration. Clinoptilolite ion exchange is utilized to sequester ammonia from the AnMBR process stream. Clinoptilolite is a naturally occurring zeolite that acts as a molecular sieve with high selectivity and capacity for ammonia (~2 meq/g). The ammonia brine waste stream created during regeneration of spent clinoptilolite media is delivered to the GreenBox™ ammonia electrolysis system. GreenBox™ is a newly developed system that converts ammonia to useful hydrogen fuel using a small current and metal electrodes to drive the thermodynamically favorable electrolytic conversion of ammonia to hydrogen and nitrogen gas (Bonnin et al. 2008, Vitse et al. 2005).
Conventional AnMBR operation at an elevated temperature of 35°C reduced chemical oxygen demand (COD) by 98% when treating highly concentrated waste streams at a hydraulic retention time (HRT) of 21 days. Operation of an AnMBR for treating FOB wastewater (1360 mg/L COD) at low temperature (20°C) resulted in COD reduction levels of 65–80%. Gas production was less efficient under these conditions, but conversion of organic nitrogen to ammonia was still achieved. The final design recommendation for integration of AnMBR technology into the system was HRT 24 hours; ambient temperature; bioseeding of initial reactor and utilization of granulated activated carbon (GAC) and TanexTM resin to decrease startup times and decrease sensitivity to variable operating conditions.
Bench-scale examination of clinoptilolite-mediated removal of ammonium by ion exchange indicated that this is an effective and robust approach for control and concentration of ammonium. Column regeneration optimized using a 10% NaCl/0.5% NaOH mixture, making it consistent with downstream brine processing plans. The final design recommendation for integration of clinoptilolite ion exchange is multi-column arrangement; a stopflow regeneration process to maximize ammonia concentration in the regenerant brine; and load times adjusted for local calcium maximums.
Ammonia electrolysis studies using the GreenBox™ were performed to assess feasibility of this technology as an alternative to conventional biological nitrification/denitrification processes. Current output is optimized with increasing ammonia concentration. Improved performance was obtained by lowering the flow rate. Reducing NaOH concentrations from 5 M (recommended) to 0.5 M lowered chemical requirements and increased safety. The final design recommendation for integration is flow rate of 150 mL/min; 0.5 M NaOH background solution; multiple passes of ammonia solution to align with clinoptilolite regeneration cycles; and return of treated brine for subsequent clinoptilolite regeneration.
The wastewater treatment system designed under this project will benefit the Department of Defense by reducing the costs, logistical burden, and risks associated with wastewater management at FOBs. Further development of the technology into a full-scale unit is expected to yield an efficient system for onsite treatment that is simple to operate and produces fuels for electrical and thermal energy generation. This positive-net-energy approach will support self-sufficient-FOB design goals. The proposed system will reduce the FOB environmental footprint and impact on indigenous populations, demonstrating innovative and effective environmental stewardship.