Complete nitrogen removal by simultaneous nitrification and denitrification in flat-panel air-cathode microbial fuel cells treating domestic wastewater

Microbial fuel cells (MFCs) can treat organic compounds from domestic wastewater without aeration, but an additional procedure is required to remove nitrogen. This study developed a flat-panel air-cathode MFC (FA-MFC) that was comprised of five MFC units connected in series and operated to remove organic and nitrogen compounds from domestic wastewater with a short hydraulic retention time (HRT) of 2.5 h. During eight months of operation, the removal efficiencies of chemical oxygen demand (COD) and total nitrogen (TN) increased, reaching 85% and 94%, respectively, and the effluent COD and TN concentrations were 20.7 ± 2.5 mg/L and 1.7 ± 0.1 mg/L, respectively. The greatest removals of COD and TN were in the first and second unit (0.62 kg-N/m³/d of TN removal rate). The FA-MFC system allowed simultaneous removals of COD and TN from domestic wastewater, although it led to minimal power output (6.3 W/m³in the first unit). Because any abiotic ammonia loss was not found under the supplied potential of ∼1.1 V at a short HRT of 30 min, the biological nitrogen removal was thought as a dominant mechanism for TN removal in the FA-MFCs. Microbial community analysis revealed that, near the cathode, Nitrosomonas-like strains contributed to nitrification and Nitratireductor-like strains led to denitrification. Acidovorax-like strains, known for their metabolic diversity, were ubiquitous and appeared to contribute to organics and nitrogen removal in anode and cathode biofilms. This study provides proof of concept that the FA-MFC system has a promise for energy sustainable wastewater treatment.