Nitrite accumulation in a denitrifying biocathode microbial fuel cell

Microbial fuel cells (MFCs) are a potential treatment technology-energy requirements for treatment can be offset with electricity production, biomass yield can be minimized, and microbial electron donors can be decoupled from acceptors, expanding treatment options. One potential MFC configuration uses an organic-oxidizing anode biofilm and a denitrifying cathode biofilm. However nitrite, a denitrification intermediate with environmental and public health impacts, has been reported to accumulate. In this study, before complete denitrification was achieved in a bench-scale, batch denitrifying cathode, nitrite concentrations reached 66.4 +/- 7.5% of the initial nitrogen. Common environmental inhibitors such as insufficient electron donor, dissolved oxygen, insufficient carbon source, and pH, were considered. Improvement in these conditions did not mitigate nitrite accumulation. We present an Activated Sludge Model with an integration of the Nernst-Monod model and Indirect Coupling of Electrons (ASM-NICE) that effectively simulated the observed batch data, including nitrite-accumulation by coupling biocathodic electron transfer to intracellular electron mediators. The simulated half-saturation constants for mediated intracellular transfer of electrons during nitrate and nitrite reduction suggested a greater affinity for nitrate reduction when electrons are not limiting. The results imply that longer hydraulic retention times (HRTs) may be necessary for a denitrifying biocathode to ensure complete denitrification. These findings could play a role in designing full-scale MFC wastewater treatment systems to maximize total nitrogen removal.