Microbial and environmental factors affecting nitrate removal in bio-electrochemical systems (BES)

Excess nitrogen has caused environmental issues by polluting the air and water. Many different processes help remove nitrogen compounds from contaminated soils and waters, and the presence of oxygen is one of the most decisive factors. Denitrification in anaerobic conditions is considered the main removal processes of excessive nitrogen, although lately discovered anaerobic ammonium oxidation (ANAMMOX) and dissimilatory nitrate reduction to ammonium (DNRA) may also have an important role in nitrogen elimination of different systems. To a lesser extent, also nitrification can contribute to nitrogen elimination in watery systems. All previously pointed out removal mechanisms occur in the constructed wetlands and could even be enhanced with the bio-electrochemical systems (BES). BES exploit the ability of the electroactive microorganisms to reduce the oxides of nitrogen.

We analyzed various articles treating nitrate (NO₃) polluted water in BES and normalized their NO₃ removal efficiencies to a common unit (mg liter⁻¹ day⁻¹). We analyzed the effect of various factors such as electrode materials, working mode, type of inoculum, number of chambers, systems’ capacity and the microbial community structure on the NO₃ removal efficiencies. The highest removal efficiencies were displayed by granular carbon and carbon cloth used as cathode and anode material, respectively. The electrode materials and operational parameters, such as working mode and number of chambers, were deemed important by the random forest classification algorithm. Continuous mode of operation, denitrifying microbes as inoculum type, and two chamber systems have displayed optimum NO₃ removal efficiencies. Feature selection using random forest classification showed the type of inoculum and capacity of the BES were unimportant factors. Proteobacteria and Firmicute were the prominent phyla observed in BES treating NO₃ polluted water. Besides the denitrification (abundance of narG, nirS, nirK, nosZI, and nosZII genes) process in BES, there is evidence of electrochemical support for anaerobic ammonium oxidation (ANAMMOX) (abundance of hzsB or ANAMMOX specific 16S rRNA gene) and dissimilatory NO₃ reduction to ammonium (DNRA) (abundance of nrfA gene) processes. The results of this work aid in understanding the prevalent processes in the BES and help to build efficient BES for optimum NO₃ removal.