A state-of-the-art review and guidelines for enhancing nitrate removal in bio-electrochemical systems (BES)

Excess unmanaged nitrogen pollutes the environment. A sustainable wastewater treatment system must achieve better pollutant removal efficiency at a lower cost, and the feasibility of integrating biological nitrogen removal into bio-electrochemical systems (BES) has been reported as a tool in green technology. Nitrogen in the form of nitrate (NO3) is a common pollutant in both surface and ground waters, and a high level of NO3 makes water unsuitable for drinking water. This analysis and review of BES for treating NO3 polluted water investigate BES's components and operational factors and their importance on the NO3 removal efficiency to design more powerful but economic systems. The NO3 removal efficiencies were analyzed by the influence of electrode materials, working mode, number of chambers, type of inoculum, capacity, and microbial community structure. Overall, the electrode materials, significantly influence the NO3 removal rate. The operational parameters, such as working mode, the number of chambers, inoculum type and the systems' capacity, were deemed important and have significantly influenced the NO3 removal efficiencies when analyzed by the random forest classification algorithm. 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 genes) and dissimilatory NO3 reduction to ammonium (DNRA) (abundance of nrfA genes) processes. Our analysis suggest that BES, as a continuous two-chamber system with cathode and anode materials as granular carbon and carbon paper, respectively, with denitrifying microbes as inoculum type, would contribute to optimum NO3 removal efficiencies.