Applying constructed wetland-microbial electrochemical system to enhance NH4+ removal at low temperature

Abstract

NH4+ removal at low temperature (<10 °C) has baffled researchers and engineers for decades. Bioelectrochemical process has been increasingly valued as a promising approach to enhance NH4+ removal by both electrochemical and stimulated microbial processes. The feasibility and the mechanism of enhanced NH4+ removal were investigated in Constructed Wetland-Microbial Electrochemical System (CW-MES) with different electrode spacings including Constructed Wetland-Microbial Fuel Cell (CW-MFC) and Constructed Wetland-Microbial Electrolysis Cell (CW-MEC) at low temperature. Solar cell panel was firstly implemented in CW-MEC to enhance NH4+ removal. The low-temperature operation lasted for about four months, CW-MEC successfully enhanced NH4+ removal while CW-MFC did not exhibit positive effect. The NH4+-N removal efficiency of CW-MEC achieved 88.2 ± 7.0%, which was 11.7 ± 6.5% higher than conventional constructed wetland (CCW). The maximum NH4+-N removal efficiency of CW-MEC achieved 100%. The average NH4+-N mass removal rate was 436.02 mg m−2 d−1. It was found that NH4+ was mainly removed by the nitrification-autotrophic denitrification process in CW-MES while it was mainly converted to NO3− in CCW. Ammoxidation and denitrification were both enhanced by electricity while NH4+ was used as the main substrate for electricity generation. AOA (Candidatus Nitrosocosmicus) and NOB (Nitrospira) were the main contributors to nitrification. This study provided a cost-effective and sustainable method for electrochemically enhanced microbial NH4+ removal at low-temperature and revealed the relevant mechanism.