Enhanced nitrogen removal of constructed wetlands by coupling with the bioelectrochemical system under low temperature: Performance and mechanism

Constructed wetland (CW) integrated with the bioelectrochemical system (BES-CW) is a novel and efficient low-strength wastewater treatment technology. In this study, the integrated CW (E-CW), open-circuit BES-CW (O-CW) and conventional CW (Control) were operated to investigate the performance and mechanism of nitrogen removal in E-CW under low temperature (10 °C). Results showed that low temperature nearly completely inhibited nitrate removal in Control and O-CW, but improved ammonia oxidation efficiencies (99.46% and 99.63%, respectively) due to high dissolved oxygen (DO) concentration in cold influent and enhanced ammonia oxidation archaea (AOA) growth. However, nitrite oxidation became the limited step of nitrification and partial nitrification and denitrification (PND) played the main role in TN removal under low temperature in Control and O-CW. On the contrary, nitrate was almost completely removed in E-CW (98.12%) on the 2nd day even at 10 °C due to its enhanced cold-resistant autotrophic denitrifying bacteria growth by applying low voltage and high DO consumption rate, and accordingly further improved its TN removal efficiency (82.26%), while the average nitrate and TN removal efficiencies in Control were only 11.10% and 37.26%, respectively. Thermomonas, Arenimonas, Gallionella and Thiobacillus were the main denitrifying bacteria attributed to nitrate removal in E-CW under low temperature involving autotrophic denitrification and cathode-driven bioelectrochemical denitrification. Moreover, extremely lower N2O emission from E-CW (0.81%) was obtained than the Control (12.76%), which indicated that the integration of CWs and the bioelectrochemical system is a viable technology for mitigation of greenhouse gas emission from CWs.