Nitrogen removal and microbial community characteristics of CANON process enhanced by static magnetic field at ambient temperature

In this study, a completely autotrophic nitrogen-removal over nitrite (CANON) process was started in a membrane bioreactor at ambient temperature, to which a static magnetic field (SMF) at a 20 ​mT intensity was applied to evaluate the enhancement of the CANON process by SMF. The results indicated that the SMF was capable of enhancing the nitrogen removal rate and shortening the start-up period of the CANON process. After running for about 8 months, the average nitrogen removal rate of the CANON process with SMF exposure was 324.8 ​± ​64.8 gN/m³/d, which was 30.4% higher than that of the control. Moreover, the SMF has the potential to protect the CANON process from being inhibited by low temperatures. Importantly, the SMF induced the synthesis and secretion of the extracellular polymeric substance (EPS) of the activated sludge and simultaneously reduced the zeta potential. This was conducive to the aggregation of the sludge material and the formation of an oxygen gradient in the sludge flocs, which promoted the coexistence of ammonium-oxidizing bacteria (AOB) and anaerobic ammonium-oxidizing bacteria (AnAOB). The high-throughput 16S rRNA gene sequence analysis confirmed that the abundances of the Nitrosomonas and Candidatus Brocadia genera, respectively the main AOB and AnAOB, were enhanced by SMF exposure.