Successful start-up of the anammox process in constructed wetland microcosms: influence of the electron acceptors on performance, microbial community, and functional genes

Nitrogen removal by anammox process has been recognized as efficient, cost-effective, and low-energy alternative removal. The longer start-up periods of anammox process hindered the widespread application of anammox-based technology. In this study, four identical unplanted subsurface-flow constructed wetlands (USFCWs) were built up to investigate the effects of electron acceptors (Fe3+, Mn4+, SO4 (2-)) on the start-up of anammox process. Results indicated that the start-up time of anammox process was shortened to 105 days in R1 (with Fe3+ addition) and 110 days in R2 (with Mn4+ addition) with nitrogen removal efficiencies of above 75%, compared with 148 days in R0 (control). The addition of SO4 (2-) had no significant effect on start-up process. High-throughput sequencing results demonstrated that Shannon index increased significantly from 2.87 (R0) to 5.08 (R1) and 5.00 (R2), and the relative abundance of Candidatus Anammoxoglobus rose from 3.6 to 5.3% in R1. Denitratisoma increased significantly in R2 under addition of Mn4+, which was beneficial for the occurrence of anammox process. The functional genes that related to signal transduction mechanisms and secondary metabolite biosynthesis, transport, and catabolism were upregulated after addition of electron acceptors. These results demonstrated that adding electron acceptors Fe3+ or Mn4+ could be an effective way to accelerate the start-up of anammox process.