Start-up and operational strategies for side-stream nitrification reactors in a cyclic up-concentration nitrogen recovery process: Enhancing targeted nitrate conversion and deciphering nitrogen metabolism

Various start-up strategies of side-stream nitrification reactors (SNR) with different initial salinities and nitrogen loading rate (NLR) were evaluated to investigate nitrogen conversion dynamics and microbial evolution during long-term operation. During start-up, the combined inhibition of nitrite-oxidizing bacteria (NOB) by salinity and free ammonia or free nitrite acid was mitigated through microbial adaptation and NLR reduction, but excessive inhibition may prevent recovery even after long-term regulation. The saline and nitrogen-rich autotrophic environment enriched Nitrosomonas and halotolerant bacteria (Luteimonas, Wenzhouxiangella, Thioalkalivibrio, Truepera) and shifted the predominant NOB from Nitrospira to Nitrobacter. The abundance of nitrogen metabolism genes (hao, amo, nxr) and energy metabolism genes increased significantly in the SNRs. Additionally, Na+/H+ antiporters and antioxidant genes were enriched to maintain osmotic and redox balance in functional bacteria.