Mitigating the detrimental effects of salt stress on anammox process: A comparison between glycine betaine and mannitol

The use of seawater to alleviate water shortages causes an increase of salinity in municipal pipe networks, posing challenges for biological wastewater treatment. The impacts of two compatible solutes on the anammox process under salt stress (20 g L−1) were compared here at the genetic and microbial levels. The findings revealed that both 0.3 mM glycine betaine (GB) and mannitol (MA) could alleviate the salt stress on anammox process, with GB exhibiting a better effect. Specifically, the addition of GB recovered the nitrogen removal efficiency (NRE) from 40 % to >80 % within 13 days. The addition of MA caused the reduction of the absolute abundance of hdh and hzsA, implying that 0.6 mM was not the optimal concentration. Moreover, salt stress induced an increase in the absolute abundance of nitrification functional genes and a decrease in the abundance of denitrification functional genes. Notably, compared with the initial level, the abundance of Candidatus Kuenenia increased by 7.1 % and 4.3 % after adding GB and MA, respectively. According to the network analysis, two compatible solutes promoted the bacterial interactions in anammox systems, which promoted the nitrogen circulation and further the nitrogen removal performance. This work provides a feasible strategy to relieve the salt stress on anammox process and then facilitates its application for treating saline wastewater.