Successional dynamics of low C/N activated sludge system under salinity shock: Performance, nitrogen removal pathways, microbial community, and assembly.

Limited carbon (low C/N) and salinity stress affect the stability of wastewater treatment plants. However, the effect of salinity shock on activated sludge systems with low C/N ratio wastewater remains unclear. An anaerobic/aerobic/anoxic sequencing batch reactor treating low C/N wastewater was established to investigate the effects of salinity shock on system performance, nitrogen removal pathways, microbial community, interactions, and assembly. The results showed that the effluent COD concentration could maintain a stable level, and the average COD removal efficiency was 94.9%. However, total nitrogen removal was significantly inhibited. With the addition of salinity, efficiencies of total nitrogen removal and simultaneous nitrification and denitrification decreased from 91.4 to 73.8% to 86.7 and 39.7%, respectively; however, nitrite reduction capacity increased by 25.4%. After removing salinity, ammonia oxidation capacity further deteriorated, evidenced by the increase in effluent NH-N from 8.0 to 11.8 mg/L. During the salinity shock, partial nitrification became the main nitrogen removal pathway because of the inhibition of Nitrospira and high nitrite accumulation ratio (>99.0%). Molecular ecological network analysis indicated that increased competition, decreased total modules, and disappearance of keystone taxa were related to the deterioration of ammonia oxidation capacity and simultaneous nitrification and denitrification. Moreover, the abundant denitrification module and increased denitrifiers contributed to the increase in nitrite reduction capacity. Salinity shock under low C/N conditions resulted in a stronger stochastic community assembly. This study provided information that can help enable stable operations for treating low C/N wastewater.