Metagenomic analysis and nitrogen removal performance evaluation of activated sludge from a sequencing batch reactor under different salinities

The effect of salinity on the nitrogen removal performance and microbial community of activated sludge was investigated in a sequencing batch reactor. The NH4+-N removal efficiency was over 95% at 0–4% salinity, indicating that the nitrification performance of activated sludge was slightly affected by lower salinity. The obvious nitrite accumulation was observed with the increment of the salinity to 5%, followed by a notable decline in the nitrogen removal performance at 6% salinity. The salinity inhibited the microbial activity, and the specific rate of nitrification and denitrification was decreased by the increasing salinity obviously. Additionally, the lower activity of superoxide dismutase and peroxidase and higher reactive oxygen species content in activated sludge might account for the deteriorative nitrogen removal performance at 6% salinity. Metagenomics analysis revealed that the genes encoding the ABC-type quaternary amine transporter in the ABC transporter pathway were abundant in the activated sludge at 2% and 4% salinity, and the higher salinity of 6% led to the loss of the genes encoding the p-type Na+ transporter in the ABC transporter pathway. These results indicated that the salinity could weaken the ABC transporter pathway for the balance of osmotic pressure in activated sludge. The microbial activity and nitrogen removal performance of activated sludge were decreased due to the unbalanced osmotic pressure at higher salinity. [Display omitted] •N removal efficiency was excellent at 0–4% salinity and worsened at 5%–6% salinity.•SOD and POD was the key antioxidase to response oxidative stress arising from salt.•Genes encoding ABC-type quaternary amine favored N removal at 2%–4% salinity.•Loss of genes encoding p-type Na+ transporter worsened N removal at 6% salinity.