Enhanced nitrogen removal at low temperature with mixed anoxic/oxic process

Different hydraulic retention times (HRTs) were tested in a mixed anoxic/oxic (A/O) system at 5°C and 10°C to investigate the effects of HRT and carrier on nitrogen removal in wastewater at low temperatures. The results showed that the addition of the fillers improved the treatment effect of each index in the system. With an optimal HRT of 7.5 h at 5°C, the removal efficiencies of ${\text{NH}}_4^{+}-\text{N}$ and total nitrogen (TN) reached 91.2% and 75.6%, respectively. With an HRT of 6 h at 10°C, the removal efficiencies of ${\text{NH}}_4^{+}-\text{N}$ and TN were 96.7% and 82.9%, respectively. The results of high-throughput sequencing showed that the addition of the suspended carriers in the aerobic zone could improve the treatment efficiency of nitrogen at low temperatures. The microbial analysis indicated that the addition of the suspended carriers enhanced the enrichment of nitrogen removal bacteria. Nitrospira, Nitrotoga, and Nitrosomonas were found to be the bacteria responsible for nitrification, and their relative concentrations on the biofilm at 5°C and 10°C accounted for 98.11%, 92.79%, and 69.98% of all biological samples, respectively.