Combined Effects of C/N Ratio and NO3--N Concentration on the Partial Denitrification (PD) Performance Under Lower Temperature: Substrate Variation, Nitrite Accumulation and Microbial Transformation

Using an intermittent SBR process with sodium acetate as additional carbon source, the combined effects of influent carbon-to-nitrogen ratio (C/N=0.8, 1.5, 2.5, 3.5, 4.5) and nitrate (NO3--N) concentration (40 and 80 mg/L labeled R40 and R80) on the nitrite (NO2--N) accumulation during the partial denitrification (PD) process were investigated at low temperatures (4 - 12 ℃), where the microbial diversity and functional bacteria evolution were also explored by high-throughput sequencing. The results showed that the 3.5-R40 and 2.5-R80 systems both presented the optimal NO2--N accumulation at temperature of 10 ℃, with the NO2--N transformation rate (NTR) of 66.89% and 76.79%, respectively. In addition, as the temperature reduced from 10 ℃ to 5 ℃, the NO2--N accumulation performance was significantly suppressed, where the average effluent NO2--N of 3.5-R40 (20.00 → 11.00 mg/L) and 2.5-R80 (43.00 → 18.90 mg/L) systems reduced by nearly half. It's worth noting that there was almost no NO2--N accumulation at C/N of 0.8, although higher NO3--N concentration promoted NTR under the same C/N ratio. The high-throughput sequencing showed that Proteobacteria and Bacteroides increased significantly from 35.31%, 18.34% to 51.69% - 60.35% and 18.08% - 35.21% as compared with the seeding sludge. Thauera and Flavobacterium as the main contributors to NO2--N accumulation accounted for 31.83% and 20.30% at the C/N ratio of 2.5 under low temperature of 5 ℃. The above discussion suggested that higher temperature (10 ℃), lower C/N ratio (2.5 - 3.5), and higher NO3--N concentration (80 mg/L) were more favorable for the stable PD formation.