Achieving partial nitrification in a continuous post- denitrification reactor treating low C/N sewage

To achieve advanced nitrogen removal from municipal sewage with a low carbon/nitrogen (C/N) ratio, a novel Sludge Double Recirculation-Anaerobic/Aerobic/Anoxic process (SDR-AOA) was developed in a continuousflow reactor. Low C/N (3.19 in average) sewage with the chemical oxygen demand (COD) concentration of 180.14 mg/L and the total nitrogen (TN) concentration of 56.19 mg/L was applied for long-term treatment in the SDR-AOA process. Results showed that, the average nitrite concentration in the effluent of the aerobic zone was increased from 0.15 mg/L (phase 1) to 16.46 mg/L (phase 3), and the nitrite accumulation ratio (NAR) remained between 85.87% and 94.85% in phase 3, for which the TN removal efficiency was increased from 91.40% (C/N: 5.14) to 95.60% (C/N: 3.19), and the low effluent TN concentrations was as low as 2.47 mg/L. The inhibitory effect of anoxic disturbance on nitrite oxidizing bacteria (NOB) might contribute to the effectiveness of partial nitrification, with the percentage of NOB accounting for total bacteria being declined from 4.66% (day 79) to 1.51% (day 135) on the genus level. Nitrogen mass balance analysis demonstrated that 27.77% of TN was removed through simultaneous nitrification and denitrification in the aerobic zone, while the remaining TN (62.25%) was removed through endogenous denitrification in the post-anoxic zone. Further Illumina MiSeq sequencing analysis demonstrated that the predominant bacteria were Rhodocyclaceae, Saprospiraceae and Comamonadaceae on the family level, and they were the key to nitrogen removal processes. In the end, the advanced nitrogen removal was realized in the SDR-AOA process for treating low C/N municipal sewage through the successful achievement of partial nitrification and endogenous denitrification.