Effect of C/N ratio in the Processes of Anoxic-Oxic Membrane Bioreactor System on Advanced Wastewater Treatment

Excessive discharge of nitrogen and phosphorus is the main cause of eutrophication in water bodies. To enhance the performance of existing anaerobic-anoxic-oxic activated sludge processes exhibiting low nitrogen removal due to the low carbon-to-nitrogen (C/N) ratio in wastewater, a pilot-scale plant experiment was conducted. This experiment comprised anoxic, selector, and aerobic membrane bioreactor (MBR) processes with a hydraulic retention time (HRT) of 5.7 h, aiming to assess the system's performance in treating domestic wastewater containing night soil (pH of 6.8±0.5, BOD of 148.6±43.7 mg/L, COD of 94.6±26.4 mg/L, SS of 157.3±51.2 mg/L, color of 232.6±126.4 unit, and T-N of 39.223±13.257 mg/L). As a result, the residual ratios of substrates with C/N (BOD/T-N) ratio were 0.03-0.08 for BOD, 0.05-0.10 for COD, 0.07-0.14 for NH4-N, and 0.28-0.38 for NO3-N. The substrate residual ratio of the upper membrane module was 15±3% lower than that of the lower membrane module in the aerobic MBR process. When applying a C/N ratio in the range of 2.5-5.0, the nitrification and denitrification rates in the MBR process decreased with increasing C/N ratio at a returned sludge ratio of 1.0 Q. Conversely, the permeate flux decreased with increasing HRT, whereas both transmembrane pressure and resistance increased with increasing HRT.