Enhanced biological nutrient removal in modified carbon source division anaerobic anoxic oxic process with return activated sludge pre-concentration

A pilot-scale modified carbon source division anaerobic anoxic oxic (AAO) process with pre-concentration of returned activated sludge (RAS) was proposed in this study for the enhanced biological nutrient removal (BNR) of municipal wastewater with limited carbon source. The influent carbon source was fed in step while a novel RAS pre-concentration tank was adopted to improve BNR efficiency, and the effects of an influent carbon source distribution ratio and a RAS pre-concentration ratio were investigated. The results show that the removal efficiency of TN is mainly influenced by the carbon source distribution ratio while the TP removal relies on the RAS pre-concentration ratio. The optimum carbon source distribution ratio and RAS pre-concentration ratio are 60% and 50%, respectively, with an inner recycling ratio of 100% under the optimum steady operation of pilot test, reaching an average effluent TN concentration of 9.8 mg·L− 1 with a removal efficiency of 63% and an average TP removal efficiency of 94%. The mechanism of nutrient removal is discussed and the kinetics is analyzed. The results reveal that the optimal carbon source distribution ratio provides sufficient denitrifying carbon source to each anoxic phase, reducing nitrate accumulation while the RAS pre-concentration ratio improves the condition of anaerobic zone to ensure the phosphorus release due to less nitrate in the returned sludge. Therefore, nitrifying bacteria, denitrifying bacteria and phosphorus accumulation organisms play an important role under the optimum condition, enhancing the performance of nutrient removal in this test.

Graphical AbstractNH4+-N was oxidized primarily in the aerobic tank by nitrifying bacteria and transformed to NO2−-N and NO3−-N, which returned with mixed liquor to the anoxic zone and removed by denitrification. Most of NOx−-N in the returned sludge was removed by the endogenous denitrification in a pre-concentration tank and a pre-anoxic tank and less of them, about 0.03 mg·L− 1 of NO3-N, entered the anaerobic zone and mixed with the influent. The accumulation of NO2−-N was not observed in steady operation. With sufficient carbon source for nitrifying bacteria and denitrifying bacteria to develop ammoniation, nitrification and denitrification and the optimum carbon source distribution ratio under the optimum operation condition, a good nitrogen removal effect was achieved in this test, which is consistent with other studies.Figure optionsDownload full-size imageDownload as PowerPoint slide