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 ratiowere investigated. The results showthat 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.