Biological nutrient removal by applying modified four step-feed technology to treat weak wastewater

For weak municipal wastewater (COD ⩽ 200 mg L−1, NH4+-N ⩽ 40 mg L−1) with low influent C/N, a pilot modified four step-feed process was applied for simultaneous biological nitrogen (N) and phosphorus (P) removal under different inflow distribution ratios. It was designed with a short hydraulic retention time of 8.7 h to raise influent load, and the optimal effluent performance of COD, NH4+-N, total nitrogen (TN) and total phosphorus (TP) were 33.05, 0.58, 9.26 and 0.46 mg L−1, respectively with inflow distribution ratio of 20:35:35:10%. More than 74% of carbon sources were utilized effectively for phosphorus release and denitrification. About 16.7% of TN was removed through simultaneous nitrification and denitrification in oxic zones. Moreover, the commendable sludge settling with a sludge loading of 0.04–0.1 kg COD/kg MLSS d, attributed to the higher mixed liquor suspended solids (MLSS) and the alternating anoxic/oxic operational mode. In addition, the pre-anoxic zone designed was beneficial for both nitrate reduction and anaerobic phosphorus release.