A step-feed cyclic activated sludge technology (CAST) with a working volume of 72 L treating real municipal wastewater was operated to examine the effect of varying ratios of influent COD/TN and COD/P on the nutrient removal. With the increased COD/P and COD/TN, the phosphorus and nitrogen removals exhibited an upward trend. The TN removals had a positive linear correlation with the phosphorus removal efficiencies, mainly because the presence of nitrate in the anaerobic zone negatively affected the phosphorus release thus the nitrogen removal process took priority over the phosphorus removal process to utilize the limited carbon source in the influent in step-feed CAST where simultaneous removals of nitrogen and phosphorus were achieved. By employing the effective step-feed strategy with alternating anoxic/oxic operation, efficient phosphorus and nitrogen removals of 95.8 and 89.3% were obtained with lower influent COD/P and COD/TN ratios of 61.9 and 5.2, respectively. It was also found that lower temperature, e.g. 13 ∼ 16°C, did not deteriorate the phosphorus removal, though the nitrogen removal decreased significantly due to incomplete nitrification. As the temperature increased further, TN removal efficiency increased gradually and nitrogen removal via nitrite pathway was successfully achieved with average nitrite accumulation rate above 90% in the system.

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