A bench-scale study was undertaken to examine the effects of easily biodegradable organic carbon substrate on denitrification reaction and overall nitrogen removal from domestic wastewater under a modified sequencing batch reactor (SBR) system. The operation strategy of the SBR consisted of 0.75 h FILL, 8 h REACT separated into 4 h aerobic, 3 h anoxic and 1 h aerobic stages, 1.5 h SETTLE, 1 h DRAW and 0.75 h IDLE. Methanol, sodium acetate and sodium propionate, at the concentrations equivalent to theoretical COD values of 50, 100 and 150 mg O2 1−1 were used as the external carbon sources and added to the reactors prior to the anoxic stage. The study reveals that 4 h aerobic stage was sufficient to nitrify more than 98% NH4+-N and carbon addition caused slightly more nitrification than the control. Addition of sodium propionate at a low concentration (50 mg O2 1−1) significantly enhanced the denitrification process, the nitrate content in this reactor dropped to 3 mg 1−1 (89% reduction) at the end of the anoxic stage. Among the three substrate added at low dose, sodium propionate was the most effective carbon source, followed by acetate and the least effective one was methanol. When the carbon substrate were added at the doses of 100 and 150 mg O2 1−1, the denitrification rates of the acetate reactors recorded at the first hour of the anoxic stage were similar to those of the propionate's and significantly higher than the methanol reactors. When high dose (150 mg O2 1−1) of acetate or propionate was used, 95% reduction in wastewater NOx-N was found after 1 h anoxic stage while 3 h anoxic stage was required when the carbon dose was at 100 mg O2 1−1, indicating that addition of external carbon substrate at large quantity could shorten the denitrification time. However, the final effluent discharged from reactors treated with high dose of acetate and propionate contained more than 20 mg 1−1 BOD5 which might cause a contamination problem. Therefore, addition of sodium acetate or propionate at the concentration equivalent to theoretical COD values of 100 mg O2 1−1 appeared to be the most economical and reliable option.

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