Controlling factors for simultaneous nitrification and denitrification in a two-stage intermittent aeration process treating domestic sewage

This bench-scale research investigated the controlling factors for simultaneous nitrification and denitrification (SND) in a 2-stage, intermittent aeration (IA) process, designed for nitrogen and phosphorus removal. The 2-stage process consisted of an anaerobic zone followed by an oxidation–reduction potential (ORP) controlled, intermittently aerated, completely mixed (IACM), tank. The three independent variables examined were the average ORP level, organic substrate (acetate and methanol) dosage and aeration cycle. The sewage used was completely of residential origin. Nitrogen balance clearly indicated that the nitrogen loss, due to SND in the aeration tank, contributed 10% to 50% of the influent TKN to the overall nitrogen removal. Significant differences in both nitrification and denitrification in the IACM tank were observed, when different average ORP levels were applied to the aeration control; this proved that absolute ORP can be used as a real-time control parameter for SND. Under low DO and intermittent aeration conditions, acetate and methanol additions improved nitrification over the entire dosage range and denitrification at relatively low dosages. Finally, a longer aeration cycle, with a zero-DO period, appeared to favor sequential nitrification and denitrification (SQND), not SND.