Biological nitrogen removal in a step-feed CAST with real-time control treating municipal wastewater
The performance of a 18 L step-feed cyclic activated sludge technology (CAST) combined with real-time control treating real municipal wastewater was evaluated. The operation strategies employed pH and oxidation reduction potential (ORP) as on-line control parameters, which can control the durations of oxic and anoxic phases flexibly. The obtained results showed that the studied process had achieved advanced and enhanced nitrogen removal by several phases of consecutive oxic/anoxic periods. Total nitrogen in effluent was lower than 2 mg/L and the average TN removal efficiency was higher than 98%, while only requiring small amount of external carbon source. Unexpected characteristic points in pH and ORP profiles denoting the depletion of nitrate were also observed during the last anoxic phase. Denitrification rate was found to be more dependent on the system temperature compared to nitrification rate. Moreover, a stable and efficient phosphorus removal rate above 90% was achieved by using step-feed strategy which enabled the influent carbon source to be fully used and the favourable condition for phosphorus releasing to be created during the anoxic phases.
Keywords: advanced nitrogen removal, cyclic activated sludge technology, real-time control, step-feed