High-rate nitrogen removal and its behavior of granular sequence batch reactor under step-feed operational strategy

Alternating anoxic/oxic (A/O) combined with the step-feed granular sequence batch reactor (step-feed SBR) was operated in laboratory scale to investigate nitrogen removal. The results showed that when the total inorganic nitrogen (TIN) and chemical oxygen demand (COD) levels were 55 and 320 mg/L in the influent, the TIN removal efficiencies were 89.7-92.4% in the step-feed mode and 48.1-59.5% in the conventional alternating A/O single-feed mode within a 360 min cycle. The pH and dissolved oxygen (DO) were used to optimize the process of denitrification and nitrification in the step-feed mode. The optimized operational condition was achieved by shortening the cycle time to 207 min, resulting in a nitrogen removal rate of 0.27 kg N/m3 d, which was much higher than those achieved using activated sludge systems. The dominant community in the aerobic granules was coccus-like bacteria, and filamentous bacteria were hardly found. Granules were well maintained throughout the 90 days of continuous step-feed operation.