The effects of aerobic/anoxic period sequence on aerobic granulation and COD/N treatment efficiency

The effects of period sequence (anoxic-aerobic and aerobic-anoxic) on aerobic granulation from suspended seed sludge, and COD, N removal efficiencies were investigated in two sequencing batch reactors. More stable granules with greater sizes (1.8-3.5 mm) were developed in R1 (anoxic-aerobic sequence). Yet, no significant difference was observed between the reactors in terms of removal efficiencies. Under optimum operational conditions, 92-95% COD, 89-90% TAN and 38-46% total nitrogen removal efficiencies were achieved. The anoxic-aerobic period sequence (R1) resulted in almost complete denitrification during anoxic periods while aerobic-anoxic sequence (R2) led to nitrate accumulation due to limited-carbon source and further granule disintegration. NH3-N concentration of 15-28 mg/L was found to inhibit COD removal up to 30%. This study also revealed the inhibitory sulfide production during anoxic periods. Sulfate concentration of 52.6-70.2 mg/L was found to promote sulfate reduction and sulfide generation (0.24-0.62 mg/L) which, together with free-ammonia, inhibited TAN oxidation by 10-50%.