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%.