Multi-stage anoxic/oxic sequencing batch reactor realizes shortcut nitrogen removal for anaerobically co-digested liquor of municipal sludge and urban organic wastes

Nitrogen removal from the combined anaerobic digestion dehydration liquor (CADDL) of municipal sludge and urban organic wastes is challenging due to high ammonium concentrations, low C/N ratio, and poor biodegradability. This study proposes a multi-stage anoxic/oxic (A/O) sequencing batch reactor with step feeding to realize partial nitrification and denitrification for shortcut nitrogen removal from the CADDL. We investigated the effects of external carbon source (acetate), dissolved oxygen (DO), A/O duration ratio, and A/O stage number on biological nitrogen removal. Moreover, we assessed the microbial community structure and nitrogen removal pathway. The results showed that the C/N consumption ratio for nitrite reduction to dinitrogen was 3.0 mg COD/mg N, and denitrifying bacteria yielded about 0.43. The optimal dosage of acetate was 2.2 mg COD/mg N. High DO concentration (1.5∼3.0 mg/L) in the aerobic stage improved the ammonia-oxidizing bacteria activity and nitrogen removal rather than worsening the nitritation. A high A/O duration ratio (50 min/60 min) was conducive to complete denitrification of nitrite. The three-stage A/O had an excellent nitrogen removal performance. Under optimal conditions, the nitrite accumulation ratio of nitritation and the total inorganic nitrogen removal reached 100% and 90.1%, respectively. The dominant ammonia-oxidizing bacteria was the genus Nitrosomonas (0.76% abundance), and the dominant denitrifying bacteria was Thauera (0.24% abundance). The nitrite-oxidizing bacteria were not detected, confirming that the biological nitrogen removal pathway was partial nitrification and denitrification. These findings provide a feasible option for the low-carbon nitrogen removal treatment for the CADDL of municipal sludge and urban organic wastes.