A2O Process

The A2O (Anaerobic-Anoxic-Oxic) process is an advanced biological nutrient removal (BNR) system achieving simultaneous removal of organics (BOD/COD >90%), nitrogen (total nitrogen removal 70-85%), and phosphorus (total phosphorus removal 80-95%) through carefully sequenced biological environments. With typical zone volume ratios of 1:1:3 (anaerobic:anoxic:oxic) and total hydraulic retention times of 12-18 hours, the process meets stringent discharge limits (TN <10 mg/L, TP <0.5 mg/L) with minimal chemical dosing.

Wastewater flows through three zones maintaining distinct redox conditions: an anaerobic zone (DO <0.2 mg/L, 1-2 hour HRT) where phosphorus-accumulating organisms (PAOs) release phosphorus and store volatile fatty acids; an anoxic zone (DO <0.5 mg/L, 2-4 hour HRT) where denitrifying bacteria convert nitrate to nitrogen gas (removing 60-80% of influent TN); and an oxic zone (DO 1.5-2.5 mg/L, 6-12 hour HRT) where carbonaceous BOD is oxidized, ammonia is nitrified to nitrate (>85% nitrification efficiency), and PAOs take up excess phosphorus achieving net removal. Internal recirculation ratios of 100-400% (nitrate recycle) and 50-100% (mixed liquor recycle) optimize nutrient removal efficiency.

The A2O process is one of the most widely adopted BNR configurations globally, treating flows from 1 MLD to 500 MLD+ capacities with volumetric loading rates of 0.2-0.6 kg BOD/m³/day. Operating at MLSS concentrations of 3,000-5,000 mg/L with SRT 15-25 days, the system achieves stable performance with energy consumption of 0.7-1.3 kWh/m³. Chemical consumption for supplemental phosphorus removal (if needed) is reduced by 60-80% compared to chemical precipitation alone, with sludge production of 0.4-0.6 kg TSS/kg BOD removed being 30-40% lower than conventional activated sludge due to biological nutrient incorporation into biomass.