A2O Process (Anaerobic-Anoxic-Oxic): Technical Principles and Applications

The A2O process, also known as the AAO process, takes its name from the initials of "Anaerobic-Anoxic-Oxic" (anaerobic-anoxic-aerobic). It is a commonly used secondary wastewater treatment process suitable for secondary or tertiary treatment and reclaimed-water reuse, offering excellent nitrogen and phosphorus removal.

The process was developed in the 1970s by U.S. experts based on the AO nitrogen-removal process.

Process flow

1. Anaerobic reactor: raw sewage enters together with phosphorus-rich return sludge from the settling tank. Its main function is phosphorus release, while part of the organic matter undergoes ammonification.

2. Anoxic reactor: its primary function is denitrification. Nitrate nitrogen is supplied via internal recirculation from the aerobic reactor; the recirculated mixed-liquor flow is relatively large, generally 2Q (Q = raw sewage flow).

3. Aerobic reactor (aeration tank): this unit is multi-functional — BOD removal, nitrification and phosphorus uptake all occur here. Mixed liquor at a flow of 2Q is returned from here to the anoxic reactor.

4. Settling tank: its function is sludge-water separation; part of the sludge returns to the anaerobic reactor, and the supernatant is discharged as treated water. [1]

Advantages

1. This process is arguably the simplest simultaneous nitrogen- and phosphorus-removal process, with a total hydraulic retention time shorter than other processes of its kind.

2. Under alternating anaerobic (anoxic) and aerobic operation, filamentous bacteria cannot proliferate extensively, so sludge filamentous bulking is unlikely; the SVI value is generally below 100.

3. The sludge is high in phosphorus and has high fertilizer value.

4. No chemicals need to be dosed during operation; the two A-zones only require gentle mixing so as not to increase dissolved oxygen, keeping operating costs low.

Limitations

1. Further improvement of phosphorus removal is difficult; sludge growth is limited and hard to increase, especially when the P/BOD ratio is high.

2. Further improvement of nitrogen removal is also difficult; the internal recirculation is generally limited to 2Q and should not be too high.

3. The treated water entering the settling tank must retain a certain dissolved-oxygen concentration and a short retention time to prevent anaerobic conditions and phosphorus release by the sludge; however, the DO concentration should also not be too high, to avoid disturbing the anoxic reactor with the recirculated mixed liquor.

Reference: Drainage Engineering (4th ed., Vol. 2), ed. Zhang Zijie, China Architecture & Building Press, June 2000.