A2O (Secondary Wastewater Treatment Process): An Introduction to a Core Wastewater Treatment Process

The A2O process, also called the AAO process, takes its name from the first letters of Anaerobic–Anoxic–Oxic (anaerobic-anoxic-aerobic). It is a commonly used secondary wastewater treatment process, applicable to secondary or tertiary treatment and reclaimed-water reuse, with good nitrogen and phosphorus removal.

This method was developed in the 1970s by American experts based on the AO nitrogen-removal process. Process flow:

1. Anaerobic reactor: raw wastewater and phosphorus-rich return sludge from the settling tank enter together; the main function is phosphorus release, while part of the organic matter is ammonified.

2. Anoxic reactor: primary function is denitrification; nitrate nitrogen is sent via internal recirculation from the aerobic reactor, with a large mixed-liquor recirculation volume, generally 2Q (Q is the raw wastewater flow).

3. Aerobic reactor—aeration tank: a multi-functional unit where BOD removal, nitrification, and phosphorus uptake all occur. Mixed liquor at flow 2Q recirculates from here back to the anoxic reactor.

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

1. This process can be called the simplest simultaneous nitrogen- and phosphorus-removal process in the system, with total hydraulic retention time shorter than other processes.

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

3. The sludge is phosphorus-rich, with high fertilizer value.

4. No chemicals are needed in operation; the two A stages use only gentle mixing, without increasing dissolved oxygen, giving low operating costs.

1. Phosphorus removal is hard to improve further; sludge growth has a certain limit and is hard to raise, especially when the P/BOD value is high.

2. Denitrification is also hard to improve further; internal recirculation is generally limited to 2Q and should not be too high.

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

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