1. AO (Anoxic-Oxic) process: Also known as the anaerobic-aerobic process, A (Anaerobic) is the anaerobic stage, used for ammonia and phosphorus removal; O (Oxic) is the aerobic stage, used for removing organic matter from the water.
Its advantage is that in addition to degrading organic pollutants, it also has a certain ammonia and phosphorus removal function. It uses anaerobic hydrolysis technology as a pretreatment for activated sludge, so the AO method is an improved activated sludge process.
A stage DO: not greater than 0.2 mg/L
O stage DO: 2~4 mg/L
Decomposed into: small molecule organic matter
2. Basic principle
The AO process connects the anoxic stage and the aerobic stage in series. The DO in the A stage is not greater than 0.2 mg/L, and the DO in the O stage is 2~4 mg/L. In the anoxic zone, heterotrophic bacteria hydrolyze suspended pollutants such as starch, fiber, and carbohydrates, as well as soluble organic matter, in wastewater into organic acids. This breaks down large organic molecules into smaller ones, and insoluble organic matter into soluble organic matter. When these products of anoxic hydrolysis enter the aerobic tank for aerobic treatment, the biodegradability and oxygen efficiency of the wastewater are improved. In the anoxic zone, heterotrophic bacteria ammonify pollutants such as proteins and fats (by releasing ammonia (NH3, NH4+) through nitrogen (N in organic chains or amino groups in amino acids). Under sufficient oxygen supply, nitrification by autotrophic bacteria oxidizes NH3-N (NH4+) to NO3-, which is then returned to tank A via reflux control. Under anoxic conditions, denitrification by heterotrophic bacteria reduces NO3- to molecular nitrogen (N3), completing the C, N, and O cycle in the ecosystem and achieving harmless wastewater treatment.
3. Characteristics of the A/O Nitrogen Removal Process
(a) Simple process, no need for external carbon source or post-aeration tank, using raw wastewater as the carbon source, resulting in lower construction and operating costs.
(b) Denitrification precedes nitrification, with internal circulation, using organic substrates in the raw wastewater as the carbon source, resulting in good efficiency and thorough denitrification reaction.
(c) The aeration tank is located after the aeration tank, further removing denitrification residues and improving the quality of treated water. Strong aeration is used in the first stage of the O stage, while the aeration volume is reduced in the later stage, lowering the DO content in the internal circulating liquid to ensure anoxic conditions in the A stage.
Stirring in the A stage only serves to suspend the sludge, avoiding an increase in DO.
4. Problems with the A/O Process
(a) Due to the lack of an independent sludge return system, sludge with unique functions cannot be cultivated, resulting in a low degradation rate of recalcitrant substances.
(b) To improve nitrogen removal efficiency, the internal circulation ratio must be increased, thus increasing operating costs. In addition, the internal circulating liquid comes from the aeration tank and contains a certain amount of DO, making it difficult for section A to maintain an ideal anoxic state, which affects the denitrification effect and makes it difficult to reach 90% of the ammonia removal rate.
