The two-stage A/O process is an efficient sewage treatment technology, which further strengthens the denitrification effect by adding a two-stage anoxic reactor and a rapid aerobic reactor on the basis of the traditional A/O process, and is especially suitable for the treatment of industrial wastewater containing high concentration of ammonia nitrogen. The following is a detailed introduction to the two-stage A/O process:
The principle of the two-stage A/O process
一级缺氧阶段（Anoxic Phase）：
Under anoxic conditions, heterotrophic bacteria hydrolyze suspended pollutants and soluble organic matter in sewage into organic acids, decompose macromolecular organic matter into small molecule organic matter, and convert insoluble organic matter into soluble organic matter.
一级好氧阶段（Oxic Phase）：
Under aerobic conditions, the nitrification of autotrophic bacteria oxidizes NH3-N (NH4+) to NO3-, which is returned to the anoxic pond through reflux control, and the denitrification of heterotrophic bacteria reduces NO3- to molecular nitrogen (N2).
二级缺氧阶段（Anoxic Phase）：
Under the secondary hypoxic condition, heterotrophic bacteria hydrolyze the suspended pollutants and soluble organic matter in the sewage into organic acids again, so that the macromolecular organic matter is decomposed into small molecular organic matter, and the insoluble organic matter is converted into soluble organic matter.
二级好氧阶段（Oxic Phase）：
Under secondary aerobic conditions, the nitrification of autotrophic bacteria oxidizes NH3-N (NH4+) to NO3-, which is returned to the secondary anoxic pool through reflux control, and the denitrification of heterotrophic bacteria reduces NO3- to molecular nitrogen (N2).
Advantages and disadvantages of the two-stage A/O process
Merit:
High treatment efficiency: the two-stage A/O process can effectively remove organic matter and ammonia nitrogen in wastewater, and the removal rate of ammonia nitrogen and total nitrogen can exceed 95%;
High resistance to load impact: the process can still operate stably even if the water quality fluctuates greatly or the pollutant concentration is high;
Low investment and simple maintenance management: Compared to other advanced treatment processes, the two-stage A/O process has low investment costs and simple maintenance management.
Shortcoming:
Low degradation rate of refractory substances: Since there is no independent sludge reflux system, the degradation rate of refractory substances is low.
The denitrification effect is not easy to control: in order to improve the denitrification efficiency, it is necessary to increase the internal circulation ratio, which increases the operating cost.
Application examples
The two-stage A/O process has been successfully applied to the treatment of a variety of industrial wastewater, such as slaughtering wastewater, fine chemical wastewater, etc. Taking slaughtering wastewater as an example, after being treated by the two-stage A/O process, the COD and ammonia nitrogen concentrations of the effluent were lower than 60mg/L and 9mg/L, respectively, and the discharge standards were stably.
At present, the two-stage A/O process is widely used in industrial wastewater treatment, especially those wastewater containing high concentrations of ammonia nitrogen and organic matter, such as chemical wastewater, pharmaceutical wastewater, etc. Through the treatment of the two-stage A/O process, the nitrogen content in the wastewater can be significantly reduced, meeting the discharge standards and reducing the environmental impact.
In summary, the two-stage A/O process is an ideal choice for small and medium-scale industrial wastewater treatment due to its efficient treatment capacity, stable operating performance, and low investment and operating costs.