A2O Process of Sewage Treatment Technology

A2O Process of Sewage Treatment Technology

Summary

A2/O process is the abbreviation of Anaerobic Anoxic Oxic, which is the abbreviation of biological nitrogen and phosphorus removal process. The treatment efficiency of this process can generally reach: BOD5 and SS are 90%~95%, total nitrogen is over 70%, and phosphorus is about 90%. It is generally suitable for large and medium-sized urban sewage plants that require nitrogen and phosphorus removal

A2O Process of Sewage Treatment Technology
A2O Process of Sewage Treatment Technology
A2O Process of Sewage Treatment Technology
A2/O process is the abbreviation of Anaerobic Anoxic Oxic, which is the abbreviation of biological nitrogen and phosphorus removal process. The treatment efficiency of this process can generally reach: BOD5 and SS are 90%~95%, total nitrogen is over 70%, and phosphorus is about 90%. It is generally suitable for large and medium-sized urban sewage plants that require nitrogen and phosphorus removal
Process Flow
The A2/O process was developed by American experts in the 1970s based on the anaerobic aerobic phosphorus process (A~/O), which also has the function of nitrogen and phosphorus removal.
Process Flow
This process adds an anoxic tank in the aerobic phosphorus process (A/O), and a portion of the mixed liquid flowing out of the aerobic tank is refluxed to the front end of the anoxic tank. This process also has the purpose of nitrogen and phosphorus removal.
Process Principle
1. The first section of the anaerobic tank is filled with raw sewage and phosphorus containing sludge that flows back from the secondary sedimentation tank. The main function of this tank is to release phosphorus, increase the concentration of P in the sewage, absorb dissolved organic matter by microbial cells, and reduce the concentration of BOD5 in the sewage; In addition, a portion of NH3-N is removed due to cellular synthesis, resulting in a decrease in NH3-N concentration in the wastewater, but the NO3-N content remains unchanged.

2. In the anoxic tank, denitrifying bacteria use organic matter in wastewater as a carbon source to introduce a large amount of NO3-N and NO2-N into the reflux mixture, which are reduced to N2 and released into the air. Therefore, the concentration of BOD5 decreases significantly, the concentration of NO3-N decreases significantly, and the change in phosphorus is minimal.

3. In the aerobic tank, organic matter is biodegraded by microorganisms and continues to decrease; Organic nitrogen is ammoniated and then nitrified, resulting in a significant decrease in NH3-N concentration. However, as the nitrification process increases the concentration of NO3-N, P also decreases at a faster rate with excessive uptake by polyphosphate accumulating bacteria.

The A2/O process can simultaneously perform functions such as removal of organic matter, nitrification denitrification, and removal of excess phosphorus uptake. The prerequisite for denitrification is complete nitrification of NO3-N. The aerobic tank can complete this function, while the anoxic tank completes the denitrification function. The anaerobic tank and aerobic tank are combined to achieve phosphorus removal function.
Process characteristics
(1)The organic combination of anaerobic, anaerobic, and aerobic environmental conditions and different types of microbial communities can simultaneously remove organic matter, nitrogen, and phosphorus.

(2)In the process of simultaneous deoxygenation, phosphorus removal, and organic matter removal, this process flow is the simplest, and the total hydraulic retention time is also less than other similar processes.

(3)Under the alternating operation of anaerobic anaerobic aerobic, filamentous bacteria do not reproduce in large numbers, with an SVI of generally 100 and no sludge swelling occurring.

(4) The phosphorus content in sludge is high, generally above 2.5%. (5) The denitrification effect is affected by the reflux ratio of the mixed solution, while the phosphorus removal effect is affected by the presence of dissolved oxygen (DO) and nitrate oxygen in the refluxed sludge. Therefore, the denitrification and phosphorus removal efficiency cannot be very high.
Problems Exist
When the A2/O process has good denitrification effect, the phosphorus removal effect is poor, and vice versa, it is difficult to achieve good deoxygenation and phosphorus removal effect at the same time.
The reason is that all the returned sludge in this process enters the anaerobic section. In order to maintain a low sludge load, a high return ratio (usually between 40% and 100%) is required to ensure good nitrification in the system. However, the returned sludge also brings a large amount of nitrate into the anaerobic tank, and the conditions for phosphorus release by polyphosphate accumulating bacteria are anaerobic, with the presence of soluble BOD5.
But when there is a large amount of nitrate in the anaerobic section, denitrifying bacteria will use organic matter as a carbon source for denitrification, and only begin anaerobic release of phosphorus after complete denitrification. This greatly reduces the effective volume of phosphorus release in the anaerobic section, resulting in poor phosphorus removal efficiency and better denitrification efficiency. On the contrary, if the nitrification in the aerobic stage is not good, the nitrate entering the anaerobic stage with the reflux sludge decreases, improving the anaerobic environment of the anaerobic stage and allowing phosphorus to be fully released anaerobically.
Therefore, the phosphorus removal effect is good, but due to incomplete nitrification, the denitrification effect is not good. So the A2/O process cannot achieve good results in both nitrogen and phosphorus removal simultaneously.
Improvement measures
To address the issues with the A2/O process mentioned above, the following improvements should be made to the design and operation of the process:
(1) Divide the reflux sludge into two points and reduce the amount of reflux sludge added to the anaerobic section, thereby reducing the amount of nitrate and dissolved oxygen entering the anaerobic section. Under the condition of ensuring a total sludge return ratio of 60% to 100%, the return sludge ratio to the anaerobic section is generally 10%, which can meet the demand for phosphorus, while the remaining return sludge is returned to the anoxic section to ensure the demand for nitrogen.

(2) The residual sludge in the A2/O process system has a high phosphorus content, and phosphorus will be re released and dissolved during its digestion process. Meanwhile, due to the good sedimentation performance of the remaining sludge, the digestion tank can be eliminated and directly used as fertilizer after concentration and pressure filtration.

(3) In the aerobic nitrification stage, the sludge loading rate should be less than 0.18kgBOD5/(kgMLSS · d), while in the anaerobic phosphorus removal stage, the sludge loading rate should be above 0.10kg BOD5/(kgMLSS · d).
Conclusions
The A2/O process not only removes organic carbon pollution (BOD pollution) from wastewater, but also effectively removes nitrogen and phosphorus pollution from wastewater, opening up new avenues for wastewater reuse and resource utilization. Compared with the ordinary reflux sludge method that undergoes secondary treatment followed by tertiary physicochemical treatment, it not only has lower investment and operating costs, but also does not have a large amount of difficult to treat chemical sludge, which has good environmental and economic benefits.