In the conventional A²/O process, efficient nitrogen removal usually comes with poor phosphorus removal, and vice versa, making it difficult to achieve excellent simultaneous nitrogen and phosphorus removal. Accordingly, optimized measures are proposed to enhance the overall treatment efficiency of the A²/O process.
Maintain the sludge return ratio at 60%~100% during design and operation
10%~20% of the returned sludge flows back to the anaerobic zone, and the rest is diverted to the anoxic zone. This reduces nitrate and dissolved oxygen entering the anaerobic zone to sustain an optimal anaerobic environment, while maintaining the required sludge concentration.
Feed raw wastewater into both the anaerobic and anoxic zones
Adjust the wastewater flow to the two zones via gates based on the organic carbon demand for biochemical nitrogen and phosphorus removal. Research shows that diverting one-third of raw wastewater to the anoxic zone helps achieve high-efficiency nitrogen and phosphorus removal.
Substitute submersible pumps for screw pumps for returned sludge lifting
Both returned sludge and raw wastewater adopt submerged inlet modes when entering the anaerobic and anoxic zones to minimize oxygen reaeration.
Set the power of submersible mixers in anaerobic and anoxic zones at 3~5 W/m³
Excessive mixer power will generate eddies and raise dissolved oxygen in the mixed liquor, impairing nitrogen and phosphorus removal. Insufficient power may lead to sludge sedimentation.
Eliminate the digestion tank; directly thicken and filter excess sludge into sludge cakes for fertilizer utilization
This prevents re-release and dissolution of phosphorus from high-phosphorus excess sludge during anaerobic digestion, which would otherwise degrade phosphorus removal performance.
Determine sludge retention time to balance nitrogen and phosphorus removal requirements
The optimal sludge retention time is generally 15~20 days.
Set the mixed liquor return ratio to balance high nitrogen removal efficiency and low operational cost
The recommended ratio is 300%~400%, which delivers a nitrogen removal rate above 70% with reasonable energy consumption.
The anoxic tank and aerobic tank can be designed in a concentric circle structure: the outer ring serves as the aerobic tank with brush aeration for plug flow, and the central circular area acts as the anoxic denitrification tank equipped with submersible mixers. Mixed liquor from the anaerobic zone flows into the aerobic tank through openings on the partition wall, while mixed liquor in the aerobic tank returns to the anoxic tank via adjustable rotating gates. This design eliminates pumping for mixed liquor recirculation, greatly cutting energy consumption while ensuring high nitrogen removal efficiency. This configuration has been successfully applied at Kunming No.2 Wastewater Treatment Plant in China.
Adopt reasonable design parameters for stable treatment performance and flexible operation
Sludge loading rate of anaerobic zone: > 0.10 kgBOD₅/(kgMLSS·d)
Influent S-P/S-BOD₅ ratio of anaerobic zone: < 0.06
C/N ratio of anoxic zone: > 6
Sludge loading rate of aerobic zone: < 0.10 kgBOD₅/(kgMLSS·d)
TKN/MLSS ratio of aerobic zone: < 0.15 kgTKN/(kgMLSS·d)
Control total hydraulic retention time and proportional distribution for each zone
The total hydraulic retention time is 6~8 hours. The optimal hydraulic retention time ratio of anaerobic : anoxic : aerobic zones is 1 : 1 : (3~4).
