Anaerobic-Anoxic-Oxic Biological Nutrient Removal: HJ 576-2010 Design Values and the Carbon Constraint

1. Process Definition and Zone Boundaries

The A2/O process, called the AAO process in the Chinese code, removes organic matter, nitrogen and phosphorus through various combinations of anaerobic, anoxic and oxic zones together with different sludge return arrangements. HJ 576-2010 defines the three zones by dissolved oxygen:

• Anaerobic zone: non-aerated, dissolved oxygen generally below 0.2 mg/L; its function is phosphorus release.
• Anoxic zone: non-aerated, dissolved oxygen generally 0.2-0.5 mg/L; its function is denitrification.
• Oxic zone: aerated, dissolved oxygen generally not below 2 mg/L; its function is organic matter degradation, ammonia nitrification and luxury phosphorus uptake.

2. Design Values from Table 5 of HJ 576-2010

For municipal wastewater or industrial wastewater of similar character, the main design parameters are as follows. Where industrial wastewater differs substantially, parameters must be fixed by testing or by reference to comparable projects.

• BOD5 sludge loading: 0.07-0.21 kg BOD5 per kg MLVSS per day, or 0.05-0.15 kg BOD5 per kg MLSS per day.
• Average MLSS in the reactor: 2.0-4.5 g/L.
• Average MLVSS: 1.4-3.2 g/L.
• MLVSS to MLSS ratio: 0.65-0.7 with a primary clarifier; 0.5-0.65 without one.
• Design sludge retention time: 10-25 days.
• Sludge yield coefficient Y (VSS/BOD5): 0.3-0.6 kg/kg with a primary clarifier; 0.5-0.8 kg/kg without one.
• Hydraulic retention time: anaerobic zone 1-2 h; anoxic zone 2-4 h; oxic zone 8-12 h; total 11-18 h.
• Sludge return ratio R: 40%-100%.
• Mixed liquor (nitrate) recycle ratio Ri: 100%-400%.
• Oxygen requirement: 1.1-1.8 kg O2 per kg BOD5.

3. Removal Efficiencies Expected by the Code

• BOD5: 85%-95%.
• Ammonia nitrogen: 80%-90%.
• Total nitrogen: 55%-80%.
• Total phosphorus: 60%-80%.

These ceilings cannot be reached simultaneously. Nitrogen removal demands anoxic volume and nitrate recycle, while phosphorus removal demands a strictly anaerobic release environment and short sludge age for phosphorus-rich waste sludge; the two compete for carbon and for sludge age.

4. Recycle Splitting in Practice

• Total sludge return is generally 60%-100%, of which 10%-20% goes to the anaerobic zone and the remainder to the anoxic zone, so that less nitrate and dissolved oxygen enter the anaerobic zone.
• Raw wastewater should enter both the anaerobic and anoxic zones through adjustable gates; studies suggest designing for roughly one third of the flow to the anoxic zone to satisfy the carbon demand of both processes.
• Submersible mixers in the anaerobic and anoxic zones are generally sized at 3-5 W per cubic metre. Too much power creates vortices that raise dissolved oxygen; too little allows sludge to settle.
• A mixed liquor recycle ratio of 300%-400% typically delivers nitrogen removal above 70% without excessive operating cost.
• Sludge age of 15-20 days balances nitrification against phosphorus removal.
• Sludge digestion is best avoided: phosphorus in the phosphorus-rich waste sludge is released again during digestion and degrades effluent phosphorus performance.

5. Nitrogen Removal Collapse at Low C/N and the Fixes

Results from a 52.5 litre A2/O pilot reported in the Journal of Chemical Industry and Engineering are directly applicable:

• At an anaerobic/anoxic/oxic volume ratio of 1/1/2, HRT 8 h, sludge return 70% and internal recycle 300%, treating domestic wastewater with a C/N of 7.89, total nitrogen and soluble orthophosphate removal reached 85.4% and 93.3%, with anoxic phosphorus uptake accounting for 25.3% of total uptake.
• Under the same operating conditions but with a C/N of 4.20, phosphorus removal was almost unaffected while total nitrogen removal fell to 62.2% and average effluent total nitrogen exceeded 20 mg/L.
• Holding the anaerobic volume constant and enlarging the anoxic zone to an anoxic/oxic ratio of 5/8 raised total nitrogen removal to 70.7%, with anoxic uptake rising to 55.2% of total phosphorus uptake.
• An internal recycle ratio of 250% maximised denitrifying phosphorus removal and lifted total nitrogen removal to 77.3%.

The lesson: when carbon is scarce, enlarging the anoxic zone and tuning the internal recycle ratio to promote denitrifying phosphorus removal works better than simply raising the recycle ratio.

6. Design Checkout List

• Back-calculate anoxic volume and internal recycle from the total nitrogen target first, then verify that the anaerobic zone can hold dissolved oxygen below 0.2 mg/L.
• Re-check sludge age at the worst-case water temperature: nitrification rate falls in cold water, so the required sludge age rises.
• Verify margins against the instantaneous Class 1A limits in Table 4 of the GB 18918-2002 amendment (COD 75, total nitrogen 20, ammonia nitrogen 10 (15), total phosphorus 1 mg/L), not against daily means alone.
• Where phosphorus removal is unstable, provide chemical phosphorus removal as a fallback, as clause 6.1.4 of the specification allows.

References

• HJ 576-2010, Technical Specifications for Anaerobic-Anoxic-Oxic Activated Sludge Process (MEE release copy, in Chinese)
• Journal of Chemical Industry and Engineering: A2O treatment of low C/N domestic wastewater (in Chinese)
• XJJ 156-2022, Design Standard for Municipal Wastewater Treatment Plants (AAO parameters, in Chinese)
• Amendment to GB 18918-2002 (in Chinese)