Anaerobic–Anoxic–Oxic Process for Wastewater Treatment
Designing an A2O (Anaerobic–Anoxic–Oxic) system is a critical step in achieving efficient biological nutrient removal (BNR), especially for municipal and industrial wastewater treatment. The A2O process is particularly effective for removing both nitrogen and phosphorus from wastewater, making it a popular choice for modern treatment facilities.
The A2O process consists of three main biological zones:
Anaerobic Zone
No dissolved oxygen or nitrates are present. In this zone, phosphorus-accumulating organisms (PAOs) release phosphorus.
Anoxic Zone
Oxygen is absent, but nitrates are available. Denitrifying bacteria reduce nitrates to nitrogen gas.
Oxic (Aerobic) Zone
Oxygen is present, allowing for nitrification (conversion of ammonia to nitrate) and phosphorus uptake by PAOs.
Typical Flow:
Influent → Anaerobic → Anoxic → Oxic → Clarifier → Effluent
| Component | Design Range |
|---|---|
| Flow Rate (Q) | Depends on population/load |
| Anaerobic HRT | 0.5 – 1.5 hours |
| Anoxic HRT | 1 – 2 hours |
| Oxic HRT | 4 – 8 hours |
| MLSS | 3000 – 5000 mg/L |
| SRT | 10 – 20 days |
| DO in Oxic Zone | 2.0 – 3.0 mg/L |
| Internal Recycle | 200% – 400% of influent flow |
| RAS (Return Activated Sludge) | 50% – 100% of influent |
Note: HRT = Hydraulic Retention Time; MLSS = Mixed Liquor Suspended Solids; SRT = Solids Retention Time
To build an A2O system, you need:
Anaerobic tank with mixer (no aeration)
Anoxic tank with internal recirculation pump
Aerobic tank with fine bubble diffusers
Clarifier for solid-liquid separation
Internal recycle and RAS pumps
SCADA or manual control systems for DO, flow, and return rates
Optional: Chemical dosing systems (e.g. for phosphorus polishing with alum or ferric)
Ensure true anaerobic conditions by preventing nitrate return to the anaerobic zone.
Maintain a stable internal recycle rate to enhance denitrification.
Provide adequate mixing in anaerobic and anoxic tanks to keep solids suspended.
Control DO levels precisely in the aerobic zone to prevent oxygen leakage into anoxic areas.
Use baffle walls or compartments to prevent short-circuiting.
Municipal WWTPs: Efficient for cities seeking to meet TN and TP discharge standards.
Industrial Wastewater: Especially useful for food & beverage, dairy, or pharmaceutical wastewater.
Retrofit Projects: A2O can upgrade existing conventional systems for BNR compliance.
Simultaneous nitrogen and phosphorus removal
Simple process flow with proven reliability
Lower chemical consumption compared to chemical phosphorus removal
Scalable to small and large plants alike
For a 500 m³/day treatment plant:
Anaerobic Tank: 30 m³
Anoxic Tank: 60 m³
Aerobic Tank: 160 m³
Clarifier Surface Area: ~50 m²
Internal Recycle Pump: 1000 m³/day
RAS Pump: 250 m³/day
(Exact volumes and dimensions depend on influent characteristics and discharge limits.)
Designing an A2O system requires a solid understanding of microbial processes, hydraulic balance, and reactor design. With proper design and control, A2O systems can achieve excellent nutrient removal with low operational cost and high process stability.
Looking to design an A2O system?
Leave us a message via live chat or email with your flow rate and effluent targets — we’ll help with layout, calculations, and equipment selection
