With global standards getting stricter, the challenge is how to remove nitrogen and phosphorus at the same time. Recently, the Inverted A²O process has gained wide attention to wastewater treatment.
This article explains its core features, compares it with the traditional A²O process, and gives advice for engineers.
🔹 Core Features of Inverted A²O
The Inverted A²O is an upgrade to the classic A²O. 👉 Its key change: the anoxic tank comes first. Flow path: Anoxic → Anaerobic → Aerobic.
Main advantages:
- ✅ Faster denitrification: Influent enters the anoxic tank first, using organic matter to remove nitrogen.
- ✅ Better anaerobic condition: After oxygen and nitrate are consumed in anoxic stage, the anaerobic tank is more stable, helping PAOs release phosphorus.
- ✅ Higher phosphorus removal: After release, phosphorus is quickly absorbed in the aerobic tank.
- ✅ Efficient return flow: Mixed liquor and sludge return to the anoxic tank, avoiding oxygen shock in the anaerobic stage.
🔹 Inverted A²O vs. A²O: Four Key Differences
- ⚙️ Process Flow
- Inverted A²O: Anoxic → Anaerobic → Aerobic
- Traditional A²O: Anaerobic → Anoxic → Aerobic
- 💧 Nitrogen Removal
- Inverted A²O starts with denitrification first.
- More efficient in high ammonia wastewater or when carbon source is limited.
- 🧬 Phosphorus Removal
- Inverted A²O makes use of oxygen consumption order.
- This gives PAOs higher activity.
- Usually achieves better phosphorus removal than A²O.
- 🔄 Return Flow Strategy
- Inverted A²O: Mixed return to anoxic tank, avoiding DO shock.
- Traditional A²O: Separate inner and outer return. Poor control may bring DO into anaerobic stage, lowering efficiency.
🔹 Application Suggestions
The Inverted A²O is suitable for:
- 🏭 Industrial wastewater with high nitrogen standards and low carbon source.
- 🏙️ Municipal wastewater plants with high phosphorus removal demand (expansion projects).
- ⚠️ Old systems where return flow control is difficult.
