Improve Phosphorus Removal with Inverted A²O

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

1. ⚙️ Process Flow

• Inverted A²O: Anoxic → Anaerobic → Aerobic
• Traditional A²O: Anaerobic → Anoxic → Aerobic

 

2. 💧 Nitrogen Removal

• Inverted A²O starts with denitrification first.
• More efficient in high ammonia wastewater or when carbon source is limited.

 

3. 🧬 Phosphorus Removal

• Inverted A²O makes use of oxygen consumption order.
• This gives PAOs higher activity.
• Usually achieves better phosphorus removal than A²O.

 

4. 🔄 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.