In wastewater treatment, a sudden COD shock is one of the biggest risks. ⚠️ If not controlled, it may lead to ammonia nitrogen exceeding limits, poor effluent quality, and even system collapse.
This guide explains how high COD affects nitrification and provides five emergency recovery measures.
🔹 Why High COD Affects Nitrification
When COD suddenly rises (e.g. high influent load, excess carbon dosing), heterotrophic bacteria grow fast. 👉 They compete with nitrifiers for oxygen and nutrients, suppressing nitrification. Result: low nitrification rate → high ammonia nitrogen → risk of collapse.
🔹 Five Key Measures to Restore Nitrification
- Pause or Reduce Influential Load
- Cut or stop influent flow.
- Reduce carbon dosing.
- Prevent further organic accumulation.
- 🧪 Increase Sludge and Add Nitrifiers
- Use short-term aeration (batch mode).
- Add fresh sludge or special nitrifying bacteria.
- Boost nitrifier population quickly.
- 💨 Raise Aeration and DO
- COD shock causes DO to drop fast.
- Increase aeration to maintain >2 mg/L DO.
- Ensure nitrifiers have enough oxygen.
- ⚖️ Adjust pH and Alkalinity
- Low pH or poor alkalinity inhibits nitrification.
- Add alkali (NaHCO₃, lime).
- Keep pH 7.5–8.5.
- 🔄 Batch Aeration + Slow Influent Recovery
- Apply 1–2 days of intermittent aeration (batch).
- Resume influences step by step.
- Avoid full-load recovery at once.
🔹 Real Case Study
At a food processing wastewater plant, influent COD spiked.
- Result: COD and ammonia exceeded discharge limits.
- Recovery: Batch aeration + alkali dosing + nitrifier addition.
- Outcome: System stabilized within 72 hours, effluent back to standard.
