✅ Anaerobic Phosphorus Release: COD Too Low
In the anaerobic stage, polyphosphate-accumulating organisms (PAOs) use COD as a carbon source. They absorb small organic molecules like volatile fatty acids (VFAs) to produce PHB and release phosphorus.
⚠️ If influent COD concentration is too low, substrate is not enough. This limits PHB production and weakens phosphorus release. Later in the aerobic stage, PAOs cannot take up phosphorus effectively. This makes it hard to meet phosphorus discharge standards.
💧 Denitrification: COD Shortage Leaves Nitrate Residue
The denitrification process depends on COD as a carbon source. Without enough organic matter, microorganisms lack electron donors.
🔹 As a result, nitrate cannot be fully reduced. This lowers nitrogen removal efficiency and may cause excess nitrogen in the effluent.
👉 This issue is common in industrial wastewater such as textile and electroplating. Extra carbon sources are often added to solve the problem.
⚡ Aerobic Stage: COD Too High or Too Low Brings Risks
- COD too high:
- Shock load on the system
- Overgrowth of heterotrophic bacteria
- Inhibition of nitrifying bacteria → poor ammonia removal
- High oxygen consumption → anoxic zones → foam and sludge bulking
- COD too low:
- Low load operation → microbes lack nutrients
- Sludge aging and efficiency drop
- Long-term → filamentous bacteria dominate → poor sludge settling
🔑 Keep the Right C:N:P Balance
The COD level directly decides the carbon supply in wastewater treatment. For stable operation, influent COD should stay in the proper range. A balanced ratio of C:N:P = 100:5:1 is usually recommended. ✅ This improves nitrogen and phosphorus removal and gives the system stronger shock resistance.
