ETP outlet COD high: troubleshooting the biological stage

The biology does the work

In an activated-sludge ETP, bacteria in the aeration tank consume dissolved organics measured as COD and BOD. When outlet COD rises, ask what has happened to the bacteria: are they starved of oxygen, shocked by a toxic load, overloaded, or being washed out of the system?

Daily measurements that reveal the cause

• Dissolved oxygen: keep 1.5–2.5 mg/L in the aeration tank
• MLSS and SV30: a settling test in a one-litre cylinder shows sludge health
• pH: biology prefers 6.5–8.5; dyeing and resin effluents can swing widely
• Inlet COD and flow: calculate the food-to-microorganism ratio
• Temperature: above 38 °C many bacteria struggle

Typical fixes

If DO is low, clean diffusers and check blower capacity. If sludge is bulking (SV30 high, poor settling), filamentous bacteria are often the cause — usually linked to low DO or nutrient deficiency. Dose urea and DAP to maintain roughly BOD:N:P of 100:5:1. After a toxic shock, reduce inlet flow, use the equalisation tank to dilute, and re-seed with healthy sludge if necessary. Recovery normally takes one to two weeks, so prevention through equalisation and monitoring is far cheaper.