ETP High COD Troubleshooting: A Practical Guide for Industrial Wastewater

A high COD result at an effluent treatment plant is a warning, not a diagnosis. The result may come from a real process load, poor equalisation, unsuitable pH, low biomass activity, chemical overdosing, sludge carryover or an unrepresentative sample. Randomly increasing chemicals can hide the pattern and raise operating cost without restoring compliance.

This guide gives plant teams a disciplined ETP high COD troubleshooting sequence. Always compare findings with the limits and monitoring conditions in the unit's consent order and use an accredited laboratory where required.

1. Confirm the COD result and sampling point

Check whether the sample was collected from the consent-defined outlet, whether the stream was fully mixed and whether preservation and holding time were controlled. Compare COD with BOD, total suspended solids, pH, flow and production records for the same period. A high COD with high suspended solids can indicate clarifier carryover, while high soluble COD may point to incomplete biological or chemical treatment.

2. Map production loads before blaming the ETP

Review batch drains, cleaning cycles, rejected product, solvent-bearing streams and maintenance discharges. Prepare an hourly flow and load profile, not only a daily average. A small concentrated dump can upset biology even when the total daily volume is within design capacity. Segregate unusual streams and assess recovery or controlled dosing into equalisation.

3. Check equalisation, pH and nutrient balance

Verify working volume, mixing and level-control operation in the equalisation tank. Measure pH at several times rather than relying on one shift reading. Biological systems also need an appropriate nutrient balance; both deficiency and uncontrolled addition can reduce performance. Confirm dosing-pump calibration and actual chemical consumption against design.

4. Review aeration and biomass health

Trend dissolved oxygen, MLSS, settling behaviour, return sludge and excess sludge wasting. Inspect diffusers, blowers and dead zones. A healthy-looking aeration tank can still be overloaded or poorly mixed. Microscopy, oxygen uptake and sludge-settling observations can help distinguish toxicity, filamentous growth and insufficient biomass age.

5. Inspect clarification and tertiary treatment

Clarifier overflow, short-circuiting and poor sludge withdrawal can carry organics into the outlet. Check weirs, scraper operation, sludge blanket and hydraulic peaks. For tertiary systems, review filter pressure, carbon exhaustion, membrane condition and cleaning history. Do not use tertiary treatment to compensate permanently for a failed biological stage.

ETP corrective-action checklist

• Verify laboratory result and sampling chain.
• Prepare a production-to-pollution load timeline.
• Measure equalisation performance and peak loads.
• Trend pH, DO, MLSS, sludge settling and flow.
• Calibrate dosing pumps, flowmeters and online instruments.
• Record corrective action, owner, deadline and verification result.

For a structured capacity and performance review, see our ETP consultancy services and environmental monitoring support.

Frequently Asked Questions

Can COD be reduced only by increasing aeration?

No. Aeration helps only when oxygen transfer is the limiting factor and the organics are biodegradable. Toxic, refractory or highly variable loads need source control and an appropriate treatment route.

Why is outlet COD high when BOD is low?

The remaining organics may be poorly biodegradable, or the sample may contain chemicals that contribute to COD but not BOD. Confirm the result and characterise the wastewater before selecting treatment.

Should a plant dilute effluent to meet the limit?

No. Compliance should come from source reduction and effective treatment, not dilution. Follow the consent conditions and applicable pollution-control requirements.