COD stands for Chemical Oxygen Demand. It is a measure of the amount of oxygen required to chemically oxidize organic and inorganic matter in water or wastewater.
Purpose:
COD is used to assess water quality and the level of pollution, especially in wastewater. A higher COD indicates more oxidizable (polluting) material is present.
How it’s measured:
A strong chemical oxidizing agent (typically potassium dichromate, K₂Cr₂O₇) is used under acidic conditions and high temperature to oxidize organic compounds in a water sample. The amount of oxygen consumed during this reaction is expressed in milligrams per liter (mg/L).
What it includes:
Organic pollutants (e.g., sugars, fats, proteins, hydrocarbons)
Some inorganic substances (e.g., sulfides, ferrous iron, nitrites)
Comparison with BOD (Biochemical Oxygen Demand):
| Feature | COD | BOD |
|---|---|---|
| Oxidation type | Chemical | Biological (microorganisms) |
| Time required | ~2–3 hours | 5 days (BOD₅) |
| Scope | Oxidizes both biodegradable and non-biodegradable organics | Only measures biodegradable organics |
| Result | Usually higher than BOD | Lower than COD |
Typical Uses:
Monitoring industrial wastewater
Evaluating treatment plant efficiency
Regulatory compliance
Clean river water: COD ≈ 10–20 mg/L
Raw domestic sewage: COD ≈ 250–800 mg/L
Industrial effluents: COD can exceed 10,000 mg/L
⚠️ Note: High COD levels can deplete dissolved oxygen in natural waters, harming aquatic life.
In summary, COD is a rapid, comprehensive indicator of water pollution based on total oxidizable material, making it essential in environmental engineering and water quality management.
