What is Chemical Oxygen Demand (COD)?

Chemical Oxygen Demand (COD) utilizes chemical oxidation to measure the amount of oxygen required to oxidize both organic and inorganic matter in wastewater.

In other words, COD measures the oxygen consumed when strong chemicals break down pollutants in a water sample, making it a reliable indicator of water quality.

Why it matters: COD provides a fast and comprehensive estimate of pollution levels—often in under 4 hours—making it a preferred tool for real-time process control in wastewater treatment plants.

How the COD Test Works

The test involves adding a 2 ml wastewater sample to a strong chemical oxidant—potassium dichromate in sulfuric acid. The mixture is then heated under reflux conditions, accelerating the oxidation reaction.

The amount of oxygen required to fully oxidize the sample is directly related to the COD value, which is expressed in milligrams of oxygen consumed per liter (mg O₂/L).

COD measures oxygen consumed by chemical reactions, whereas BOD measures oxygen consumed by living organisms.

COD vs. BOD: Key Differences

COD offers a broader picture of pollution by capturing both biodegradable and non-biodegradable materials. It’s faster, more consistent, and ideal for operational decisions.

BOD, on the other hand, measures only the oxygen used by aerobic organisms to decompose organic matter—usually over a 5-day period—making it more specific to biological treatment performance.

• COD: Chemical oxidation of all compounds (fast: 3–4 hours).
• BOD: Biological oxidation of biodegradable organics (slow: 5 days).