BOD and COD — Water Pollution Indicators

BOD stands for Biochemical Oxygen Demand (or Biological Oxygen Demand).

• Definition: It is the amount of dissolved oxygen needed by aerobic biological organisms (like bacteria) to break down organic material present in a given water sample at a specific temperature over a specific time period (usually 5 days at 20°C).
• What it measures: It ONLY measures the amount of biodegradable organic matter in the water (stuff that bacteria can naturally eat and decompose, like sewage or food waste).

COD stands for Chemical Oxygen Demand.

• Definition: It is the total amount of oxygen required to chemically oxidize all organic and inorganic compounds in water, using a strong chemical oxidizing agent (like potassium dichromate) in an acidic medium.
• What it measures: It measures BOTH biodegradable AND non-biodegradable organic matter (like toxic industrial chemicals, plastics, or pesticides that bacteria cannot eat).

Testing both tells environmental scientists about the type of pollution.

• If BOD and COD values are almost the same, the pollution is mostly natural sewage.
• If COD is much, much higher than BOD, it means the water is heavily polluted with toxic, non-biodegradable industrial chemicals that bacteria cannot clean up naturally.