Introduction

• Biochemical Oxygen Demand (BOD) is the amount of dissolved oxygen used by microorganisms while they decompose organic matter in water under aerobic conditions.
• It is one of the most important indicators of organic pollution in water.

Basic meaning

• When sewage, decaying plant material, food waste, or other organic matter enters water, bacteria and other microorganisms begin breaking it down. In doing so, they consume oxygen dissolved in the water.
• BOD measures this oxygen demand.

Why it is important

• BOD matters because aquatic life such as fish and other organisms also depend on dissolved oxygen. If microorganisms use too much of it, less oxygen is left for higher aquatic life.
• So, higher BOD usually means poorer water quality and greater organic pollution.

What BOD indicates

• BOD is used to estimate:
  • the amount of biodegradable organic matter in water
  • the likely oxygen depletion in a water body
  • the level of organic pollution
  • the performance of wastewater treatment systems.

Standard BOD test

• A common laboratory measure is BOD₅, which means the oxygen consumed over 5 days at 20°C.
• This 5-day test is widely used as a standard reference in water-quality analysis.

Unit

• BOD is generally expressed in milligrams per litre (mg/L) of oxygen.

Interpretation

• Low BOD means:
  • less biodegradable organic pollution
  • relatively better water quality.
• High BOD means:
  • more organic waste in water
  • more oxygen being consumed by microorganisms
  • greater stress on aquatic ecosystems.

Sources of high BOD

• BOD rises when water receives large quantities of organic matter from sources such as:
  • domestic sewage
  • industrial wastewater
  • agricultural runoff
  • decaying vegetation or animal waste.

Relation with dissolved oxygen (DO)

• BOD and dissolved oxygen (DO) are closely related.
• If BOD is high, microorganisms consume more oxygen, so DO tends to fall.
• This is why a high-BOD water body may become unfit for fish and other aerobic organisms.

Difference from COD

• BOD measures oxygen used mainly through biological decomposition by microorganisms.
• COD (Chemical Oxygen Demand) measures oxygen equivalent needed to chemically oxidize pollutants, including substances that may not be biologically degradable. Britannica notes BOD and COD as related but distinct oxygen-demand concepts used as indicators of water quality.
• So, COD is usually broader, while BOD is more focused on biodegradable organic pollution.

Environmental significance

• BOD is important in:
  • river-water assessment
  • lake pollution studies
  • sewage treatment monitoring
  • effluent regulation.
• EPA also lists BOD among conventional pollutants for regulatory purposes.

In simple words

• BOD tells us how much oxygen water will lose because microbes are busy breaking down organic waste.
• More waste means more microbial activity, and that means higher BOD.

Conclusion

• Biochemical Oxygen Demand (BOD) is a key water-quality parameter used to judge the level of organic pollution and its effect on dissolved oxygen. It is especially useful in assessing sewage pollution, wastewater treatment, and the health of aquatic ecosystems.