Biochemical oxygen demand


QUESTION ASKED IN UPSC EXAM ON-Biochemical oxygen demand

UPSC 2017

QUES – Biological Oxygen Demand (BOD) is a standard criterion for
(a) Measuring oxygen levels in blood
(b) Computing oxygen levels in forest ecosystems
(c) Pollution assay in aquatic ecosystems
(d) Assessing oxygen levels in high altitude regions

Answer – (c)

Biochemical Oxygen Demand (BOD or Biological Oxygen Demand) is the amount of dissolved oxygen needed  by aerobic biological organisms to break down organic material present in a given water sample at certain temperature over a specific time period.

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It is expressed in milligrams of oxygen consumed per litre of water and is often used as a indicator of the degree of organic pollution of water.


Most natural waters contain small quantities of organic compounds. Aquatic microorganisms have evolved to use some of these compounds as food. Microorganisms living in oxygenated waters use dissolved oxygen to oxidatively degrade the organic compounds, releasing energy which is used for growth and reproduction. Populations of these microorganisms tend to increase in proportion to the amount of food available. This microbial metabolism creates an oxygen demand proportional to the amount of organic compounds useful as food.

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Under some circumstances, microbial metabolism can consume dissolved oxygen faster than atmospheric oxygen can dissolve into the water or the autotrophic community (algae, cyanobacteria and macrophytes) can produce. Fish and aquatic insects may die when oxygen is depleted by microbial metabolism.