BOD (Biochemical Oxygen Demand) is the amount of oxygen that bacteria would use up if all the organic matter in one litre of water were broken down (oxidised) by them. More organic waste means bacteria need more oxygen, so a higher BOD means the water is more polluted. Memory hook: "Bacteria's Oxygen Diet" — the more food (organic waste) in the water, the more oxygen bacteria eat, the higher the BOD.
Bacteria break down organic matter in one litre of water using oxygen. The oxygen they consume is the BOD. More organic waste means more oxygen used, so a higher BOD signals more pollution.
Your doubts, answered
Does BOD measure oxygen or organic matter?
BOD directly measures the oxygen that micro-organisms take up in a sample of water. But because bacteria only use oxygen to break down organic matter, BOD indirectly tells you how much organic matter is in the water. So it measures oxygen uptake directly and organic load indirectly. NCERT states BOD is 'a measure of the organic matter present in the water.'
Is a high BOD good or bad?
A high BOD is bad. It means a lot of organic waste is present, so bacteria are using a lot of oxygen to break it down. NCERT says 'the greater the BOD of waste water, more is its polluting potential.' Clean water has a low BOD.
Why does polluted water have a high BOD?
Polluted water (like sewage) is full of organic matter — food for bacteria. Bacteria multiply and use up lots of dissolved oxygen to oxidise this matter. The more organic waste, the more oxygen they need, so BOD goes up. That is why sewage is treated until its BOD is reduced.
Is BOD the actual amount of oxygen present in the water?
No. BOD is not the oxygen currently dissolved in the water. It is the amount of oxygen that would be consumed if all the organic matter in one litre were oxidised by bacteria. It is a demand (what bacteria would need), not the oxygen already available.
Does BOD work for every kind of polluted water?
No. BOD only counts organic matter that microbes can break down (biodegradable waste). Effluent that is mostly non-biodegradable, like from a petroleum industry, is not measured well by BOD — the test underestimates its true pollution. This exact point was asked in NEET 2016.
⚠️ The NEET trap ✗ BOD is the amount of oxygen present in a water sample. ✓ BOD is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria — an oxygen demand, not oxygen already present. 🧠 NTA loves swapping 'oxygen consumed to break down organic matter' with 'oxygen present in the water.' Read the option fully — BOD is a demand created by bacteria, and higher BOD means dirtier water.
Real NEET questions
2016
Biochemical Oxygen Demand (BOD) may not be a good index for pollution for water bodies receiving effluents from:
A · Domestic sewage
B · Dairy industry
C · Petroleum industry ✓
D · Sugar industry
Solution: BOD measures only the biodegradable (organic) load that microbes oxidise. Petroleum-industry effluent is largely non-biodegradable, so the BOD test underestimates its true pollution and is not a good index for it. Domestic sewage, dairy and sugar effluents are rich in biodegradable organic matter, so BOD reflects their pollution well.
2016
A river with an inflow of domestic sewage rich in organic waste may result in:
A · Drying of the river very soon due to algal bloom
B · Increased population of aquatic food web organisms
C · An increased production of fish due to biodegradable nutrients
D · Death of fish due to lack of oxygen ✓
Solution: Sewage rich in organic waste raises the BOD. Aerobic microbes multiply and consume the organic matter, using up the dissolved oxygen in the water. With oxygen exhausted, fish and other aquatic organisms die. Higher BOD means more polluting potential, so options predicting more fish or food-web growth are wrong.
Solved Microbes in Human Welfare NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
BOD is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria.
What does a high BOD indicate?
A high BOD indicates a large amount of organic waste and therefore more pollution. NCERT: 'the greater the BOD of waste water, more is its polluting potential.'
Why is sewage treated until BOD is reduced?
High-BOD water depletes oxygen and kills aquatic life. Treatment lowers the organic load, so the released effluent uses less oxygen and harms rivers less.
When is BOD not a good index of pollution?
When effluent is mostly non-biodegradable (for example from a petroleum industry), because BOD only measures organic matter that microbes can break down.
