Biology · Microbes in Human Welfare · NEET
High BOD means polluted water. BOD is the oxygen that microbes DEMAND to oxidise organic waste. More organic waste in the water means microbes need more oxygen, so BOD goes up. NCERT states it plainly: the greater the BOD of waste water, more is its polluting potential. So high BOD equals high pollution, not clean water.
BOD is the oxygen DEMANDED (needed), not the oxygen present. It is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. Do not confuse it with dissolved oxygen (DO), which is the oxygen actually present. When organic waste is high, microbes use up the dissolved oxygen fast, so DO falls while BOD is high.
Domestic sewage is rich in organic waste, so it has a high 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 from lack of oxygen. This is a common NEET question and the answer is death of fish due to lack of oxygen.
BOD only measures the biodegradable (organic) load that microbes can oxidise. Petroleum-industry effluent is largely non-biodegradable, so microbes cannot break it down and the BOD stays low even though the water is heavily polluted. This makes BOD underestimate the real pollution. For domestic sewage, dairy and sugar industry (all rich in biodegradable organic matter) BOD works well.
The BOD test measures the rate of uptake of oxygen by micro-organisms in a water sample. Microbes take up oxygen in proportion to how much organic matter they have to break down. So by measuring oxygen uptake, we indirectly measure the organic matter present. More organic matter means more oxygen uptake means higher BOD.
A river with an inflow of domestic sewage rich in organic waste may result in:
Biochemical Oxygen Demand (BOD) may not be a good index for pollution for water bodies receiving effluents from:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
BOD stands for Biochemical Oxygen Demand. It is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria.
BOD is directly related to pollution. Higher BOD means more organic matter and more polluting potential. Lower BOD means cleaner water.
Untreated sewage water has the highest BOD because it has the most organic waste. Treated (secondary) effluent has a low BOD, and relatively clean river water has an intermediate-to-low BOD. In the NCERT example the values 20, 8 and 400 mg/L belong to river water, treated effluent and untreated sewage respectively, so 400 mg/L (untreated sewage) is the most polluted.
BOD is the oxygen DEMANDED by microbes to break down organic waste. DO (dissolved oxygen) is the oxygen actually PRESENT in the water. High BOD usually goes with low DO, because microbes use up the available oxygen.
BOD is a repeated NEET topic from Microbes in Human Welfare. Questions test whether you know high BOD means high pollution, that BOD is oxygen demanded (not present), and that BOD fails as an index for non-biodegradable petroleum waste.
