Biology · Microbes in Human Welfare · NEET
Sewage is the waste water made every day in cities and towns. NCERT calls it municipal waste water. Its major part is human excreta (urine and faeces). It holds a large amount of organic matter and many microbes, and a lot of those microbes are pathogenic, meaning they can cause disease. So sewage is not just dirty water — it is water full of organic waste plus germs.
Sewage carries organic matter and pathogenic microbes. If it is poured straight into a river, the germs can spread diseases like cholera and typhoid, and the organic matter feeds microbes that use up the oxygen in the water. Because of this danger, NCERT says sewage must first be treated in Sewage Treatment Plants (STPs) to make it less polluting before it is released.
When sewage rich in organic waste enters a river, aerobic microbes multiply and break down the organic matter. To do this they consume dissolved oxygen in the water. The oxygen level drops sharply, so fish and other aquatic animals suffocate and die. For humans, the pathogenic microbes in untreated sewage cause water-borne diseases. This is why untreated sewage is both an ecological and a health threat.
No. Sewage is the raw incoming waste water (dirty water plus organic matter plus microbes). Sludge is the settled solid mass of bacterial cells (flocs) and particles that forms during sewage treatment. Sewage is the input; sludge is a solid by-product produced while cleaning the sewage. Do not mix the two in a NEET matching question.
No special microbes are added at the start. NCERT clearly states that treatment of waste water is done by the heterotrophic microbes naturally present in the sewage itself. These natural microbes digest the organic matter during treatment. This is a common NEET one-liner: the microbes are natural, not artificially introduced.
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.
Yes. NCERT uses the words interchangeably: the municipal waste water of cities and towns is called sewage. Its major component is human excreta.
Two things: a large amount of organic matter (which uses up water oxygen when microbes break it down) and many pathogenic microbes (which cause water-borne diseases).
BOD (Biochemical Oxygen Demand) measures the amount of oxygen microbes would use to break down the organic matter in the water. High organic sewage means high BOD, which means more oxygen removed and more pollution.
STP stands for Sewage Treatment Plant. Sewage is treated in STPs to make it less polluting before it is released into natural water bodies.
The heterotrophic microbes that are naturally present in the sewage. No microbes are added from outside at the beginning of treatment.
