Which of the following is considered as major source of pollution in rivers of…

Concept: A pollution source is ranked by the total pollution load it delivers to a water body — the volume of wastewater discharged multiplied by the concentration of pollutants carried in it — and not by how hazardous a single litre of that discharge happens to be. A stream that is high in volume, continuous through the year and largely untreated therefore dominates a river’s load even when each litre of it is chemically milder than an industrial effluent.

Application: Applying that load rule to Indian rivers, step by step.

1. Volume: urban centres in India generate on the order of 72,000 million litres of sewage per day, while the installed capacity of sewage treatment plants covers well under half of that generation, and the capacity actually operated is lower still (Central Pollution Control Board national inventory of sewage generation and treatment).

2. Treatment gap: the shortfall is released, largely untreated, into nalas and municipal drains that outfall directly into rivers. Because households discharge every day of the year, this input is continuous rather than episodic. For the Ganga basin the Government of India has put untreated or partially treated sewage at roughly 75–80 per cent of the total pollution load.

3. Signature in the monitoring data: the parameter on which the Central Pollution Control Board identifies a river stretch as polluted is biochemical oxygen demand, and faecal coliform is the accompanying indicator — both are markers of domestic wastewater, not of metals or heat. The polluted-river-stretch inventories are built on that BOD criterion.

Cross-check: Comparing the other listed sources against the same load rule.

• Unregulated small scale industry is far more hazardous per litre — chromium, dyes and acids — and can devastate a stretch locally, but the volume of effluent it discharges is a much smaller share of the total load than domestic wastewater, so it is not the dominant source nationally.

• Agricultural run-off delivers a large diffuse nutrient input, but it is tied to irrigation cycles and monsoon rainfall, so it is seasonal and spread thinly across the catchment rather than a continuous concentrated outfall.

• Thermal power plants mainly return once-through cooling water at a raised temperature and seepage from ash ponds; they add heat and trace metals but contribute very little of the organic load that drives BOD.

Result: Untreated sewage — domestic wastewater discharged into rivers without treatment — is considered the major source of pollution in India’s rivers.