The primary source of organic pollution in fresh water bodies is

Concept: Organic pollution is the addition of biodegradable organic matter — material that micro-organisms can decompose — to a water body. The decomposers consume dissolved oxygen as they break this matter down, so the size of an organic pollution load is measured as biochemical oxygen demand (BOD). A source therefore counts as a source of organic pollution in proportion to the biodegradable organic mass it delivers, not in proportion to the toxicity or the total volume of what it carries.

Application: Of the discharges that reach a freshwater body, domestic sewage is the one whose bulk is biodegradable organic matter — human excreta, kitchen and washing waste, and detergents. It is also a continuous point discharge: every settlement along a river or lake releases it every day of the year, wet season and dry. India’s Central Pollution Control Board attributes the great majority of the BOD load carried by Indian rivers to untreated and partially treated municipal sewage, and WHO guidance likewise describes inadequately treated sewage as the major point source of pollution of the world’s waters. Hence the primary source of organic pollution in fresh water bodies is sewage effluents.

Cross-check — why the other discharges contribute less organic load:

• Run-off from urban areas is storm-driven and therefore intermittent, and its bulk is silt, grit, oil and grease, road salts and litter — largely mineral and hydrocarbon material rather than readily biodegradable organic matter.

• Run-off from agricultural farms delivers mainly dissolved nitrate and phosphate fertiliser plus pesticide residues. These are nutrients and toxicants: they enrich a water body and drive eutrophication rather than adding a large oxygen-demanding organic mass directly, and being a diffuse source they arrive seasonally with irrigation and monsoon rain.

• Industrial effluents are chemically diverse — acids, alkalis, heavy metals, dyes and heated water. Only particular industries (distilleries, paper and pulp, sugar, food processing) discharge a heavily organic waste, so across all industry the aggregate biodegradable organic share stays smaller than the sewage share.

Result: sewage effluents.