Water Pollution — Scientific Principles

Scientific Principles

Begin with source, pathway and receptor. Sewage can introduce biodegradable matter and microorganisms; runoff can carry nutrients or other contaminants; an industrial discharge must be assessed for its particular constituents. The same visible colour or odour does not identify the cause, and the absence of either does not establish safety.

Dissolved oxygen, or DO, is oxygen available in the water. Biochemical oxygen demand, or BOD, measures oxygen used through biological decomposition under the specified test conditions. A high organic load can increase demand and contribute to oxygen depletion. BOD is not the oxygen already present, and a low BOD result does not rule out pathogens or a non-biodegradable toxic chemical.

Nutrient enrichment can stimulate growth of algae and other primary producers. Subsequent respiration and decomposition can increase oxygen demand. This eutrophication pathway depends on the water body's conditions; sunlight, flow and other factors influence the response. A short-term daytime oxygen reading should not be treated as a complete account of the ecological process.

CPCB's designated-best-use criteria distinguish uses. Class A concerns a drinking-water source without conventional treatment but after disinfection, whereas Class B concerns organised outdoor bathing. Their DO and BOD limits differ. Meeting two measured parameters does not establish that a sample meets the entire class, because microbial and other specified criteria also matter. Treatment requirements remain part of the classification.