Environment / Pollution Control — UPSC Mains Model Answer

A high BOD value signifies that a water body carries a heavy load of decomposable organic matter — typically from sewage, agricultural runoff, or industrial effluents. Microorganisms consume dissolved oxygen while breaking down this matter, depleting the oxygen available to aquatic life. India's Central Pollution Control Board uses BOD as a primary criterion for classifying river water quality into designated best-use categories.

BOD is measured by incubating a water sample at 20°C for five days under aerobic conditions and recording the oxygen consumed — commonly termed BOD₅. This standardised protocol enables consistent cross-site and cross-temporal comparisons. Effluent discharge norms under the Environment Protection Act prescribe permissible BOD limits before treated wastewater may be released into water bodies.

As BOD rises, microbial oxygen consumption intensifies, causing Dissolved Oxygen (DO) levels to fall — not rise. Low DO triggers hypoxic or anoxic zones, threatening fish and other aerobic organisms. This inverse relationship is central to the oxygen-sag curve concept used in river self-purification modelling and in designing sewage treatment plant capacities.

Real-time BOD and DO monitoring through the National Water Quality Monitoring Programme informs enforcement action against polluting industries and municipalities. Achieving the National Mission for Clean Ganga targets requires sustained reduction of BOD loads from untreated sewage, which still constitutes the dominant pollution source in most Indian rivers.