How a river is judged polluted: the classes, the standards and the sewage gap

Prelims and MainsCurrent affairs on this: Water pollution and river rejuvenation

A river is not clean or dirty; it is fit or unfit for a use, and the Central Pollution Control Board grades water by its designated best use in five classes, A to E, from a source drinkable after disinfection alone to water fit only for irrigation and cooling. Three measures do most of the work: dissolved oxygen, which fish need; biochemical oxygen demand, the oxygen that microbes use up eating the organic matter in sewage; and coliform bacteria, the sign of faeces. A river stretch is listed as polluted when its biochemical oxygen demand exceeds 3 milligrams a litre, the bathing standard, and the Board's count of such stretches is the national scorecard. The cause is arithmetic: Indian cities generate about 72,000 million litres of sewage a day and treat under a third of it, so every river programme is at bottom a sewage programme.

• The five classes, in the figure: A, drinking water source without conventional treatment but after disinfection; B, outdoor bathing; C, drinking water source with conventional treatment; D, propagation of wildlife and fisheries; E, irrigation, industrial cooling and controlled waste disposal. Class B, the bathing standard, is the one a river is judged by.
• The measures: dissolved oxygen falls as biochemical oxygen demand rises, and below about 4 milligrams a litre fish begin to die; chemical oxygen demand counts what chemicals as well as microbes would oxidise, so it is always the higher figure; faecal coliform is counted as the most probable number per 100 millilitres. Eutrophication is the chain from nitrate and phosphate to an algal bloom to night time oxygen collapse; the froth on the Yamuna is phosphate from detergents whipped at a barrage.
• The polluted river stretches: the Board lists a stretch when two or more monitoring stations show biochemical oxygen demand above 3 milligrams a litre; it counted 351 stretches in 2018 and 296 now, and Maharashtra has the most. The Namami Gange subject tracks the Ganga's own stretches.
• The law and the programme: the Water (Prevention and Control of Pollution) Act, 1974 created the Board and the State boards, which set discharge standards and can close an industry; grossly polluting industries such as tanneries, distilleries and textile dyeing face zero liquid discharge norms. Namami Gange, from 2014, is run by the National Mission for Clean Ganga under the National Ganga Council chaired by the Prime Minister, and the Ganga has had notified minimum environmental flows since October 2018.
• Groundwater contaminants are judged against the Bureau of Indian Standards drinking water standard IS 10500: arsenic above 0.01 milligrams a litre, geogenic in the Ganga Brahmaputra plains; fluoride above 1.5, causing fluorosis in Rajasthan, Gujarat and Telangana; nitrate above 45, from fertiliser and sewage; uranium in Punjab; and chromium from tanneries, the Kanpur subject.

Mains: Building a treatment plant is the easy half of cleaning a river: the plant must be connected to the drains, run at capacity and paid for after the scheme ends, which is why treated capacity and treated sewage are different numbers and why reuse policies that make treated water worth selling matter as much as new plants.

Further reading: Water Quality Standards (CPCB)

See also: Namami Gange · Reuse of treated wastewater · Urban River Management Plans · Hexavalent chromium in the Kanpur region · The Jojari · The National Green Tribunal and the pollution boards · Jal Jeevan Mission 2.0