Polluted river stretches

Also called: Polluted river stretches in India · Topic: Geographical Issues: Pollution, Urban Waste, Slums and Land Degradation · NCERT: Class 12, Ch 4 "Water Resources"; Class 12, Ch 9 "Geographical Perspective on Selected Issues and Problems"

Meaning

A polluted river stretch is a part of a river where biochemical oxygen demand (BOD) is above 3 mg/l. This means the water fails the Central Pollution Control Board's (CPCB) standard for outdoor bathing. Polluted stretches show where cities and industries are dumping more waste than a river can clean on its own. They are also the main targets for river clean-up work such as sewage treatment plants and Namami Gange (2014).

Explanation

How a stretch becomes polluted

• Every river has a self-purifying capacity. This is its natural ability to clean itself.
• Flowing water takes in oxygen from the air.
• Microbes (tiny living things such as bacteria) use this oxygen to break down dirt.

• Sunlight and dilution (more water mixing in) also help.

• A river gets polluted when the waste is more than it can handle. The waste can be suspended (floating), organic (from living things, such as sewage) or inorganic (minerals, metals and chemicals).

• Chain of the problem:
• Cities grow fast, and sewers and sewage treatment plants (STPs) cannot keep up. Raw sewage flows into drains and then into rivers.
• Microbes use up the dissolved oxygen to break down this waste. BOD rises and dissolved oxygen (DO) falls.
• Fish die, the water smells, and the river can no longer clean itself.
• Faecal coliform rises, and water-borne diseases spread downstream.

How pollution is measured: BOD and bathing criteria

• Biochemical oxygen demand (BOD): the amount of dissolved oxygen that microbes use to break down the organic matter in water.
• More sewage means microbes need more oxygen, so BOD is higher and the water is dirtier.

• Low BOD means clean water. High BOD means organic pollution.

• CPCB primary water quality criteria for outdoor bathing:

Parameter	Limit for bathing water
pH (how acidic or basic the water is)	6.5–8.5 [4]
Dissolved oxygen (DO) (oxygen in water that fish need)	5 mg/l or more [4]
BOD	3 mg/l or less [4]
Faecal coliform (bacteria from human and animal faeces, a sign of sewage)	500 MPN/100 ml desirable; 2,500 MPN/100 ml maximum [4]

• Why is the BOD limit 3 mg/l? At 3 mg/l or less, the water is fairly free of pollutants that use up oxygen, and foul-smelling gases do not form [4].
• Why is the DO limit 5 mg/l? It stops sediment from rotting without oxygen and giving off bad-smelling gases (anaerobic gases) [4].

Priority classes of polluted stretches

• CPCB puts each stretch into one of five classes (I to V), based on the highest BOD recorded there [2].

Priority class	BOD (mg/l)	Number of stretches (2022)
I (worst)	More than 30	46 [2]
II	20–30	16 [2]
III	10–20	39 [2]
IV	6–10	65 [2]
V (least bad)	3–6	145 [2]

• CPCB assessment (2022): 311 polluted stretches on 279 rivers [2]. This is down from 351 stretches in 2018 [2].
• Priority V has the most stretches. Most polluted stretches are only mildly polluted, but 46 are in the worst class [2].

The core cause: the sewage treatment gap

• Sewage treatment gap: the difference between the sewage a city produces and the sewage it actually treats.
• CPCB National Inventory of STPs (March 2021):
• Cities produce 72,368 MLD of sewage (MLD: million litres a day) [3].
• Installed treatment capacity is 36,668 MLD, about half of what is needed [3].
• Capacity actually used is only 20,235 MLD, about 28% of the sewage produced [3].

• 52,133 MLD goes into rivers, lakes and the ground untreated [3].

• STP status (2021): there are 1,631 STPs in 35 states and UTs. Of these, 1,093 are working, 102 are not working, 274 are under construction and 162 are proposed [3].

• Of the 900 working STPs whose status is known, only 578 STPs (12,200 MLD) met the norms set by the state boards [3].

• Other sources make it worse:

• Industrial: tanneries (leather), pulp and paper, textiles and chemicals release effluents (liquid factory waste) and heavy metals. Heavy metals such as lead, mercury, cadmium and chromium do not break down.
• Agricultural: extra fertilisers and pesticides wash into streams. Fertilisers raise nitrate levels.
• Cultural: bathing and offerings at pilgrimages and religious fairs add organic waste to stretches that are already under stress.

Where it occurs

• India, by state (2022): Maharashtra has the most polluted stretches with 55. It is followed by Madhya Pradesh (19), Bihar (18), Kerala (18) and Karnataka (17) [2].

• Maharashtra has 4 Priority I, 5 Priority II, 18 Priority III and 17 Priority IV stretches [2].

• Some of the worst stretches:

• Yamuna below Delhi. The Yamuna rises at the Yamunotri glacier in Uttarakhand and joins the Ganga at Prayagraj, Uttar Pradesh. NCERT shows toxic white foam floating on the Yamuna (Fig. 9.1).
• Sabarmati at Ahmedabad, Gujarat.
• Musi at Hyderabad, Telangana. It is a tributary of the Krishna.
• Gomti at Lucknow, Uttar Pradesh. It is a tributary of the Ganga.
• Cooum at Chennai, Tamil Nadu.

• Ganga at Kanpur (tanneries) and at Varanasi.

• Spatial pattern: the worst stretches lie downstream of big cities and industrial clusters. The main cause is untreated city sewage, not natural sources.

• Policy response: CPCB identifies and ranks the stretches. Namami Gange (2014) builds STPs and checks industrial outflow in the Ganga basin.

Don't confuse with

• BOD vs dissolved oxygen (DO): BOD is the oxygen that microbes use up, so higher BOD means dirtier water. DO is the oxygen present in the water, so higher DO means cleaner water. For bathing water, BOD must be 3 mg/l or less and DO must be 5 mg/l or more [4].
• Priority I vs Priority V: Priority I is the worst class (BOD above 30 mg/l). Priority V is the least bad class (BOD 3–6 mg/l), and it has the most stretches (145) [2].
• Polluted river stretch vs groundwater contamination: a polluted stretch is surface water with high organic load, caused mostly by sewage. Groundwater contamination by arsenic (Ganga-Brahmaputra plains) or fluoride (Rajasthan, Telangana, Gujarat) is often geogenic, meaning it comes from natural rocks, and BOD does not measure it.
• Biggest source of water pollution (NCERT) vs main cause of polluted stretches (CPCB): NCERT names industry as the biggest human source of water pollution. For river stretches today, CPCB data show untreated urban sewage as the main cause.

Prelims Hooks

• A stretch counts as polluted when BOD is above 3 mg/l. The bathing standard also needs DO of 5 mg/l or more, pH of 6.5–8.5, and faecal coliform of 2,500 MPN/100 ml at most (500 desirable) [4].
• CPCB 2022: 311 polluted stretches on 279 rivers, down from 351 in 2018 [2].
• Priority I means BOD above 30 mg/l (46 stretches). Priority V means BOD of 3–6 mg/l (145 stretches, the largest class) [2].
• Order of states: Maharashtra (55) > Madhya Pradesh (19) > Bihar = Kerala (18) > Karnataka (17) [2].
• River–city pairs: Musi–Hyderabad, Cooum–Chennai, Gomti–Lucknow, Sabarmati–Ahmedabad, Ganga–Kanpur (tanneries).
• Urban sewage (2021): 72,368 MLD produced, 36,668 MLD installed capacity, and only about 28% (20,235 MLD) actually treated [3].

Mains Points

• Cause and distribution: polluted stretches cluster below big cities and industrial belts, such as the Yamuna below Delhi, the Ganga at Kanpur and the Musi at Hyderabad.
• Cities have grown faster than their sewers and STPs. Only about 28% of 72,368 MLD of sewage is treated, and 52,133 MLD flows out untreated [3].

• Even existing plants underperform. Of 900 working STPs checked, only 578 met the norms [3].

• Health and economic cost: WHO estimates that about one-fourth of India's communicable diseases (diseases that spread from person to person) are water-borne. These include diarrhoea, typhoid, cholera and hepatitis.

• Heavy metals build up in fish and crops. Over time they harm the nerves, kidneys and bones.

• The poor who live downstream and depend on river water are hit hardest.

• Policy response and gaps (GS-III): the Water (Prevention and Control of Pollution) Act 1974 made industries get "consent to discharge", but enforcement was weak (NCERT counted 251 polluting industries in 1997). Namami Gange (2014) moved the focus to building STPs, and polluted stretches fell from 351 (2018) to 311 (2022) [2].

• Still needed: run existing STPs at full capacity, reuse treated sewage, monitor effluents in real time, and control farm runoff and crowd pollution at religious fairs.

Related concepts

• Water pollution
• Water-borne diseases
• Biochemical oxygen demand
• Sewage treatment gap
• Water (Prevention and Control of Pollution) Act 1974

Read more

• Water pollution: sources, measurement and health