Water pollution

Topic: Geographical Issues: Pollution, Urban Waste, Slums and Land Degradation · NCERT: Class 10, Ch 3 "Water Resources"; Class 10, Ch 6 "Manufacturing Industries"; Class 12, Ch 4 "Water Resources"; Class 12, Ch 9 "Geographical Perspective on Selected Issues and Problems"

Meaning

Water pollution happens when suspended (floating), organic (from living things, such as sewage) and inorganic (minerals, metals, chemicals) substances enter water faster than the water can clean itself (its self-purifying capacity), so the water is no longer fit for use. It matters because almost all surface water in India is unfit to drink without treatment. It also matters because water-borne diseases make up about one-fourth of India's communicable diseases (diseases that spread from person to person), according to WHO.

Explanation

How water cleans itself, and how it fails

• Surface water is never fully pure. River, lake and pond water always carries some soil, plant matter and dissolved salts.
• Self-purifying capacity means water's natural ability to clean itself:
• Flowing water takes in oxygen from the air.
• Microbes use this oxygen to break down dirt.

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

• How the system breaks down:

• Too much waste enters the water, so microbes use up the dissolved oxygen.
• Fish die and the water starts to smell.
• The water can no longer clean itself, so it is now polluted.

Sources of water pollution

Source	Examples	Main pollutants	How important
Natural	Erosion, landslides, decay of plants and animals	Silt, organic matter	Minor
Industrial	Leather (tanneries), pulp and paper, textiles, chemicals	Effluents, heavy metals, poisonous gases	Biggest human source (NCERT)
Agricultural	Chemical fertilisers, pesticides, herbicides	Nitrates, toxic chemicals	Large and spread out, with no single outlet
Cultural	Pilgrimages, religious fairs, tourism	Organic waste, ashes, flowers, plastics, bathing load	Seasonal, but heavy at holy sites

• Industry:
• Effluent is the liquid waste a factory releases.

• Heavy metals are metals such as lead, mercury, cadmium and chromium. They do not break down. They build up in fish and then in humans.

• Agriculture:

• Farmers use too much fertiliser, and crops cannot take it all up.
• Rain and irrigation water wash the extra into streams. Some of it also seeps into groundwater (water stored under the ground in rocks and soil).

• As a result, nitrate levels rise.

• Cultural activities: big crowds bathe in rivers and throw offerings into them. This adds organic load to river stretches that are already stressed.

• Urban sewage: this is the main cause of polluted river stretches today (see the sewage treatment gap below).

Measuring water pollution: BOD and bathing standards

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

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

• CPCB primary water quality criteria for outdoor bathing. CPCB is the Central Pollution Control Board, India's national pollution watchdog.

Parameter	Limit for bathing water
pH (how acidic or basic 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 a BOD of 3 mg/l? At this level the water is fairly free of pollutants that use up oxygen, and foul-smelling gases do not form [4].
• Why a DO of 5 mg/l? It stops sediment from rotting without oxygen and giving off bad-smelling gases (anaerobic gases) [4].
• Polluted river stretch: a part of a river where BOD is above 3 mg/l, so the water fails the bathing standard. Stretches are put into five priority classes by the highest BOD recorded there [2]:

Priority class	BOD (mg/l)	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]

The sewage treatment gap, the core cause

• Sewage treatment gap is the difference between the sewage a city produces and the sewage it actually treats.
• CPCB National Inventory of STPs, March 2021 (STP means sewage treatment plant; MLD means million litres a day):
• Cities produce 72,368 MLD of sewage [3].
• Installed treatment capacity is 36,668 MLD, about half of what is needed [3].
• Only 20,235 MLD of capacity is actually used. That is 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 with known status, only 578 STPs (12,200 MLD) met the norms set by the state boards [3].

• How the problem builds up:

• Fast urban growth: cities grow faster than sewers and STPs, so raw sewage flows into drains and rivers.
• More organic load: BOD rises and DO falls, so the river can no longer clean itself.
• More faecal coliform: water-borne diseases spread downstream.

Where it occurs

• Polluted river stretches (CPCB 2022): 311 stretches on 279 rivers, down from 351 stretches in 2018 [2].
• States with the most polluted stretches (2022): Maharashtra 55, then Madhya Pradesh 19, then Bihar 18 and Kerala 18 (tied), then 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 (Fig. 9.1) shows toxic white foam floating on the Yamuna.
• Ganga at Kanpur (tanneries) and at Varanasi, both in Uttar Pradesh.
• 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.

• Pattern: the worst stretches lie downstream of big cities and industrial clusters.

• Groundwater contamination:
• Arsenic is found mainly in the Ganga-Brahmaputra delta and plains: West Bengal, Bihar and Assam.
  • It has been found at certain points in 221 districts of 25 states/UTs [5]. The earlier figure was 152 districts in 21 states [6].
  • The worst districts are in West Bengal: Nadia (86.4% of samples), Murshidabad (76.7%) and North 24 Parganas (52.2%) [7].
• Fluoride is found mainly in Rajasthan, Telangana and Gujarat. Pockets in 370 districts in 23 states have fluoride above the permissible limit [6].

• Nitrate comes from fertilisers and sewage. The BIS (Bureau of Indian Standards) permissible limit is 45 mg/l [7].

• Law and programmes:

• The Water (Prevention and Control of Pollution) Act 1974 applies across India and is enforced by the pollution control boards. Under it, an industry needs consent from the board before it sets up a unit or discharges effluent into a river, a sewer or land.
• Namami Gange (2014) works in the Ganga basin. It mainly builds STPs and checks industrial outflow.

Don't confuse with

• BOD vs DO: Dissolved oxygen (DO) is the oxygen present in water, and high DO (5 mg/l or more) means healthy water [4]. BOD is the oxygen demanded by microbes, and high BOD (above 3 mg/l) means polluted water [4].
• Sewage (organic) pollution vs heavy-metal pollution: Organic pollution raises BOD, and microbes can break it down. Heavy metals (lead, mercury, cadmium, chromium) do not break down and do not show up in BOD. Instead they build up in fish and people.
• Surface water pollution vs geogenic groundwater contamination: surface pollution comes mostly from human waste and effluents. Arsenic and fluoride in groundwater come mainly from the rocks and soil (geogenic). Nitrate in groundwater comes from fertilisers and sewage.
• Water-borne disease vs fluorosis: water-borne diseases (diarrhoea, intestinal worms, hepatitis, typhoid, cholera) are caused by germs. Fluorosis (mottled teeth and bent, stiff bones) is caused by a chemical, excess fluoride, and not by germs.

Prelims Hooks

• Bathing water standard: BOD 3 mg/l or less, DO 5 mg/l or more, pH 6.5–8.5, and faecal coliform 500 MPN/100 ml desirable, 2,500 maximum [4].
• CPCB 2022 found 311 polluted stretches on 279 rivers, down from 351 in 2018 [2]. Priority I means BOD above 30 mg/l. Priority V means BOD of 3–6 mg/l and has the most stretches (145) [2].
• Most polluted stretches: Maharashtra (55), then Madhya Pradesh (19), then Bihar and Kerala (18 each), then Karnataka (17) [2].
• River–city pairs: Musi–Hyderabad, Cooum–Chennai, Gomti–Lucknow, Sabarmati–Ahmedabad. Contaminant–region pairs: arsenic → West Bengal, Bihar, Assam; fluoride → Rajasthan, Telangana, Gujarat.
• Urban sewage (2021): 72,368 MLD produced, 36,668 MLD capacity, and only about 28% (20,235 MLD) actually treated [3].
• Trap: NCERT names industry as the biggest human source of water pollution. But CPCB data show untreated urban sewage as the main cause of polluted river stretches today. Also, the Water Act 1974 came before the Air Act (1981) and the Environment (Protection) Act (1986), and its key tool is consent to discharge.

Mains Points

• Causes and pattern (GS-I/GS-III): polluted stretches cluster below big cities and industrial belts, such as the Yamuna at 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 [3], and only 578 of the 900 working STPs with known status met the norms [3]. Industrial effluents, farm runoff and crowds at religious fairs add to the load.
• Health and regional effects: WHO estimates that about one-fourth of India's communicable diseases are water-borne. Arsenic (221 districts [5]) and fluoride (370 districts [6]) in groundwater cause long-term disease such as skin lesions, cancer and fluorosis. Nitrate causes "blue baby syndrome" in infants. The rural poor in the Ganga plains and in dry western India are hit hardest.
• Policy response and gaps: the Water Act 1974 consent system was weakened by poor enforcement. NCERT (1997) noted 251 polluting industries along rivers and lakes. Namami Gange (2014) shifted the focus to building STPs, and polluted stretches fell from 351 (2018) to 311 (2022) [2]. Still needed:
• run existing STPs at full capacity and to the norms
• reuse treated sewage
• monitor effluents in real time
• control farm runoff and pollution at religious fairs

Related concepts

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

Read more

• Water pollution: sources, measurement and health