Q.Dissolved oxygen in water is responsible for aquatic life. What processes are responsible for the reduction in dissolved oxygen in water?

To combat pollution of India's major rivers, the Ministry of Environment and Forests launched the Ganga Action Plan and the Yamuna Action Plan.

Seeing the alarming levels of pollution (from sewage, industrial effluents and religious/social practices) in India's major rivers, the Government of India's Ministry of Environment and Forests initiated:

• Ganga Action Plan (GAP), launched in 1985, aimed at reducing the pollution load on the river Ganga through interception and diversion of sewage, setting up sewage treatment plants, and low-cost sanitation schemes along its course.
• Yamuna Action Plan (YAP), a similar programme targeting pollution abatement of the river Yamuna, particularly the heavily polluted stretch through Delhi.

Note: this topic (river-pollution-control programmes) belongs to the 'Environmental Issues' chapter, which is not part of the current rationalised NCERT syllabus supplied for this platform - it is flagged out-of-syllabus here, though it may still appear in JAC papers that predate that syllabus change.

Organic pollutants in sewage are broken down by microbes that consume dissolved oxygen, so oxygen-depleted water near a sewage discharge point suffocates aquatic animals.

Sewage is rich in biodegradable organic matter. When discharged into a water body, heterotrophic (saprotrophic) microorganisms multiply rapidly and decompose this organic load, consuming large amounts of dissolved oxygen in the process — this oxygen demand is measured as the Biochemical Oxygen Demand (BOD) of the water, and is highest closest to the discharge point. As dissolved oxygen falls sharply near the outfall, fish and other aquatic animals that depend on dissolved oxygen for respiration are unable to survive, causing a spike in mortality in that zone; further downstream, as organic matter is progressively degraded and diluted, oxygen levels and animal life gradually recover. Toxic chemicals/pathogens present in sewage may add to the effect.

BOD measures how much dissolved oxygen microbes consume decomposing organic pollutants; a low BOD (< 5 ppm) means little organic pollution, so the water retains high dissolved oxygen and is considered relatively clean.

Biochemical Oxygen Demand (BOD) is defined as the amount of dissolved oxygen (in ppm, i.e. mg/L) required by microorganisms to biologically oxidise/decompose the organic matter present in a water sample, measured over a standard period of 5 days at 20 degrees C (BOD5).

The logic connecting BOD to pollution is:

• More organic waste (sewage, industrial effluent) in the water → more food for aerobic bacteria → bacteria consume more dissolved oxygen decomposing it → HIGH BOD, and correspondingly LOW leftover dissolved oxygen (bad for aquatic life).
• Less organic waste → bacteria have little to decompose → LOW BOD, and the water RETAINS most of its dissolved oxygen.

So BOD and "how rich the water is in dissolved oxygen" move in opposite directions: a value below 5 ppm is the accepted benchmark for relatively unpolluted/clean water, meaning the sample is rich in dissolved oxygen rather than depleted of it. (Highly polluted water typically shows BOD values well above 17 ppm.)

Abundant E. coli signals sewage (faecal) pollution.

Escherichia coli is a coliform bacterium normally found in the human intestine. Its presence in large numbers in water indicates that the water has been contaminated with faecal matter/sewage and may carry disease-causing pathogens. Coliform count is therefore used as an indicator of sewage pollution and the sanitary quality of drinking water. It does not indicate hardness, industrial pollution or chlorination.

Excess nutrients (nitrates, phosphates) entering water bodies cause algal blooms, oxygen depletion, and a decline in aquatic biodiversity — this process is eutrophication.

When water bodies receive excess nutrients — commonly nitrates and phosphates from agricultural runoff, sewage, and detergents — it triggers rapid, excessive growth of algae and other producers, called an algal bloom. When these organisms die and decompose, decomposers consume large amounts of dissolved oxygen, causing hypoxic ('dead zone') conditions. This oxygen depletion and altered water chemistry causes fish and other aquatic organisms to die off, sharply reducing species diversity in the water body. This process of nutrient over-enrichment followed by biodiversity loss is termed eutrophication.

Healthy water bodies typically show a Dissolved Oxygen level of roughly 4–6 mg/litre; values well below this indicate pollution/organic overload.

Dissolved Oxygen (D.O.) is the amount of free oxygen gas dissolved in water, essential for aquatic organisms (fish, zooplankton, aerobic decomposers) to respire. In a clean, well-oxygenated water body, D.O. levels normally range around 4–6 mg per litre. When a water body receives heavy organic pollution (e.g. sewage, industrial effluents), aerobic microbes that decompose this organic matter consume large amounts of oxygen, causing D.O. to fall sharply — a low D.O. (and correspondingly high Biochemical Oxygen Demand, BOD) is therefore used as an indicator of water pollution.

Eutrophication is the specific term for the process of nutrient over-enrichment of a water body that leads to excessive plant/algal growth, oxygen depletion, and a consequent decline in species diversity.

Reasoning

Eutrophication is the process by which a water body (lake, pond, river) becomes progressively enriched with excess plant nutrients — mainly nitrates and phosphates — typically from agricultural runoff (fertilisers), sewage or detergents. This nutrient enrichment first stimulates excessive growth of algae and other aquatic plants, which — as they die and decompose — leads to depletion of dissolved oxygen, causing a decline in aquatic species diversity (and often fish kills). While an algal bloom (c) is a visible symptom/consequence of eutrophication, and biological magnification (a) is a distinct phenomenon (progressive concentration of a non-biodegradable toxin up a food chain), the overall named process of "nutrient enrichment leading to loss of species diversity" is specifically called eutrophication, making (d) an overreach.

The Damodar river is heavily polluted by coal-mining discharge, industrial effluents and untreated sewage from the coalfield and industrial towns it flows through in Jharkhand.

The Damodar river flows through the coal-rich Dhanbad-Bokaro-Ranchi industrial and mining belt of Jharkhand before entering West Bengal. Decades of coal mining, thermal power plants, steel and other heavy industries situated along its course have discharged large quantities of untreated industrial effluents, coal washery waste and domestic sewage into the river, severely degrading its water quality with heavy metals, suspended solids and organic pollutants. This has earned it the reputation of being one of the most polluted rivers in Jharkhand (and among the most polluted in India), sometimes referred to locally as the 'Sorrow of Bengal/Jharkhand' due to this pollution and past flooding history.

Water polluted with organic waste supports a large population of decomposer microbes that respire aerobically to break down that waste, consuming dissolved oxygen much faster than in clean, unpolluted water — this rate is measured as the Biochemical Oxygen Demand (BOD).

Reasoning

Biochemical Oxygen Demand (BOD) is the amount of dissolved oxygen needed by microorganisms to biologically decompose the organic matter present in a water sample, and is widely used as a measure of the degree of organic pollution of water. Polluted water — typically containing high loads of organic waste from sewage, industrial effluent or agricultural runoff — supports a large, actively respiring population of decomposer bacteria and fungi. These organisms consume dissolved oxygen at a much higher rate while breaking down the abundant organic matter, giving polluted water a high BOD (high rate of oxygen consumption). In contrast, unpolluted, spring, or tap water, having little organic load, has a correspondingly low BOD.

Primary treatment removes solid material from sewage by purely physical means.

Sewage treatment typically proceeds in stages. Primary treatment is the first, purely mechanical/physical stage: raw sewage passes through bar screens to remove large floating debris, then through grit chambers where sand and heavier particles settle out, and finally into large sedimentation (settling) tanks, where suspended solids settle as sludge under gravity and oils/grease are skimmed from the surface. This stage removes a large fraction of suspended solids but does not remove dissolved organic pollutants or pathogens — that biological breakdown of dissolved matter is the job of the subsequent secondary (biological) treatment stage.

Reverse osmosis is used to desalinate brackish water.

Reverse osmosis (RO) is a membrane-based purification technique in which high pressure is applied to force water molecules from a saline/brackish solution through a semi-permeable membrane, against the natural osmotic gradient, while dissolved salts and other solutes are held back by the membrane's fine pores. This yields purified, low-salinity water on one side and a concentrated brine reject on the other, making it the standard technology for desalinating brackish or sea water for drinking and irrigation use. Ion-exchange and adsorption are used for softening/removing specific ions or trace contaminants, while electrodialysis (option d) is a related but distinct electrical-membrane desalination method — reverse osmosis remains the most widely used technique for large-scale desalination.