Water quality is the chemical, physical and biological characteristics that determine whether water is fit for drinking, irrigation or aquatic life. CBSE Class 12 and NEET test this in the environmental issues chapter.
Core Concepts
Sources of water pollution
Domestic sewage, industrial effluent, agricultural runoff (fertilizers, pesticides), oil spills, thermal pollution from power plants, eroded sediment from construction sites.
Point sources: Pollution from a single, identifiable location — a factory drain, sewage pipe, or oil tanker leak. Easier to monitor and regulate.
Non-point sources: Diffuse pollution from large areas — agricultural runoff carrying fertilizers and pesticides, urban stormwater carrying oil and chemicals, atmospheric deposition. Harder to control.
Types of pollutants:
| Type | Example | Effect |
|---|---|---|
| Organic matter | Sewage, food waste | Increases BOD, depletes oxygen |
| Nutrients | Nitrates, phosphates (fertilizers) | Eutrophication, algal blooms |
| Heavy metals | Mercury, lead, cadmium | Bioaccumulation, neurological damage |
| Pathogens | E. coli, Vibrio cholerae | Waterborne diseases |
| Pesticides | DDT, BHC | Biomagnification, endocrine disruption |
| Thermal | Hot water from power plants | Reduces dissolved oxygen, harms aquatic life |
Biological Oxygen Demand (BOD)
The amount of oxygen required by microbes to decompose organic matter in water. High BOD means lots of organic pollution. Clean water has BOD below 3 mg/L; sewage can be 300 mg/L or more.
How BOD is measured: A water sample is sealed in a dark bottle at 20°C for 5 days (called BOD). The dissolved oxygen (DO) is measured before and after. The difference is the BOD.
What BOD tells us: Higher BOD means more organic pollutants that bacteria are consuming oxygen to break down. When DO drops below 4-5 mg/L, fish start dying. Below 2 mg/L, only anaerobic organisms survive — producing foul-smelling hydrogen sulphide and methane.
Chemical Oxygen Demand (COD)
Total oxygen needed to chemically oxidise all organic and inorganic matter. Always higher than BOD. Measures total pollution load including non-biodegradable substances.
BOD vs COD: BOD measures only biodegradable organic matter (what bacteria can eat). COD measures everything that can be chemically oxidised — including non-biodegradable organics (plastics, pesticides) and some inorganic substances. COD is always BOD. The ratio BOD/COD indicates biodegradability: a high ratio means most pollutants are biodegradable (sewage), a low ratio means many are not (industrial effluent).
Eutrophication
Excess nutrients (especially nitrates and phosphates) cause algal blooms. Algae die, decomposers use up oxygen, fish and other aquatic life suffocate. Common in lakes downstream of farm runoff.
Stages of eutrophication:
Nitrates and phosphates from fertilizers, sewage, or detergents enter a water body.
Excess nutrients cause rapid growth of algae (and sometimes cyanobacteria). The surface turns green.
The dense algal mat blocks sunlight from reaching submerged plants, which die.
Dead algae and plants are decomposed by aerobic bacteria, which consume dissolved oxygen rapidly.
Dissolved oxygen drops to near zero (hypoxia/anoxia). Fish and aerobic organisms die. Only anaerobic bacteria survive, producing HS (rotten egg smell) and CH.
Natural vs cultural eutrophication: Natural eutrophication takes thousands of years as nutrients gradually accumulate. Cultural (human-caused) eutrophication happens in decades due to fertilizer runoff and sewage discharge.
Biomagnification
The concentration of persistent pollutants (like DDT, mercury) increases at each trophic level of the food chain. A pollutant at 0.003 ppm in water can reach 25 ppm in fish-eating birds — a 10,000× concentration.
Classic example: DDT in a lake ecosystem:
- Water: 0.003 ppm
- Phytoplankton: 0.04 ppm
- Zooplankton: 0.5 ppm
- Small fish: 2 ppm
- Large fish: 10 ppm
- Fish-eating birds: 25 ppm
This is why the bald eagle population crashed in the 1960s — DDT caused eggshell thinning. And why Minamata disease devastated Japanese fishing communities — methylmercury in fish reached toxic levels.
Drinking water standards
WHO and BIS set limits on turbidity, pH, hardness, fluoride, arsenic, lead, coliform bacteria. Indian standard IS 10500 specifies acceptable limits for each parameter.
| Parameter | Acceptable limit (IS 10500) |
|---|---|
| pH | 6.5 - 8.5 |
| Turbidity | <5 NTU |
| Fluoride | <1.0 mg/L (excess causes fluorosis) |
| Arsenic | <0.01 mg/L |
| Lead | <0.01 mg/L |
| Coliform bacteria | 0 per 100 mL (for drinking) |
| Total dissolved solids | <500 mg/L |
Water treatment
Primary — screens and sedimentation. Secondary — biological degradation by aerobic bacteria (activated sludge). Tertiary — chemical or advanced treatment. Disinfection with chlorine or UV at the end.
Sewage treatment in detail:
Primary treatment (physical): Screening removes large objects. Grit chambers settle sand and gravel. Primary settling tanks allow suspended solids to settle as primary sludge. Removes ~60% of suspended solids.
Secondary treatment (biological): The primary effluent is aerated in the presence of aerobic bacteria (activated sludge process) or passed over a trickling filter. Bacteria decompose the dissolved organic matter. BOD is reduced by 85-90%. Settled sludge from secondary clarifier is partly recycled as “return sludge.”
Tertiary treatment (advanced): Removes nutrients (N, P), heavy metals, and remaining organic compounds. Techniques include chemical precipitation, membrane filtration, activated carbon adsorption. Not always done due to high cost.
Disinfection: Chlorination ( or NaOCl) kills remaining pathogens. Residual chlorine of 0.2 mg/L ensures safety in distribution. UV treatment is an alternative — effective but leaves no residual.
Worked Examples
Bloom → death → decomposition → oxygen depletion → fish kill. The oxygen consumption of dead algae is what does the damage, not the algae themselves.
Mercury from industrial effluent enters water, is taken up by microbes, concentrated in fish, then in humans who eat fish. Minamata disease in Japan was the textbook case.
Initial DO = 9 mg/L. After 5 days at 20°C, DO = 3 mg/L. BOD = 9 - 3 = 6 mg/L. This indicates moderate pollution (clean water has BOD < 3).
Common Mistakes
Saying BOD and COD are the same. COD is always higher because it includes non-biodegradable matter.
Writing that eutrophication is caused by algae. It is caused by nutrients; the algae are a symptom.
Confusing primary and secondary treatment. Primary is physical, secondary is biological.
Confusing bioaccumulation and biomagnification. Bioaccumulation is the buildup of a substance in one organism over time. Biomagnification is the increase in concentration at each successive trophic level of the food chain.
Thinking all bacteria in water are harmful. Most are harmless decomposers essential for secondary sewage treatment. Only specific pathogens (E. coli, V. cholerae) cause disease.
Exam Weightage and Revision
NEET 2023 asked about eutrophication stages. NEET 2022 tested BOD definition. CBSE boards ask about water treatment steps as a five-mark question. Environmental biology gives 2-3 NEET questions per year.
When a question gives a scenario, identify the core mechanism first, then match it to the concepts above. Most wrong answers come from reading the scenario too quickly.
Three numbers to lock — BOD under 3 for clean water, coliform zero for drinking, chlorine 0.2 mg/L residual.
Practice Questions
Q1. Why is BOD measured at 20°C in the dark?
20°C is a standardised temperature for consistent comparisons. The dark prevents photosynthesis by algae, which would produce oxygen and give a falsely low BOD reading. The 5-day period allows most biodegradable organics to be consumed by bacteria.
Q2. What is the activated sludge process?
Primary effluent is vigorously aerated in large tanks. Aerobic bacteria, fungi, and protozoa (collectively the “activated sludge”) decompose dissolved organic matter. After treatment, the mixture is settled. Clear water goes for tertiary treatment or disinfection. Settled sludge is partly recycled to maintain the microbial population in the aeration tank.
Q3. Why is DDT a biomagnification concern even though it was banned decades ago?
DDT is persistent — it breaks down very slowly in the environment (half-life of 15+ years in soil). It is also lipophilic (fat-soluble), so it accumulates in fatty tissues rather than being excreted. These properties mean DDT from decades ago is still present in ecosystems and still biomagnifying through food chains.
Q4. What causes fluorosis and how is it related to water quality?
Fluorosis is caused by excess fluoride in drinking water (above 1.5 mg/L over prolonged periods). Dental fluorosis causes mottled, discoloured teeth. Skeletal fluorosis causes joint pain and bone deformity. Parts of Rajasthan, Andhra Pradesh, and Gujarat have naturally high fluoride levels in groundwater.
Q5. Differentiate between point and non-point sources of pollution.
Point source: Single, identifiable discharge location — factory drain pipe, sewage outfall. Easy to monitor and regulate. Non-point source: Diffuse pollution from a wide area — fertilizer runoff from farms, urban stormwater, atmospheric deposition. Harder to identify and control.
FAQs
What is a dead zone in the ocean? A dead zone is a hypoxic area (very low dissolved oxygen) where most aquatic life cannot survive. The largest dead zone is in the Gulf of Mexico, caused by nutrient runoff from the Mississippi River basin. Agricultural fertilizers cause massive algal blooms; when the algae die and decompose, oxygen is depleted over thousands of square kilometres.
Is boiled water always safe to drink? Boiling kills pathogens but does not remove chemical pollutants like heavy metals (arsenic, lead), fluoride, or pesticides. Water from a chemically polluted source is not made safe by boiling alone — it needs filtration or chemical treatment.
Why do we chlorinate water if UV treatment is available? Chlorine provides a residual — it stays active in the water as it travels through pipes to your tap, killing any bacteria that enter during distribution. UV treatment is effective at the treatment plant but has no residual effect. Many treatment plants use both.
Water quality is where environmental chemistry meets public health. A village with polluted water has more disease and less productivity — the link is direct.