Water pollution

Sources of water pollution

• Domestic sewage: organic matter, pathogens, detergents (phosphates).
• Industrial effluents: heavy metals (mercury, lead, chromium from tanneries), acids, dyes, oil.
• Agricultural runoff: fertilisers (nitrates, phosphates) and pesticides.
• Thermal pollution: hot water from power plants lowers oxygen in rivers.
• Oil spills, plastics and mining waste.

India’s river-cleaning efforts include the Ganga Action Plan (started 1985–86) and Namami Gange (2014).

Water-quality indicators

• pH: acidity or alkalinity. The Indian drinking water standard (BIS IS 10500) range is 6.5–8.5.
• Dissolved oxygen (DO): oxygen dissolved in water, in mg/L. Clean, cool water holds roughly 8–10 mg/L; most fish struggle when DO falls below about 4–5 mg/L. Warm water holds less oxygen.
• Biochemical oxygen demand (BOD): the amount of oxygen used by microbes to break down organic matter in water, usually measured over 5 days at 20 °C (BOD₅). High BOD = much organic pollution.
• Chemical oxygen demand (COD): oxygen needed to oxidise organic (and some inorganic) matter chemically, using a strong oxidant such as potassium dichromate. COD covers material microbes cannot break down, so COD is normally higher than BOD, and the test takes only a few hours.
• Coliform bacteria (e.g. *E. coli*) indicate faecal contamination.
• Turbidity, hardness, total dissolved solids and nitrates are also tested.

BOD worked example: a 10 mL sample is diluted to 300 mL. DO at the start = 8.0 mg/L; after 5 days = 5.0 mg/L. BOD = (DO start − DO end) ÷ (sample volume ÷ total volume) = (8.0 − 5.0) ÷ (10/300) = 3.0 × 30 = 90 mg/L.

Key points

• High BOD → low DO → fish die.
• COD ≥ BOD for the same sample.
• BOD₅: 5 days, 20 °C.
• Drinking water pH 6.5–8.5.
• Coliforms show sewage contamination.

Exam tips

Remember the inverse link between BOD and DO. Calculation questions use the dilution formula; check that you divided by the dilution fraction.

Common mistakes

• Saying high BOD means clean water; it means heavy organic pollution.
• Thinking COD is lower than BOD.
• Forgetting that warm water holds less dissolved oxygen.