Water Pollution: Sources and Treatment Basics For students.

Water pollution is the contamination of rivers, lakes, groundwater, or seas by sewage, industrial chemicals, farm runoff, or waste, making the water unsafe for drinking, farming, fishing, or the life within it. Since less than 1% of Earth's water is easily accessible fresh water, polluting it is like poisoning a very small, shared well.

It matters because dirty water spreads cholera, typhoid, and hepatitis, kills fish and aquatic ecosystems, and raises treatment costs for every city downstream. Engineers fight it with treatment plants that clean sewage and industrial effluent before release.

Key Concepts

• BOD (Biochemical Oxygen Demand): oxygen microbes need to break down organic waste, in mg/L. High BOD means heavily polluted water. Typical raw sewage: 200–300 mg/L.
• COD (Chemical Oxygen Demand): oxygen needed to chemically oxidise all pollutants, including non-biodegradable ones. COD is always ≥ BOD.
• Dissolved Oxygen (DO): oxygen actually present in water, in mg/L. Fish need at least 4–5 mg/L; below ~2 mg/L they suffocate.
• Eutrophication: excess nutrients (nitrogen, phosphorus from fertilisers/sewage) trigger algal blooms; when algae die, decomposition strips oxygen and kills aquatic life.
• Point vs non-point sources: point = one pipe (factory outlet); non-point = spread out (farm runoff, city stormwater) — much harder to control.

How It Works

• Sewage treatment has stages: screening removes rags and plastics; primary settling removes suspended solids; secondary (activated sludge) uses bacteria to eat dissolved organics, cutting BOD by 85–95%.
• Tertiary treatment removes nutrients, pathogens, and fine particles via filtration, chlorination/UV, and nutrient removal.
• Industrial ETPs (Effluent Treatment Plants) add chemical steps: neutralisation of acids/alkalis, coagulation-flocculation, and sometimes reverse osmosis for zero liquid discharge.
• Cleaned water must meet discharge standards (e.g. BOD below 30 mg/L for inland surface water in India) before release.

Worked Example

Raw sewage enters a treatment plant with BOD 250 mg/L and leaves with BOD 20 mg/L. Calculate the BOD removal efficiency, step by step.

1. BOD removed = 250 − 20 = 230 mg/L.
2. Efficiency = (230 / 250) × 100 = 92%.
3. Treated BOD of 20 mg/L is below the 30 mg/L discharge limit, so the plant complies. If efficiency fell to 80%, outlet BOD would be 50 mg/L — a violation, and a sign the bacterial process needs attention.

Common Misconceptions

• "Clear water is clean water." Dissolved chemicals, heavy metals, and pathogens are invisible. Only lab tests (BOD, COD, coliform count) prove safety.
• "Dilution solves pollution." Rivers do dilute waste, but heavy metals and plastics do not break down — they accumulate in sediments and food chains.
• "Boiling fixes all water problems." Boiling kills germs but does NOT remove dissolved lead, arsenic, fluoride, or pesticides. Different pollutants need different treatment.

Real-World Applications

• Cities: sewage treatment plants (STPs) protect rivers — Delhi's Yamuna cleanup depends on them working at full capacity.
• Industry: textile dyeing units use ETPs with reverse osmosis to achieve zero liquid discharge in Tiruppur.
• Drinking water: municipal plants combine coagulation, filtration, and chlorination to make tap water safe.
• Agriculture: buffer strips and controlled fertiliser dosing cut nutrient runoff that causes algal blooms in lakes.

Water pollution is measured with BOD, COD, and dissolved oxygen, and treated in stages — screening, settling, bacterial digestion, then disinfection — with industries adding chemical treatment. Remember the key numbers: healthy water keeps DO above 4 mg/L, and treated effluent must leave BOD below about 30 mg/L before it meets a river.