Environmental Pollution and Control
Exam focus
Sources and effects of air, water, soil, noise and thermal pollution, plus the working principles of control devices.
Core rules
- Pollutants are primary (emitted directly, e.g. CO, SO₂) or secondary (formed in the atmosphere, e.g. ground-level ozone, PAN, sulphuric acid mist).
- London smog is reducing, formed from smoke, SO₂ and fog in winter. Los Angeles photochemical smog is oxidising, formed from NOx, VOCs and sunlight in summer.
- Water pollution is measured by DO, BOD and COD. BOD is the oxygen consumed by microbes decomposing organic matter; COD is the oxygen equivalent of chemically oxidisable matter. COD is generally greater than BOD.
- Eutrophication: nutrient enrichment → algal bloom → decay → oxygen depletion → fish kill.
- Control devices: cyclone separator for coarse particles, electrostatic precipitator for fine particles, bag filter for fine dry particles, wet scrubber for gases and particles.
- Noise is measured in dB(A); prolonged exposure above 85 dB(A) risks hearing damage.
Worked example
A water sample has an initial DO of 8 mg/L. After 5 days at 20°C, DO is 3 mg/L. BOD₅ = 8 − 3 = 5 mg/L. A nearby sample with BOD₅ 30 mg/L is more organically polluted. BOD measures the oxygen demand of biodegradable organic matter over five days.
Traps
- Reporting BOD as COD or vice versa.
- Saying photochemical smog forms in winter fog.
- Confusing an electrostatic precipitator with a scrubber; an ESP removes particles, not gases well.
- Forgetting that thermal pollution lowers DO and changes species composition.