Environmental pollution: air, water, soil, noise and waste
What this domain tests
Pollution questions test pollutant classification, the measurement used for each medium, and the control route. You should read a BOD value, place an air quality index reading in the right band, apply the residential night noise limit, and name the rule governing a hospital's discarded syringes.
Core concepts
- Air: primary pollutants (SO₂, NOx, CO, particulates) versus secondary (ozone, PAN, acid rain from SO₂ and NOx). PM2.5 is more dangerous than PM10 because it reaches the alveoli. The National Air Quality Index runs 0–50 good, 51–100 satisfactory, 101–200 moderate, 201–300 poor, 301–400 very poor, 401–500 severe.
- Water: dissolved oxygen falls as organic load rises; BOD measures oxygen demanded by microbes over five days at 20 °C. Clean water has BOD under about 3 mg/L; raw sewage runs into the hundreds. Excess nutrients cause eutrophication and algal blooms.
- Soil: pesticide residues, heavy metals, salinisation; remediation by bioremediation and phytoremediation.
- Noise: measured in dB(A) on a logarithmic scale. Ambient limits are 55 dB(A) day and 45 night for residential areas, 75/70 industrial, 65/55 commercial, and 50/40 in silence zones around hospitals and schools.
- Waste: segregation at source, the 3Rs, extended producer responsibility, and separate rules for municipal, biomedical, e-waste, battery and construction waste. Maharashtra's 2018 notification bans most single-use plastic.
Worked example
A dyeing unit discharges 200 m³/day of effluent with BOD 400 mg/L. Convert: 400 mg/L = 400 g/m³. Load = 200 × 400 = 80,000 g = 80 kg of BOD per day. That load, not the concentration alone, sizes the treatment plant, because a dilute but huge flow can carry the same load as a small concentrated one.
Traps that cost marks
Saying noise "adds up" arithmetically - two identical 60 dB sources give about 63 dB, not 120. Treating COD as smaller than BOD; COD is always higher. Swapping the day and night noise limits.