Air pollution control
Burning fuel and some factory processes put dust (particulates), sulphur oxides (SOx) and nitrogen oxides (NOx) into the air.
Make less: cleaner fuel, low-sulphur coal, burners that make less NOx, better efficiency.
Catch it: a cyclone spins dust out; a bag filter is like a giant cloth filter; an electrostatic precipitator charges dust so it sticks to plates; a wet scrubber sprays water or limestone slurry to remove SOx; a catalyst turns NOx into harmless nitrogen. A tall chimney only spreads pollution; it does not remove it.
Water pollution control
Factory waste water can carry solids, oil, acids, heavy metals and organic matter. Organic matter is measured as BOD (how much oxygen microbes need to eat it).
Typical steps: (1) screening removes big things; (2) sedimentation lets solids sink as sludge; (3) biological treatment (activated sludge with air bubbles) lets microbes eat organic matter; (4) chemical treatment (neutralise acid, make metals settle); (5) filtration and disinfection. Better still, reuse the treated water inside the factory.
Soil contamination control
Soil can be contaminated by heavy metals, oil, solvents and chemicals from leaks or old dumps. Cleaning is slow, so prevention (no leaks, lined tanks) is best.
Clean-up methods: excavation (dig out and treat or dispose), soil washing, bioremediation (microbes break down oil and solvents), phytoremediation (plants soak up metals), soil-vapour extraction (pull out volatile gases), and containment (cap or wall to stop spreading).
Noise, vibration and odour control
Noise is measured in decibels (dB). Control it at three places: the source (quieter machine, enclosure), the path (barrier wall, distance, trees do little) and the receiver (ear protection, double glazing). In open air, each doubling of distance from a point source lowers sound by about 6 dB.
Vibration travels through the ground and floor. Fix it with rubber or spring mounts, balanced machine parts, and a separate foundation.
Odour comes from gases. Remove it with activated carbon adsorption, scrubbing, burning the gas, a biofilter, or by sealing the source.
Waste management
Industry makes waste such as scrap, slag, sludge, packaging and sometimes hazardous waste. The order of preference is the 3R: Reduce (make less waste), Reuse (use again), Recycle (turn into new material). What cannot be reused or recycled can give energy (waste-to-energy) and the last remainder goes to a properly lined landfill. Hazardous waste needs special storage, labels and licensed treatment. Keeping waste types separate makes recycling much easier.
Try it: sort your own bin
For one day, put your home waste in three boxes: reuse, recycle, rest. Count how much goes to "rest". In the 3D, guess the landfill count before sliding. Also try this: put a thin cloth over a cup, pour muddy water through it, and see what the cloth catches. That is a filter.
Key formulas and definitions
- Removal efficiency (%) = (inlet − outlet) ÷ inlet × 100.
- Sound drops about 6 dB when the distance from a point source doubles (open air).
- Two equal sources add about 3 dB (80 dB + 80 dB ≈ 83 dB).
- 3R: Reduce, Reuse, Recycle.
- Diversion rate (%) = waste kept out of landfill ÷ total waste × 100.
Worked examples
1. Dust before a filter is 500 mg/m³ and after the filter it is 25 mg/m³. Find the efficiency.
(500 − 25) ÷ 500 × 100 = 95%.
2. A chimney gives 20,000 m³ of gas per hour with 200 mg/m³ of dust. How much dust per hour before a filter? What escapes if the filter is 99% efficient?
20,000 × 200 mg = 4,000,000 mg = 4 kg per hour. Escaping: 1% of 4 kg = 0.04 kg (40 g) per hour.
3. Waste water BOD is 300 mg/L before treatment and 30 mg/L after. Find the removal.
(300 − 30) ÷ 300 × 100 = 90%.
4. Machine noise is 80 dB at 5 m. What is it at 10 m (open air, point source)?
Distance doubled, so about 6 dB less: 80 − 6 = 74 dB.
5. A factory makes 24 tonnes of waste. 18 tonnes are reused or recycled. Find the diversion rate and the landfill amount.
Diversion = 18 ÷ 24 × 100 = 75%. Landfill = 24 − 18 = 6 tonnes.
6. A wall lowers the sound at a house from 90 dB by 15 dB. What is the new level? Is it a big change in loudness?
90 − 15 = 75 dB. Yes. Every 10 dB feels about twice as loud or half as loud, so 15 dB is a big drop.
Common mistakes
- Thinking a tall chimney cleans the air. It only spreads the pollution further; filters and scrubbers remove it.
- Putting treated waste in the wrong place: moving pollution from air to water or soil without treating it (for example dumping filter dust).
- Using a barrier wall with gaps. Sound leaks through gaps and the wall helps much less.
- Mixing waste types. One dirty item can spoil a whole recycling batch.