Knowledge Organiser: Water Treatment
Key Terms
- Potable water: Safe to drink — not the same as pure water
- Sterilisation: Killing microorganisms (e.g. by chlorination)
- Distillation: Heating to steam then condensing — removes all dissolved substances but uses lots of energy
- Desalination: Removing salt from seawater — by distillation or reverse osmosis
- Sedimentation: Particles settle under gravity in tanks
Must-Know Facts
- 4 stages: Screening → Sedimentation → Filtration → Chlorination (SSFC)
- Chlorination kills bacteria and pathogens
- Potable water ≠ pure water (still has dissolved minerals)
- Desalination is expensive due to high energy use
- Sewage treatment: Primary (physical) → Secondary (biological) → Tertiary (chemical, HT)
- Sludge from sewage treatment produces methane gas for energy
Key Equations
- No calculation equations — descriptive/process topic
- Chlorination: Cl₂ + H₂O → HCl + HClO (kills microorganisms)
- Water treatment order: Screening → Sedimentation → Filtration → Chlorination
Common Mistakes
- Saying potable water is pure water: Potable water is SAFE TO DRINK — it still contains dissolved minerals and is NOT chemically pure
- Getting the treatment stages in the wrong order: The correct order is Screening → Sedimentation → Filtration → Chlorination — mixing up filtration and sedimentation loses marks
- Saying distillation is the normal water treatment method: Distillation produces pure water but uses too much energy for everyday water supply — chlorination is used for potable water treatment
- Forgetting why desalination is limited: Desalination is not widely used because it requires very high energy input — it is only cost-effective in water-scarce regions with cheap energy
Memory Aid
- Screening — removes large debris with grids
- Sedimentation — particles settle under gravity
- Filtration — sand and gravel remove smaller particles
- Chlorination — kills remaining microorganisms
- Primary = Physical (screening + settling tanks)
- Biological = Bacteria break down organic matter
- Chemical/Tertiary = Chemical disinfection (Higher Tier)