Treatment process
Coagulation and flocculation
The first chemical step in conventional surface water treatment. A coagulant neutralises the electrical charge that keeps fine particles suspended, allowing them to collide and grow into settleable flocs.
Key facts
- Scale
- Municipal
- Treatment stage
- Clarification
How it works
A metal salt — usually aluminium sulphate or ferric chloride — is dosed and rapidly mixed. Charge neutralisation destabilises colloids. Slow stirring then lets particles collide and aggregate into flocs large enough to settle or be filtered.
What it removes — and what it does not
| Row | Removes | Does not remove |
|---|---|---|
| 1 | Suspended solids and turbidity | Dissolved salts |
| 2 | Natural organic matter, which reduces later disinfection by-product formation | Nitrate |
| 3 | Colour | Most dissolved organic micropollutants |
| 4 | A substantial fraction of pathogens bound to particles | Short-chain PFAS |
| 5 | Phosphate, iron, manganese and arsenic at appropriate pH | — |
Trade-offs
- Consumes alkalinity, so low-alkalinity water needs supplementation
- Produces sludge requiring disposal
- Leaves a residual aluminium concentration that is itself regulated
- Dose must be optimised continuously as raw water quality changes
Related intelligence
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Sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- NSF International / American National Standards Institute — NSF/ANSI drinking water treatment unit standards. Public but restricted · Publisher terms — public access, reuse not clearly granted
- American Water Works Association — AWWA standards, water audits and utility benchmarking. Public but restricted · Publisher terms — public access, reuse not clearly granted