How a water treatment plant works, unit by unit

From intake to tap: screening, aeration, coagulation, flocculation, sedimentation, filtration and disinfection.

Raw water from a river or lake carries silt, colloids, colour, taste and microbes. A conventional treatment plant removes them in a fixed order.

1. Intake and screening

Coarse and fine screens keep out floating debris, leaves and fish.

2. Aeration

Spraying or cascading the water adds oxygen and strips out dissolved gases. It removes tastes and odours and oxidises dissolved iron and manganese so they can settle.

3. Coagulation

Very fine particles carry negative charges that keep them apart. A coagulant, usually alum (Al₂(SO₄)₃·18H₂O), neutralises the charge. It is added in a flash mixer with a detention time of about 30 to 60 seconds. The optimum dose is found with a jar test.

4. Flocculation

Slow, gentle stirring for 10 to 30 minutes lets the destabilised particles collide and grow into visible flocs.

5. Sedimentation (clarification)

Water moves slowly through a settling tank for about 2 to 4 hours, and the heavy flocs settle. The key design value is the surface overflow rate, Q / plan area.

6. Filtration

Filter	Rate	Cleaning
Slow sand	0.1 to 0.2 m³/m²/h	Scrape top layer (schmutzdecke)
Rapid sand	5 to 7.5 m³/m²/h	Backwash with water and air

Rapid sand filters are standard after coagulation; slow sand filters work by a biological layer and suit small, clear sources.

7. Disinfection

Chlorine kills pathogens. Enough is added to pass the breakpoint, so that a free residual remains. CPHEEO recommends at least 0.2 mg/L free residual chlorine at the consumer's tap.

8. Storage and distribution

Treated water goes to a clear water reservoir and is pumped to overhead tanks and the distribution network.