Public drinking water systems use different water treatment methods to provide safe drinking water for their communities. Public water systems often use a series of water treatment steps that include coagulation, flocculation, sedimentation, filtration, and disinfection.

Water Treatment Process

During the water treatment process, raw (untreated) water is pre-treated by dosing with chemicals and then passes to the clarifiers for removal of relatively large particles and impurities in water. Clarified water then flows into filters for filtering out the more finely divided particles. The filtered water is disinfected before supply to the public. A small amount of residual chlorine is maintained in the water to prevent bacterial growth on the rest of its journey. Fluoride is also added for dental protection.

1. Pre-treatment

Various chemicals may be added into raw water as pre-treatment to facilitate the subsequent treatment process:

• Alum – to coagulate impurities

• Hydrated lime – to control pH

• Polyelectrolyte – to facilitate coagulation

• Potassium Permanganate KMnO4 – to facilitate removal of manganese

• Ozone O3 – to oxidise impurities, control algae growth and remove taste and odour

• Powdered activated carbon (PAC) – to remove taste and odour

2. Clarification

To coagulate the particles to form large flocs that can be removed. The technologies of clarification includes:

• Multi-deck sedimentation

• Solids contact clarification

• High Rate Lamella Sedimentation

• Dissolved Air Flotation (DAF)

All flocs are dewatered to produce sludge with ≥30% solid, and the water is recycled.

3. Filtration

To remove the more finely divided suspensions by "Gravity Filtration" or "Biological Filtration".

Biological nitrification process employs the naturally occurred nitrifying bacteria in the raw water to convert ammonia into nitrate under favorable conditions. This process is successfully applied in the biological filtration of water treatment.

4. Disinfection

The filtered water is disinfected by adding chlorine or ozone in contact tank.

Ozone is a powerful disinfectant as well as oxidant. Use of ozone also benefits from its other treatment effects including taste and odour control, oxidation of iron and manganese, and reduction in chlorine dose. Ozone does not leave a disinfectant residual in water and use of chlorine is required for disinfection and to maintain the disinfectant residual in distribution system. Downstream biological filtration process is usually needed for removing the assimilable organic carbon formed due to ozonation to maintain biologically stable treated water.