Treatment

1. Coagulation

In this first step, we add a chemical coagulant to the raw water. These chemicals neutralize the electrical charges of fine suspended particles, allowing them to come together.

After coagulation, the water is gently mixed to encourage the particles (like dirt and other bits) to stick together into large clumps, called flocs. Proper mixing is essential to form strong, settleable flocs.

The water then flows into a sedimentation tank where the settles to the bottom by gravity. This process removes the majority of suspended solids and significantly reduces turbidity. The settled material (sludge) is periodically removed from the tank.

The clarified water passes through pressure filters containing multiple layers of media (such as anthracite, sand, and gravel). These layers trap remaining fine particles that did not settle during sedimentation. This step further reduces turbidity and improves water clarity.

The filtered water is exposed to ultraviolet light. UV radiation inactivates microorganisms such as bacteria, viruses, and protozoa by damaging their DNA, preventing them from reproducing. UV provides effective primary disinfection without adding chemicals.

Finally, chlorine is added to the treated water. Chloring serves two main purposes:

• It provides additional disinfection.
• It maintains a residual disinfectant in the distribution system to prevent microbial regrowth before the water reaches consumers.

Together, these processes ensure the removal of physical impurities and the inactivation of harmful pathogens, producing water that is safe for use and consumption.

For more information on chlorination check out Why we chlorinate | Tararua District Council.

Dannevirke Water Supply also includes fluoridation. For more information on this, check out Fluoridation | Tararua District Council.