The Wainuiomata water treatment plant receives water from the Wainuiomata and Orongorongo rivers and three smaller creeks (Georges, Big Huia, and Little Huia). The plant's design capacity is 60 million litres per day (ML/d), but production is typically around 30 ML/d or just under 20 percent of total supply each year.
Wainuiomata is the newest treatment plant in our network, opened in 1993. It usually supplies water for Wainuiomata and, together with the Waterloo Water Treatment Plant, Wellington's business district and the city's southern and eastern suburbs.
Wainuiomata treatment process:
- Water is drawn from the rivers/creeks at weirs. These intakes have screens at their openings, which acts like a sieves, and stops any rocks, sticks and other large debris from entering into the treatment plant.
- Once the water has been strained, carbon dioxide and lime are added to adjust the pH and alkalinity of the water (this is because the water in the Wellington Region is 'soft' and the lime and carbon dioxide help reduce the corrosive effect of the water on pipes and fittings).
- The contaminants and dirt particles in the river water are very small and are negatively charged, which causes them to repel each other. To help counter this, the water is then coagulated. Coagulation is the process where positively charged chemicals are mixed into the water, which works to attract the contaminants and dirt, rather like a magnet. Coagulation reactions enable us to then better separate any extra dirt in the water.
- Once the coagulant chemical has been added, time is needed for the contaminants and dirt to bump and stick together, so forming clumps. This process is helped along using 5 Flocculation chambers. Water in the flocculation chambers is gently mixed with large paddles to help the clumps form, called 'flocs'. They often look like tiny tufts of brown cotton wool. These flocs are allowed to increase in size up to an optimum point so they do not become heavy enough to settle out of the water, so they can be easily removed by separation and filtration.
- To separate the water from the floc particles, both flow into, a clarification process called dissolved air flotation. This involves injecting pressurised water containing dissolved air into the water. The air comes out of this solution in the same way that carbon dioxide gas is released from a bottle of soft drink when you unscrew the cap. Millions of tiny air bubbles rise, clinging to the floc and lifting them to the surface where they form a ‘scum’ and can be scraped off into a waste pipe.
- The clarified water is then passed down through filter beds containing, sand. Any remaining floc* is trapped in the filter beds.
*Floc from the filters and clarifier tanks is passed through a waste treatment process, which separates the liquid and solid components and concentrates the solid waste for disposal in landfills.
- Filtered water is then disinfected by adding a small quantity of chlorine are then added. Chlorine guards against the risk of treated water becoming contaminated within the distribution system (the drinking water network) and is a precaution against the slim possibility of contaminants getting through the treatment process. The amount of chlorine used is measured and dosed to target a minimum residual of 0.2 parts per million at the far end of our distribution system.
- The pH of the treated water is adjusted again using lime, to protect our pipe network from corrosion, and flouride is added for dental protection (in Wainuiomata and at Te Marua).
For more technical information on the treatment process download Greater Wellington Regional Council's PDF below.