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Updated 29 May 2014 9:20am

Wainuiomata Water Treatment Plant

Wainuiomata Water Treatment Plant receives water from the Wainuiomata and Orongorongo Rivers and three smaller creeks. The plant's design capacity is 60 million litres per day, but production is typically around 30 million litres daily 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 Waterloo Water Treatment Plant, Wellington's business district and the city's southern and eastern suburbs.

The treatment process is similar to that used at Te Marua, and includes coagulation, flocculation, separation, filtration and disinfection. The main difference between the two plants is that the floc particles formed at Wainuiomata are smaller (pin floc) so they can be separated out from the water by floating them rather than by gravity.

Direct air floatation

This involves injecting pressurised water containing dissolved air into the water being treated as it leaves the flocculation chambers. 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, which is overflowed into a waste pipe. The clean water then passes through a filter of sand to remove any remaining floc particles. 

Chlorine and fluoride are added to the treated water prior to distribution; chlorine as a safeguard against the treated water being contaminated during distribution and fluoride to promote dental health.