Te Marua
Te Marua Water Treatment Plant
Our water treatment plant, pumping station and storage lakes at Te Marua were built between 1980 and 1987 to treat water taken from the Hutt River at Kaitoke. The plant has a design capacity of 140 million litres daily, but treats around 60 milllion litres per day on average.
Te Marua usually supplies Upper Hutt, Manor Park, Stokes Valley, Porirua and the western suburbs of Wellington, as far south as Karori. Production from Te Marua accounts for just over 40 percent of our total supply volume in a typical year. A pdf of the technical information handout is available.
The treatment process involves coagulation, flocculation, separation, filtration and disinfection.
As river water enters the treatment plant, carbon dioxide and lime are added to adjust the pH and alkalinity of the water. This makes the following treatment processes more effective and reduces the corrosive effect of the water, which is 'soft', on the pipes and fittings within the plant.
Coagulation
The contaminants and colour particles in the river water are very small and are negatively charged, which causes them to repel each other. Coagulation involves mixing a postively charged chemical into the water, which works by attracting the contaminants and dirt to it, rather like a magnet attracts iron filings. Coagulation reactions take less than a second.
Flocculation
Once the coagulant chemical has been added, time is needed for the contaminants and dirt to bump into the coagulant and stick to it, so forming clumps. Water in the flocculation chambers is gently mixed with large paddles to help this occur. The clumps that form, called 'flocs', look like tiny tufts of brown cotton wool. They are allowed to increase in size until the point that they are heavy enough to settle out of the water, so they can be easily removed by separation and filtration.
Separation and filtration
Water containing floc particles is piped to the centre of a large circular tank, called a clarifier, which has a wall dividing it into inner and outer rings. The wall goes from the top water level to about two-thirds of the depth of the tank. The water containing floc sinks in the inner circle and the floc particles settle on the floor of the tank. The water then passes under the bottom of the wall and rises, minus most of the floc particles, in the outer ring of the tank. This partially-clean water is then passed down through filter beds containing layers of gravel, sand and anthracite coal. Any remaining floc is trapped in the filter beds.
A mechanical scraper rotates slowly around the floor of the clarifier, sweeping the floc particles into a waste pipe. The filters are also cleaned at regular intervals to prevent them being clogged by floc particles.
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.
Disinfection
Filtered water is disinfected by adding a very small quantity of chlorine. Chlorine guards against the risk of treated water becoming contaminated within the distribution system and is a precaution against the slim possibility of contaminants getting through the treatment process. The amount of chlorine used is measured to ensure there is 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, to protect our pipe network from corrosion, and fluoride is added for dental protection.