The 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 up to 140 million litres daily (ML/d), but typically treats around 60 ML/d on average.
Water from the Te Marua treatment plant is also stored in the Stuart Macaskill lakes.
Te Marua usually supplies water to Upper Hutt, Manor Park, Stokes Valley, Porirua and the western suburbs of Wellington, as far south as Karori, beyond this it combines with water from the other treatment plants.
Production from Te Marua accounts for just over 40 percent of our total water supply volume in a typical year.
Te Marua treatment process
- Water is drawn from the Hutt River at the Kaitoke Weir. The water at the weir is pre-treated at the strainer house through a sand trap followed by three mechanical strainers which remove leaves, twigs and silt from the water before entering the treatment plant.
- Once the water has been strained it is then directed to be treated down one of two streams: one for smaller quantities of water or two for larger quantities of water.
- Both streams lead to rapid mix tanks. Carbon dioxide and lime are added in the mix tanks 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 called floc. They often 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.
- To separate* the water from the floc particles, both are piped to the centre of a large circular tank, called a clarifier, which has a wall dividing it into inner and outer rings. In the clarifier the clusters of ‘floc’ are heavier and sink to the bottom of the inner ring. The clarifier also has a sloping floor, which assists with gravity separating the solid particles. The walls around the inner-ring of the clarifier go from the top water level to about two-thirds of the depth of the tank. This allows a gap for the water to pass underneath and rise, minus most of the floc particles, in the outer ring of the tank.
The clarified water is then passed down through filter beds containing layers of gravel, sand and activated carbon. 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 in landfills.
- Filtered water is then disinfected by adding a small quantity of chlorine. 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 caustic soda, to protect our pipe network from corrosion, and flouride is added for dental protection.
For more technical information on the treatment process download Greater Wellington Regional Council's PDF below.