Water treatment plants

The water treatment plant, pumping station and storage lakes at Te Mārua 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 Mārua treatment plant is also stored in the Stuart Macaskill lakes.

Te Mārua 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 Mārua accounts for just over 40 percent of our total water supply volume in a typical year.

Te Mārua treatment process

1. 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.
2. 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.
3. 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).
4. 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.
5. Once the coagulant chemical has been added, time is needed for the contaminants and dirt to bump and stick together, 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.
6. 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.
7. 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.
8. 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.
9. The pH of the treated water is adjusted again using caustic soda, to protect our pipe network from corrosion, and fluoride is added for dental protection.

For more information, check out Greater Wellington Regional Council's PDF

Te Mārua Water Treatment Plant Technical Information

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:

1. Water is drawn from the rivers/creeks at weirs. These intakes have screens at their openings, which act like a sieve, and stop any rocks, sticks and other large debris from entering into the treatment plant.
2. 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).
3. 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.
4. Once the coagulant chemical has been added, time is needed for the contaminants and dirt to bump and stick together, 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.
5. 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.
6. 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.
7. 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.
8. The pH of the treated water is adjusted again using lime, to protect our pipe network from corrosion, and fluoride is added for dental protection (in Wainuiomata and at Te Marua).

For more information, check out Greater Wellington Regional Council's PDF

Wainuiomata Water Treatment Plant Technical Information

Eight wells supply water from the Waiwhetu aquifer, beneath Lower Hutt, to GWRCs Waterloo Water Treatment Plant, which was commissioned in 1981.

Waterloo has a maximum production capacity of 115 ML/d, but typically produces about 60 ML/d or roughly 40 percent of our total annual supply.

Water treated at Waterloo supplies Lower Hutt (apart from Manor Park and Stokes Valley, which are supplied from Te Marua) and, mixed with water from Wainuiomata, also supplies Wellington's business district and southern and eastern suburbs.

Waterloo treatment process:

1. The treatment process at the Waiwhetu aquifer is slightly different than the rivers, as it is an underground zone of water-holding sand, gravel and boulders beneath the Hutt Valley. Water takes more than 12 months to pass through the aquifer to our wells and is naturally filtered while underground.
2. Waterloo treatment plant draws water from eight bores located along the “Knights Road spine” collectively known as the Waterloo wellfield bores.
3. Once the bores extract the water it is then pumped to an Ultra Violet (UV) treatment unit at Waterloo treatment plant*.
   *The Drinking Water Standards NZ recognises three classes of microorganisms that may cause disease: bacteria, viruses, and protozoa. UV treatment manages the protozoa risk, and if delivered in a high enough does can also treat bacteria and viruses, whereas chlorine on its own only manages bacteriological and viral risks. A combination of both treatment measures provides a multiple barrier protection system. Chlorination of the water drawn from the Waiwhetu Aquifer began after E-coli were detected in the Waterloo wellfield bores in December 2016.
4. Once the water has been treated by UV at Waterloo it is aerated to remove carbon dioxide and lime is 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).
5. Fluoride and chlorine are then added. The amount of fluoride is measured and dosed to target a range from 0.7mg/L to 1.0mg/L. The amount of Chlorine is measured and dosed to target a minimum residual of 0.2 parts per million at the far end of our distribution system.

For more information, check out Greater Wellington Regional Council's PDF

Waterloo Water Treatment Plant Technical Information

Gear Island water treatment plant sits beside the Hutt River at the eastern end of the Petone foreshore. The plant was commissioned in 1935 and, like Waterloo, treats water from the Waiwhetu aquifer.

Gear Island is able to produce 27 ML/d from 3 bores located at the edge of the Shandon Golf Course, but since 1999 it has been used as a standby treatment facility and is typically only run on one or two days each month, to maintain operational readiness.

Gear Island Water Treatment Plant also operates as a boost plant supplementing the supply to Wellington's business district and southern and eastern suburbs.

Gear Island treatment process

1. The treatment process at the Waiwhetu aquifer is slightly different than the rivers, as it is an underground zone of water-holding sand, gravel and boulders beneath the Hutt Valley. Water takes more than 12 months to pass through the aquifer to our wells and is naturally filtered while underground.
2. Gear Island draws water from three bores along the Shandon Golf Course.
3. Once the bores extract the water it is then pumped to Gear Island treatment plant where the water at Gear Island is aerated to remove carbon dioxide and caustic soda is added to adjust the pH and alkalinity of the water.
4. Fluoride and chlorine are then added. The amount of fluoride is measured and dosed to target a range from 0.7mg/L to 1.0mg/L. The amount of chlorine is measured and dosed to target a minimum residual of 0.2 parts per million at the far end of our distribution system.

Since 1999 the Gear Island water treatment plant has only been used as a standby facility and is typically only run on one or two days each month, to maintain operational readiness.

Greater Wellington Regional Council has approved the permanent chlorination of Lower Hutt drinking water. The Council voted in favour of a recommendation from Wellington Water, based on our Waterloo Wellfield Water Quality Contamination Investigations Report.

 See more here