New Plymouth Water Treatment Plant

The water for the New Plymouth Water Treatment Plant is diverted from the Waiwhakaiho River via a tunnel which leads to Lake Mangamahoe. During the summer, water is normally drawn from an intake (called the river intake) near the tunnel outlet. The water then passes through the pipe which runs under the lake and then to the treatment plant. Plant operators use automated valves to control the water takes from either the river intake or the lake intakes in Lake Mangamahoe.

Each intake is fitted with a fish screen to ensure that large debris such as twigs and other organic matter, as well as aquatic life, are prevented from inadvertently entering the water supply.

The lake intake fish screens are fitted with an air burst cleaning system. A large volume of compressed air is forced through the screen to release any trapped material or algae growth from the screen and to keep an even flow to the treatment plant.

The raw water is piped to the plant where it may, depending on raw water conditions, be dosed with carbon dioxide or lime to control the pH to 6.7 following poly aluminium chloride (PACL) addition. This ensures the pH is optimal for coagulation.

Powdered activated carbon (PAC) is also added to remove organic taste and odour causing compounds. A mixing tank ensures enough reaction for lime and PAC.

Following the carbon dioxide and/or lime addition a small amount of a chemical called PACL (polyaluminium chloride) is added. This enables the tiny particles in the water which cause colouration, as well as other suspended particles, to clump together in a process called coagulation, which makes them easier to remove. Lime addition also ensure sufficient alkalinity is available if the natural water has an insufficient level for the coagulation process to work well.

Polyelectrolyte is then added to bind together the clumps of particles resulting from the coagulation process. This binding of coagulated particles into larger particles is called flocculation.

The water flows into the bottom of the clarifiers and rises slowly, allowing the heavier particle clumps to settle out into a sludge (floc) blanket. The floc blanket binds micro-organisms, such as giardia and other protozoa, with the sludge or into filterable floc particles. Tube settlers in clarifier tanks greatly increase their capacity.

The clear water flows out the top of the tanks via the lauders while the sludge is drained off to the sludge lagoons at the front of the plant.

As the water leaves the clarifiers, hydrated lime is added to raise the pH of the water and chlorine is added to precipitate any iron and manganese that is present for removal by the filters.

From the clarifiers, the water travels through channels to the rapid sand filters where it passes downwards through layers of anthracite and sand media which remove any remaining particles. The filters are taken out of service for back washing based either on a scheduled time or alternatively when there is either an increase in turbidity or increase in differential pressure. Backwashing the filter removes the particles trapped in the filter media and the resulting dirty water is drained to the sludge lagoons.

From the filter outlet, the treated water goes into the clear-water tank directly beneath the plant. 

Chlorine gas, hydrated lime and hydrofluorosilicic acid are added to the filtered water leaving the filters. Carbon dioxide gas may also be added.

Chlorine is added as a disinfectant to deactivate any micro-organisms that might be present in the water that is distributed to the reservoirs, and as a safeguard against further contamination of the reticulation system.

Lime is added to raise the pH of the water to 7.6, making the acidity/alkalinity level healthy for consumers and non-corrosive to reticulation and household plumbing.

Carbon dioxide may be added to the water leaving the filter to lower the pH to 7.6.

Hydrofluorosilicic acid adds fluoride to the water as directed by the Ministry of Health.

The final water leaving the water treatment plant is continuously monitored for residual chlorine and pH to ensure that adequate disinfection against contamination of the reticulation system is maintained. Fluoride concentration is also continuously monitored.

The water quality is constantly monitored at the plant, throughout the various stages of the treatment process, to ensure that it meets strict standards for public health.
Critical online monitoring equipment includes the following:

• Conductivity.
• Dissolved organic carbon.
• Ultra-violet transmittance.
• Ultra-violet intensity.
• pH meters.
• Turbidity meters.
• Chlorine monitors.
• Fluoride monitors.

Supplementing online monitoring is regular sampling and analysis the districts water supply by the water treatment technicians, and contract laboratories which are recognised by Taumata Arowai.

All our water treatment staff, including the plant Lead and seven operators, receive specialist training to gain NZQA Drinking Water Certificate and/or diploma qualifications from external authorities. Ongoing competency training is implemented to ensure staff are operating the plants in line with New Plymouth District Water Safety Plan.

The reticulation system is supervised by a qualified engineer who manages contractors who maintain the service infrastructure. This infrastructure comprises 120km of trunk mains, 560km of service mains and three pump stations. The replacement value of all the water supply assets is more than of $135m.

This management and control ensures that public health is secure, by providing water to your tap that is wholesome and free of harmful organisms.

Treated water from the New Plymouth Water Treatment Plant is piped under gravity to 12 distribution reservoirs, which in turn feed water through the reticulation network in the various areas served by the plant.

One of the great advantages of the New Plymouth water supply is that it is gravity fed and relies upon pumping stations only during times of peak demand. This makes it a cost-effective system with less opportunity for interruptions to supply due to electrical and pump failures.

Each household in New Plymouth District uses an average 250,000 litres of water per year – that’s about 685 litres per day.

Only a small percentage of this water is needed for drinking and cooking – the rest is used for bathing, washing, toilet flushing, watering the garden and recreational use. But because water is an excellent vehicle for transporting disease organisms, all water supplied to consumers must meet strict standards for public health.

Programmed sampling by water treatment staff and independent testing of water quality by IANZ accredited and Taumata Arowai recognised facilities is routinely undertaken throughout the water reticulation system, to ensure a high level of confidence in the safety of the water for all consumers. The goal is to ensure ongoing water quality that conforms to the New Zealand Drinking Water Standards.