Source of water
Lake Mangamahoe / Waiwhakaiho River.
70,000,000 L/day (2,917,000 L/hour)
- Filtration at inlet (stepper screens).
- Taste and odour removal (powdered activated carbon).
- Raw water conditioning (lime and/or CO2 addition).
- Mixing (mechanical and static mixers).
- Coagulation (polyaluminium chloride).
- Floculation (polyelectrolyte).
- Clarification (hopper bottom clarifiers fitted with plate seperators).
- Filtration (rapid gravity filters).
- Manganese removal ("green sand" filter media).
- Sterilisation (chlorine gas).
- pH correction and corrosivity contral (CO2 and lime).
Sixteen Portals hopper bottom clarifiers fitted with plate settling tubes and gravelectric cones.
Eight rapid gravity sand filters filled with 600mm of anthracite coal over 400mm silicon sand media. A wedge wire collection and backwash system is bolted to the floor of the filters. Each filter is fitted with launders to aid filter cleaning at the time of back washes and filter to waste capacity to ensure only the best quality water is sent to storage.
The water treatment process
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 to a stepper screen chamber and then to the treatment plant.
When the river intake is not in use, water is drawn from the lake intake on the north-western side of Lake Mangamahoe. This water is also piped to the stepper screening chamber and then on to the treatment plant. The stepper screens trap large debris such as twigs and other organic matter, and stop them from flowing into the plant. They also prevent aquatic life such as ducks and eels from inadvertently entering the water supply pipes.
The stepper screens are checked regularly to prevent blockages and to help maintain good, even flow. Plant operators use automated valves to control which intake is used: the river or the lake.
The raw water is piped to the plant where it may, depending on raw water conditions, be dosed with carbon dioxide or lime in order 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 make them easier to remove. Lime addition also ensure sufficient alkalinity is available if the natural water contains 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 is fed into the bottom of the settling tanks 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, for removal with the sludge or into filterable floc particles. Plate settling tubes greatly increases the capacity of the clarifiers.
The clear water flows out the top of the tanks via the decant troughs, while the sludge is drained off to the sludge lagoons in front of the plant.
On average it takes the water 4.5 hours to pass through the settling tanks. As the water leaves the tanks, hydrated lime (calcium hydroxide) is added to raise the pH of the water to 7.7 and chlorine is added in order to create the right conditions in the filters for manganese removal.
Filtration and final disinfection
From the settling tanks, the water travels in channels to the rapid sand filters where it passes downwards through layers of anthracite coal and sand which remove any remaining particles. This process usually takes about 35 minutes. The filters are taken out of service for back washing based either on a scheduled time or alternatively on service demand conditions, such as head loss or turbidity. Backwashing the filter removes the particles trapped in the coal and sand layers. The resulting dirty water is drained to the sludge lagoons, where the ‘mud’ settles out.
From the filter outlets, the treated water goes into a large clear water storage tank directly beneath the plant. Chlorine gas is added to the water before it enters the clear water tank as a final disinfection process to ensure there are no remaining micro-organisms present in the water that is distributed to reservoirs, and as a safeguard against accidental contamination of the reticulation system. CO2 and lime are added to give a final pH level of 7.8 to 8.0 and condition the water to minimise its corrosivity.
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:
- pH meters.
- Turbidity meters.
- Chlorine monitors.
- Streaming current monitors.
Supplementing online monitoring is regular testing of the water by the water treatment technicians, and contract laboratories which are IANZ accredited and recognised by Ministry of Health (MOH).
Our IANZ accredited laboratory operates under strict requirements to maintain process and quality management systems. All of our water treatment staff, including the plant superintendent and six operators, receive specialist training to gain NZQA Drinking Water Diploma/Certificate qualifications from external authorities. Ongoing training is implemented to ensure staff are operating the plant in line with best practices based on national and international standards and guidelines.
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 in excess of $135 million.
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 eight 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 200,000 litres of water per year – that’s about 600 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 MOH 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 ultimate goal is to ensure ongoing water quality that conforms to the New Zealand Drinking Water Standards.