The primary water supply for the city is taken from the catchment area of the Turitea Stream at the northern end of the Tararua ranges. Water is also supplied from four artesian wells to the city's network.
Single artesian wells each supply separate systems in Ashhurst, Longburn and Bunnythorpe.
The Council supplies some 10.5 million cubic metres of water a year to Palmerston North, Ashhurst, Longburn and Bunnythorpe. The average per-head amount of water in the population amounts to 400 litres a day. This figure includes industrial use, garden hosing, hydrant flushing, fire fighting and any system losses.
Although the water from the Turitea catchment is of good chemical quality its direct use is undesirable. This is common in natural run-off from bush catchments as the water is slightly coloured, periodically contains large numbers of algal cells, and after heavy rain may contain silt and protozoans such as Giardia and Cryptosporidium. The treatment plant (originally constructed in 1953) removes the colour, as well as algae and silt, and ensures the water is free of bacteria and protozoans. Ongoing monitoring indicates that the treatment plant provides an effective barrier to these issues.
The quality of the artesian supplies is excellent, with the only treatment required being chlorination to neutralise hydrogen sulphide that is present naturally, and to provide residual disinfectant protection in the reticulation. These are deep bores between 300mm and 450mm diameter and are up to 250 metres deep.
Fluoride is added at Turitea and the four artesian stations. The level of fluoride in the water is maintained at between 0.7 and one part per million.
The Council's laboratory maintains a close check on the water quality from all five sources. Bacteriological and general chemical quality is regularly tested and samples are collected daily in different parts of the city for microbiological quality analysis.
Turitea water source
The primary water supply for Palmerston North is taken from the catchment area of the Turitea Stream at the northern end of the Tararua Ranges. The catchment has an area of approximately 2,400 hectares which is largely covered in bush. The average rainfall is 1,400 mm per year.
This area is run under the Turitea Reserve Management Plan.
A reservoir has been formed at the lower end of the catchment by constructing a dam across the stream valley. The storage capacity of the dam is 1.7 million cubic metres, which meets the city's average daily use for about 60 days.
Turitea Water Treatment Plant
The plant is a full conventional alum-flocculation, rapid gravity dual media (pumice and sand) filtration type. The plant can also be operated in direct filtration mode during periods of low raw water turbidity, whereby the clarification process is bypassed.
The plant was substantially upgraded in 1999 and is now partly automated, requiring only one eight hour shift per day to ensure a continuous supply. Most of the plant functions are automated with most of the human input being to clean and calibrate equipment, as well as replenish chemical stocks. Turitea produces water quality equal to the best conventional plants in the world, with final water turbidity typically at 0.02 Turbidity Units.
During periods of peak demand (such as weekday mornings), the mains from Turitea are not sufficient to supply the quantity of water required. To meet this extra demand for water, four pumping stations draw water from artesian wells to boost mains pressure. These wells are located in the city at Papaioea Park, Takaro Park, Keith Street and Roberts Line.
Additionally, 15 million litres of treated water storage is provided at Ngahere Park. This storage helps provide the additional water for peak demand as well as emergency situations.
The pumps are automatically controlled, starting when local pressures fall below a pre-set level.
These pumps provide about 40% of the city's supply.
Dangers of backflow
Under the New Zealand Building code water from the water network must be prevented from returning to that system. This can happen if there is backflow (a reversal of the normal direction of flow in a pipe).
This can be especially dangerous if there is also a cross connection - a connection between a drinking water supply and a source of contamination or pollution. This situation can be created from something as simple as the common garden hose in a bucket which if the bucket has fluid in it can act like a siphon to mechanical failure of appliances, machinery or valves.
There are cases where public water supply has been contaminated with dangerous chemicals and other pollutants causing injury and illness because the potential for backflow was overlooked.
Residential prevention measures
Householders can ensure there is no risk from operating the garden hose by fitting an inexpensive hose tap vacuum breaker available from plumber's merchants to their outdoor hose taps.
Commercial prevention measures
Commercial buildings that have potential hazards are required to have adequate backflow prevention.
- Hospitals and medical centres
- Agricultural and horticultural operations
- Points where portable water tankers are filled
- Premises with boilers and cooling towers.
- Some food preparation premises
- Many other situations
Council's Water Technical Officer can assess the potential hazard of any system. For advice call 06 356 8199