Water – A Precious Commodity
The supply of drinking water in sufficient quantities and at a reasonable cost has historically been an issue for Rottnest Island Authority, particularly in recent years as the demand for freshwater increases and rainfall decreases.
Providing the Island with its water needs
A rainwater catchment area originally sealed by the Army in 1939 and located at the top of Mount Herschel, along with ground water bores, provided Rottnest Island’s water needs for many years.
Due to its deteriorating condition and low rainfall patterns over Rottnest Island, the catchment area has been decommissioned and a Desalination Plant is now the main source of potable water on Rottnest Island.
Freshwater bore field
Rottnest Island still sources drinking water from a bore field located near Wadjemup Lighthouse and consists of 30 bores, ranging from 12 – 30 metres in depth, that allow for abstraction of water. The freshwater bores, are a valuable part of the Island’s ecosystem and care must be taken not over exploit this resource.
Precautions taken by the Authority to ensure protection of the freshwater aquifer, or ground water supply include:
An annual abstraction limit of 47 000KL
- Rotation of the abstraction bores
- Abstraction from no more than 10 bores at a time
- Monthly monitoring of water quality from the aquifers and surrounding freshwater swamps and seeps
- No trees are planted on the recharge area
Desalination is the way forward
A Desalination Plant, first introduced on the Island in 1995, remains the most viable option to supply the Island’s long-term water requirements. As such, to alleviate pressures on groundwater usage doubled its capacity production in 2002 to now produce 550 kilolitres per day. This was achieved through the installation of a wind turbine which provides power to the Desalination facility. How does it work?
Saline water is pumped from 2 of the 6 beach bores at Longreach Bay, where the water is then filtered to remove any fine materials. The water then enters a pump, which increases the water pressure to approximately 6000Kpa as high pressure is required for reverse osmosis to occur.
The water then passes through a membrane, separating the fresh water from the salt molecules. Fresh water passing thought the membrane is collected and the salt water or brine is rejected and pumped into the ocean. The currents off the coast of Rottnest help to disperse the salt over a large area.
Water storage and supply
All water from the bore field and desalination plant flows directly into 4500kL storage tanks where the water is disinfected and then gravity fed to the settlement area for consumption. It is monitored carefully and adjusted to ensure safe drinking water.
Water supply is currently provided at 75% by the Desalination and 25% through groundwater abstraction. The total amount of water supplied to the Island is on average 110,000kL per year, however this amount has seen in marked increase in recent years.
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