Potable Water from Sea and Wind

Western Australia's Water Corporation is on track to produce drinking water form a 45GL/a desalination plant located in Kwinana, 25km south of Perth. The plant is being built by proAlliance - a 50/50 joint venture between West Australian construction company Multiplex and French water treatment company Degremont. Degremont will operate the 140,000m3/d plant for 25 years under a public-private partnership with the West Australian government-owned Water Corporation. The construction cost is $387 million.

To reduce the environmental impact of the project electricity for the desalination plant will be produced from a wind farm located 30km east of Cervantes in WA's midwest. The 80MW wind farm has been engineered by the Queensland government-owned power generation company Stanwell Corporation and WA private company Griffin Energy. The wind farm will be operated by the WA government-owned power utility Western Power.

When complete Perth's desalination plant will ease pressure on WA's Integrated Water Supply Scheme. 45GL represents the single biggest water source feeding into the IWSS. To maintain water supplies for Perth's growing population the Water Corporation is pursuing a strategy of "security through diversity" and so is progressing engineering on a further desalination plant as well as developing the underground Yarragadee Aquifer to the south of Perth.

The Corporation has had to accelerate installation of water desalination due to reduced rainfall in Perth's catchment areas.

The technology to be used for the desalination plant is reverse osmosis. Osmosis is a natural phenomenon that occurs when water diffuses through a semipermeable membrane to equalise the concentration of salt in a solution. The transfer of water is from the dilute to the concentrated solution. By applying energy in the form of water pressure water can be made to move in reverse from a concentrate solution to dilute solution - hence the term reverse osmosis.

A semipermeable membrane acts like a molecular sieve allowing water particles to pass through while stopping dissolved salts, viruses and bacteria. To reduce the area required to house the membrane, the membrane is wound onto a spiral. Water is pumped under pressure down the spiral and migrates to the centre.
In recent years improvements in membrane manufacture and energy recovery devices have reduced the capital and operating costs. The Perth plant will produce drinking water at less than $0.95/kl at the fence. Electrical power consumption is expected to be less than 4.5Wh/kl.

An important part of reducing power consumption is the energy recovery system which uses a ceramic pressure recovery device. A device called the Pressure Exchanger (PX), a trademark of US firm Energy Recovery, uses a cylindrical rotor with longitudinal ducts parallel to its rotational axis. The rotor spins inside the sleeve between two end covers with port openings for both streams. Pressure energy is transferred directly from the high-pressure concentrate/reject stream to the low-pressure feed/seawater stream. A liquid piston moves back and forth inside each duct creating a barrier that inhibits mixing between the streams. The low-pressure side of the rotor fills with seawater while the high-pressure side discharges seawater. This rotational action is similar to that of an old-fashioned machine gun firing high-pressure bullets that is refilled with new seawater cartridges while spinning around a central axis.

When complete the plant will employ 16 people and provide valuable technical expertise in operating a large-scale reverse osmosis plant. A comprehensive environmental monitoring program is an operating license condition to ensure that the marine environment, near the seawater inlet and brine outlet, will not be harmed.