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One of the world's biggest desalination plants is about to open in Australia's most populous - and thirstiest - city, Sydney.

The $1.7bn (£1.04bn) scheme has been driven by concerns about climate change and of erratic rainfall patterns in a fast-growing metropolitan area attracting 50,000 new residents each year. Critics say the plant, in the southern suburb of Kurnell, is a waste of money and have accused the authorities of panicking in the face of ecological challenges, while the project's architects have insisted it will help drought-proof Sydney at a time of great environmental uncertainty.

Sydney Water Corporation managing director Kerry Schott said: "The cost justification really rests on the fact that you don't want a city running out of water. "We have historically had cities run out of water and they have been abandoned and we certainly don't want it happening in a major city like Sydney.

"Sydney, unlike most Australian cities, strangely has an extremely variable rainfall,""We get pretty steady rain on the coast but our catchments are not on the coast, except for a few small ones. They are inland and it has been extremely dry inland for the last decade."

Construction of the Kurnell facility began in late 2007 and it can produce 250 megalitres of water each day, about 15% of Sydney's needs. Seawater is drawn into the system through a large 2.5km (1.5-mile) tunnel snaking under the ocean floor.

After detritus and seaweed have been removed, the water is pushed at high pressure through membranes small enough to capture the salt in a process called reverse osmosis. The desalinated reserves are then re-mineralised and slightly carbonated, while chlorine and fluoride are added, before being pumped straight into the city's main supply.

The operation is powered by a wind farm near Canberra, and officials say coastal ecosystems will not be adversely affected by the salty discharge deposited back into the sea. "We have been very careful about the marine environment. I've seen some practices in the Mediterranean that Australian plants just would not do," said Dr Schott. But John Kaye, a Greens MP in the New South Wales state parliament, said construction work in Botany Bay had stirred up heavy metals that could harm migrating whales, while other sea life could be affected by the dumping of saline waste back into the Tasman Sea.

 "Sydney's desalination plant was a huge mistake," Mr Kaye. "The historical records show we did not need it. The government says it is all powered by green energy, but that could have been used to offset coal generation elsewhere. "It is an unnecessary use of energy," he said.

Conservationists say efficiency measures in recent years have resulted in wholesale reductions to household and industrial water use and Sydney's main reservoir, the mighty Warragamba Dam, will ensure supplies for at least the next decade even as the population expands.

But with an army of new residents pouring into an increasingly demanding city each year, Kristina Keneally, the premier of New South Wales, said the desalination facility was essential. "This is about preparing for Sydney's expanding population. In the face of climate change, in the face of increasing drought it is important we are securing Sydney's water supply," she said.

On the other side of the continent, authorities in Western Australia are forging ahead with plans to open a second desalination plant near the state capital, Perth.

 
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