Skip to navigation | Skip to content

Desalination: taking the plunge


Desalination can deliver clean drinking water, but at what cost? (Image: iStockphoto)

News Analysis Australia will get the world's largest desalination plant of its kind if the drought doesn't release its stranglehold on the eastern state of New South Wales.

Like a giant vacuum cleaner, the plant will suck hundreds of millions of litres from the ocean and, through a process known as reverse osmosis, turn the salty liquid into drinking water.

"Seawater desalination has emerged as a way to provide drought-proof water source," says a Sydney Water-commissioned report released last week when Premier Bob Carr announced plans for the A$2 billion (US$1.5 billion), 500 million litre-a-day plant.

But critics say the proposed desalination, or desal, plant on the Kurnell Peninsula, will cause more problems than it solves.

How does it work?

Reverse osmosis, invented in the 1940s, was first used in the US in the 1960s. It can purify any form of water, including sewage and groundwater.

It works by forcing the water through a membrane, which lets clean water through but stops salts, viruses and microorganisms at the gate.

Desalination plants are used in the US and the Middle East. In Australia small-scale operations have been used on island resorts and navy boats for about 20 years.

"It's not a new technology, it's the scope and scale of the Kurnell proposal that's attracted attention," says Colin Creighton, director of CSIRO's Water for a Healthy Country program

Creighton says all Australian states and territories have considered desalination but only Western Australia, where work is under way on a plant in Perth, and New South Wales have taken the plunge.

The case for desal

Experts agree the best argument for the Kurnell plant is that the technology works.

"It's off-the-shelf technology we know we can bring online to put clean water directly into the arteries of Sydney," says Professor Nick Ashbolt, incoming head of civil and environmental engineering at the University of New South Wales (UNSW).

Creighton says reverse osmosis is a proven technology and part of the solution for Sydney.

"The biggest advantage is membrane technology gives you very clean water 100% of the time, and you can turn it on and off."

And Professor Greg Leslie, a member of the UNESCO Centre for Membrane Science and Technology at UNSW, says reverse osmosis is "an excellent idea to produce potable quality water from water that doesn't meet drinking water standards".

The case against

Huge amounts of energy are required to power desalination. And the proposed plant will require 906 gigawatt hours of electricity a year.

According to an independent analysis by Sutherland Shire Council, the council that covers the Kurnell Peninsula, generating amount of energy produces almost a million tonnes of carbon emission a year, the equivalent of 120,000 households.

The government plans to build a gas-fired power station to power the plant, which it says will reduce greenhouse gas emissions to about half those produced by coal.

But even then the cost is too high, Ashbolt says.

"It's incredibly irresponsible from a sustainability point of view where we should be trying to curtail our greenhouse gas production," he says.

The next big problem, critics say, revolves around what the plant will suck in and spew out.

Plankton and fish spawn will be sucked into the plant, along with water equivalent to 1100 Olympic swimming pools a day.

And larger marine life will get trapped against the mesh, Ashbolt says.

"In this environment we have a lot of jellyfish and they're known to block the screens so they can be quite problematic and close down a plant," he says.

Meanwhile, the plant will discharge the equivalent of 600 swimming pools of concentrated brine into the ocean.

If it isn't properly dispersed, this can form a salt slug of dense, deoxygenated water, which will sit at the bottom of the sea and kill sea life.

Ian Drinnen, principal environmental scientist at Sutherland Shire Council, says there are concerns for aquatic life including sea grass beds and the weedy dragon sea horse (Phyllopteryx taeniolatus).

Drinnen also says building the plant on the land will destroy unique vegetation, including fresh water wetlands, salt marsh communities and rare remnants of swamp oak and eucalypt forest.

"The vegetation at Kurnell, pretty much all of it has been listed as one endangered ecological community or another," he says.

Is recycling a better alternative?

Leslie, from UNSW, says reverse osmosis is the right solution to Sydney's water shortage crisis, but we're looking in the wrong direction.

He says we shouldn't be looking to our oceans but closer to home, to our own toilet bowls.

Leslie has played a key role in setting up waste water recycling schemes in California and Singapore.

He says it's cheaper and more environmentally friendly to recycle sewage than seawater, and it can be done using the same technology proposed for the desal plant.

Leslie says one large pipe would collect raw sewage and pump it to a wastewater treatment plant. The treated sewage would then be run through reverse osmosis and added to Warragamba Dam.

After final treatment it would make its way to Sydney's pipes.

"It is eminently more sensible ... to recycle [waste water] than to stick a straw into the ocean," he says.

But CSIRO's Creighton says a desal plant shouldn't be dismissed out of hand.

"It's a spare tyre in the back of your car and when you think you're going to run out of water you can press the button," he says.

Tags: environment, climate-change, engineering