Instant expert

Desalination

Can turning salt water into fresh water solve the worldwide water crisis?

Water droplet splash

Credit: Laszlo Ilyes

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What is desalination?

Desalination is a method of turning salty, brackish, or recycled water into drinkable fresh water. Seawater is the most common water to undergo desalination. It is a common practice in very dry countries, where for many communities it may provide the only reliable source of potable water.

How exactly does it work?

There are two primary methods that can be used to create fresh water: 'distillation' and 'membrane' desalinisation.

Distillation involves heating salty water until the water evaporates, leaving the salt behind. The salt is removed and the water vapour collected and condensed back into a fresh liquid, ready for drinking.

The more technical but increasingly popular option of membrane desalinisation involves forcing water through a specially made synthetic membrane. The pores of this membrane are so small that while water molecules can pass through it, the larger salt molecules cannot.

So do we use desalinisation in Australia?

According to the International Desalination Association, there are 13,869 desalination plants in the world. These span across 120 countries, including Australia.

We are in fact listed as the tenth top desalinating country, producing over one million cubic metres of desalinated water a day (which does, however, pale in comparison to top-of-the-list Saudi Arabia's daily 11 million cubic metres!).

Our first large-scale plant opened only in 2006, in Perth, Western Australia. It now supplies 17 per cent of the capital city's water needs, producing 45 billion litres annually. Tugun, on the Gold Coast in Queensland, is nearing completion of its own desalination plant, and one is also being built in Sydney, set for completion in 2010. It will supply up to 15 per cent of Sydney's drinking water and be powered with 100 per cent renewable energy.

Australia is an increasingly drought-affected country, and many fresh water sources are being affected by rising salinity - a downstream impact of widespread tree felling and agricultural land use. Though desalination is an option only recently been taken up here, we will likely increasingly look towards it in the future.

So far desalinisation sounds like a good option. There must be downsides, though?

There are indeed. Desalination uses a lot of energy and is therefore quite expensive in monetary and environmental terms.

Exactly how much energy is used depends on the location of the plant and the method used. For example, with the membrane desalinisation method, the saltier the water the more energy is needed to convert it to freshwater, while the warmer the water, the less energy is needed (which means a plant in cooler Hobart would require more energy than one in sweltering Darwin).

In terms of pollution, not only does the high-energy use result in greenhouse gas emissions, but there is also a constant stream of concentrated salt waste product being produced that must be disposed of.

Because of these downsides, for many countries desalinisation is generally used minimally and as a strong last resort in times of water crisis.

Can we get around these disadvantages?

Energy usage can be minimised through 'cogeneration' - where an electricity plant and a desalination plant are run together. The waste heat and excess power from the electricity production can be used to power the desalination plant more cheaply and efficiently.

We can also look towards running desalinisation plants on renewable energy.

Dealing with waste salt can be tricky. We can't simply dump it back into the ocean, as it could damage marine ecosystems. However, reintroducing it into the ocean together with wastewater could reduce the salinity to acceptable levels.

Alternatively, we could create special 'brine ponds' to dispose of the salt, or even consider the manufacturing of edible salt.