There's been a lot of debate going on recently about Desalination - the process of converting saline water, which is unfit for human or animal consumption and irrigation, into freshwater. "Why would countries worldwide need to resort to such measures?," you may ask. Consider this: over 97% of all water on Earth is oceanic therefore we have a very limited access to readily available freshwater - only about 0.014% is contained in lakes, streams and rivers, and another 0.592% is in the form of groundwater.

Unsurprisingly, one of the main reasons desalination plants keep popping up more and more frequently in the news and on Google maps is population growth. Total global water use has been increasing steadily since the 1940s, and at three times the rate of population increase. With seven billion people to sustain, our planet will be experiencing more frequent water shortages in the years to come. The second issue to consider is varying global water distribution. Developing nations are especially vulnerable to water scarcity (see Figure 1). In fact, about one in eight people (884 million of the world's population) lack access to safe water supplies (UNICEF/WHO, 2008), therefore the race is on to exploit new water resources.

We could argue that sea water is easily accessible and exploitable in most places around the world. We know that desalination has been around for centuries, so why has it not been adopted on a much larger scale? Today, only a tiny fraction of the world's water supply derives from desalination. The reason is remarkably simple: the amount of energy needed to convert saline water to potable water is excessively high. Although the technology is improving rapidly, the costs involved in setting up an energy efficient desalination plant can not always be justified in places where they are needed most (see Figure 2 & 3).

The International Desalination Association recently produced this nifty little video "to dispel some of the most common myths and misconceptions [of desalination]." However, not everyone is as upbeat about this freshwater manufacturing technology as the speakers featured in the IDA video - some such as Debbie Cook, former mayor of Huntington Beach, California, and Phil Dickie, author of WWF's 2007 desalination report, take a rather cautionary approach.

According to Gary Crisp, Australia's foremost desalination practitioner, "desalination should happen only after sustainable water, marine environment and land-use strategies have been tried to maintain the natural functioning of landscapes to provide water."

In any case, desalination offers a partial solution to fresh drinking water problems therefore it can be incorporated into an existing water management strategy in areas of acute water scarcity. It is expected that continuous research into membrane production and manufacturing will lead to more reliable and longer lasting filters to reduce long term plant operating costs.

As mentioned above, the disadvantages of desalination are usually twofold - cost and energy intensity. An average desalination plant is powered by fossil fuel electricity, therefore the operation produces vast quantities of CO2 emissions, which contribute to global warming. Pilot programmes around the world are currently exploring the potential of renewable energy for the running of desalination plants. Also, let's not forget the environmental and ecological impacts of discharging the highly concentrated brine, which can be up to twice as salty as sea water and often contains process chemicals, back into the sea/ocean at the end of the cycle (see Figure 4). Clearly, monitoring is key to minimise negative impacts on the ecosystem.

So, as The Ecologist concluded a few years ago, the argument for desalination plants isn't won yet. 

Figure 1: Physical and economic water scarcity, WRI, 2009

Figure 2: Water desalination - a global picture, World's Water, 2009

Figure 3: Freshwater scarcity by region

Figure 4: Desalination plant - how it works, WSJ, 2008