The US may have hit peak water back in the 1970s, but it continues to struggle to meet the water needs of a growing population. And elsewhere, many nations are attempting to provide their citizens with a reliable source of water, even as sporadic droughts and various forms of pollution limit the potential supply of fresh water. That challenge has driven many technological innovations in desalination, which has seen its energy requirements plummet over the last few decades. Unfortunately, according to a recent perspective in Science, we're approaching the thermodynamic limits of desalination efficiency, meaning that further significant gains will have to come from somewhere other than the desalination procedure itself.

Desalination's reputation as an energy hog started out with the initial large-scale facilities, which involved boiling salt water and simply condensing out fresh water from the resulting vapor. Although some of these plants are still in operation, all new construction relies on a more efficient process called reverse osmosis.

Osmosis is the process by which water equilibrates across a salt-impermeable membrane (it's something you may remember from your biology textbooks). Place fresh water on one side of the membrane, salt water on the other, and water will move across to the saltier side in order to equilibrate the solution. But, like all good thermodynamic processes, this can be run in reverse: put energy into the system, and you can force the water back across the membrane from the saltier side. For reverse osmosis, that energy is supplied in the form of pressure applied to the salt water, which forces some of it across the membrane, leaving the salt it once contained concentrated on the opposite side.

This still requires a fair input of energy, in the form of pumps that maintain that pressure, but it doesn't involve the most expensive step, namely driving a phase change from liquid to vapor. And it's possible to limit the energy put into the pumps by performing reverse osmosis in stages: a small increase of pressure generates a small fraction of fresh water, but the resulting brine is passed to a second pump that increases the pressure further, and so on.

The authors of the perspective point out that, because osmosis is a simple matter of thermodynamics, it's possible to calculate exactly how efficient we can make the process. And, as it turns out, we're really quite close as these things go. A state-of-the-art facility is now within a factor of two of the theoretical energy minimum, and only 25 percent higher than the realistic minimum for the current reverse osmosis process. In short, it's going to be tough to squeeze too much more energy out of reverse osmosis, and we're unlikely to find an alternative method of desalination that will provide a significant boost over that.

But that doesn't mean that there are no other ways of getting better output for our energy. The total process of desalination turns out to require three to four times the theoretical minimal energy use, since the salt water must be pumped and pretreated, the membranes maintained, and the resulting brine handled afterwards. Some of these things might be amenable to further improvements, and there has been work put into developing membranes that don't clog up as easily or better pre-filtering of biological materials.

Still, there are limits here as well: it will always take energy to pull salt water out of the ocean and deliver it to a facility that's some vertical distance away from sea level.

So, the authors conclude that one of the best things we can do is focus on ways of handling the energy requirements of desalination in a sustainable manner. These could include using waste heat from co-localized facilities (such as traditional power plants) to help drive some of the steps, which would lower the total energy requirement. In addition, many of the sites of desalination facilities are in locations of good renewable energy resources, including solar or tidal.

Still, the energy challenges will be immense. By 2016, it's estimated that we'll be producing 38 billion cubic meters of fresh water with the technique. Even though the energy cost of doing so has dropped roughly in half during the past decade, that's still a lot of energy: each billion cubic meters could require up to four terawatt-hours to produce. That's quite a hefty amount of additional power that we'll need to add during what promises to be an awkward transition away from fossil fuels.

Science, 2011. DOI: 10.1126/science.1200488 (About DOIs).