To mention the Caribbean is to conjure up images of sun, sand and sea. Yet the island region’s reputation for crystalline blue waters washes over a critical problem for many of its 30 territories: water scarcity.

Hard as it might be to believe, water-related risks in picture-postcard Caribbean islands such as Saint Lucia, Barbados, Antigua and Jamaica compare to those in the Western Sahara and parts of the Middle East, according to the World Resources Institute’s Aqueduct rankings.

Some islands, such as Curaçao and Aruba, simply lack decent rainfall. For most, however, it’s a case of “feast or famine” between wet and dry seasons, observes Paul Choules, senior vice president of business development at Water Standard, a Houston-based water treatment firm.

In addition, changes in weather patterns in the Caribbean due to climate change are exacerbating existing water challenges. “Places are not getting rain where they used to get rain, and places are getting rain where they [used not] to”, Choules says.

Desalination is not new to the Caribbean, but extracting clean water from seawater is becoming an increasingly integral part of the region’s search for water security. Since 2007, 68 new desalination plants have been built across the Caribbean, which now boasts an installed capacity of 782,000 cubic metres of purified water per day, according to the Caribbean Desalination Association (CaribDA).

Some islands, such as St. Martin, St. Thomas and the British Virgin Islands, where UK-based water treatment firm Biwater just inaugurated a new $43m (£28m) plant with a 10,400 cubic metres daily capacity, are almost entirely dependent on the technology for their domestic water supply.

“A lot of these islands could not survive without desalination. Their overall economies would collapse”, says Choules, who describes the technology as a “drought-proof solution” in a world facing ever more erratic weather patterns.

Counting the costs of freshwater

Desalination isn’t without its challenges, however. By far the biggest and most obvious relates to energy consumption. Huge amounts of power are required to operate commercial-scale desalination – power that is often produced by importing expensive fossil fuels.

With large natural gas deposits offshore, Trinidad and Tobago is a rare exception to such import dependency. The fact that the twin island country is home to the largest reverse osmosis desalination plant in the Western hemisphere - Desalcott - is therefore perhaps no coincidence.

“We’ve got industry that can afford to pay for it [desalinated water] and large water demand … but desalination in general is relatively expensive, [so] it’s more difficult for small islands that don’t have the industry”, says John Thompson, general manager of Desalcott and president of industry group CaribDA.

Energy typically amounts to one-third of a typical plant’s operating costs, increasing to as much as two-thirds in some cases, says Thompson. Being quick to adopt energy-efficiency innovations has therefore become a hallmark for the region’s desalination operators.

It’s a view echoed by Gerard Pereira, vice president of engineering at Consolidated Water, a desalination firm based in the Cayman Islands where electricity costs around $0.40 per kilowatt hour – more than four times the price in the US. “Just out of necessity, we design and build the most efficient plants in the world”, says Pereira.

Consolidated Water was among the early adopters of isobaric energy recovery devices, which Pereira credits for helping recover up to 98% of the unused energy in the waste streams of desalination plants. The Grenadine islands of Petite Martinique and Curaçao, meanwhile, recently installed two solar-power desalination plants in a $2.1m project part-funded by UK development agency DFID.

“The Caribbean has tended to be where a lot of firsts have happened”, Pereira says, pointing out that Aruba and Curaçao built the first commercial desalination plants more than 80 years ago.

Advances in membrane and pump technologies present further ways of reducing desalination’s energy requirements, but the sense from industry experts is that many of the big energy-saving wins may already have been had.

“There is a thermodynamic limit for desalination”, says David Maingot, regional sales manager at LG NanoH20, a manufacturer of nanocomposite membrane technology that has carried out about 20 retrofits in the Caribbean over the last three years. “We’re getting close to that, so it’s kind of limiting unless you change the actual reverse osmosis technology.”

Wildlife concerns

Alongside its dependency on expensive - not to mention carbon-intensive - power supply, environmentalists have raised concerns over the impacts of large-scale desalination on marine life. In particular, open intakes of seawater can cause fish and other larger organisms to become trapped.

A potentially safer alternative lies in subsurface intakes from coastal aquifers, a technique that extracts seawater from beneath the seafloor or beaches. This is now being carried out in a handful of Caribbean locations (pdf) where site conditions are suitable.

CaribDA’s Thompson admits that desalination is no panacea. Ideally it should constitute only “one choice out of a number of options”, he says, and not necessarily the top one either. However, while desalination may be more expensive and energy-intensive than some options, desalination remains the crucial answer to freshwater access in regions with heavy dry seasons.