Jury still out on desalination technology

Desalination technology to be used on seawater is attracting interest as a way of topping up the world’s depleting freshwater resources, but the technology comes at an economic and environmental price.

Desalination technology has existed for two centuries, but interest has grown quickly recently as technological advances have reduced prices while demand for potable water increases. Limited global freshwater resources are running out due to population growth and groundwater pollution, leading policymakers to consider exploiting the 97% of the Earth’s water contained in oceans.

Desalination technologies can generally be divided into thermal distillation and membrane processes. Thermal processes have been used for decades in the Middle East, involving the distillation of seawater by bringing water to the boil. The resulting vapour is then condensed back into water without the impurities of the original liquid.

However, the majority of newly-built capacity uses semi-permeable membranes to desalinate both brackish water and seawater by electro-dialysis or reverse osmosis . Membranes allow the passage of selected ions, producing freshwater and salt brine.

Although still expensive and energy-intensive, energy-efficiency improvements have made membrane processes more affordable. Cost is directly dependent on the salt concentration of the liquid, making brackish water cheaper to desalinate.

The potential is huge, but commercialising the technology brings with it significant costs. The most promising areas are the Middle East and North Africa, where other options for increasing freshwater are limited. However, only the oil-rich states of the Persian Gulf have afforded to build up significant capacity.

Moreover, critics point out that alternatives like improved water efficiency and wastewater recycling can produce similar results much more cheaply in many regions, while desalination remains the last resort in areas with droughts and periodic water shortages.

Desalination prices were between $0.5 and $0.8 per cubic metre in 2004, according to World Bank estimates. However, the Bank pointed out that while desalination is still more expensive than most conventional water sources available, it often costs less if other options require major investments like large dams.

Environmental concerns boost solar desalination

The economic costs of desalination are intricately linked with the environmental costs that result from their high energy-intensity. Energy accounts for up to 44% of the desalination cost, making it vulnerable to oil-price hikes.

Moreover, should the increased energy consumption come from fossil fuels, the resulting emissions would contradict efforts to halt global warming.

Using alternative energy systems like solar or nuclear power to run desalination plants has been floated as a means of going carbon-neutral. Solar thermal energy has been used to distil salty water for many years, but space requirements and high capital costs have proven prohibitive.

Recently, advanced desalination plants have been built to harness wind or photovoltaic electricity to initiate a reverse osmosis process. While renewable energy is still more expensive than fossil fuels, this may change in the coming decades, making solar desalination a viable option.

Experts point out that the costs and benefits of desalination must be weighed against local conditions. Solar thermal systems, for example, could be the most affordable choice in remote, arid areas.

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