Desalination Part 2 of 4: No Salt, Please

Continued from Desalination Part 1: Tapping the Ocean

This reduced the energy consumption of sugar refining by up to 80%, says James Birkett of West Neck Strategies, a desalination consultancy based in Nobleboro, Maine. But it took about 50 years for the idea to make its way from one industry to another. Only in the late 19th century did multi-effect evaporators for desalination begin to appear on steamships and in arid countries such as Yemen and Sudan.

A few multi-effect distillation plants were built in the first half of the 20th century, but a flaw in the system hampered its widespread adoption. Mineral deposits tended to build up on heat-exchange surfaces, and this inhibited the transfer of energy. In the 1950s a new type of thermal-desalination process, called multi-stage flash, reduced this problem. In this, seawater is heated under high pressure and then passed through a series of chambers, each at a lower pressure than the one before, causing some of the water to evaporate or “flash” at each step. Concentrated seawater is left at the bottom of the chambers, and freshwater vapour condenses above. Because evaporation does not happen on the heat-exchange surfaces, fewer minerals are deposited.

Countries in the Middle East with a lot of oil and a little water soon adopted multi-stage flash. Because it needs hot steam, many desalination facilities were put next to power stations, which generate excess heat. For a time, the cogeneration of electricity and water dominated the desalination industry.
Research into new ways to remove salt from water picked up in the 1950s. The American government set up the Office of Saline Water to support the search for desalination technology. And scientists at the University of Florida and the University of California, Los Angeles (UCLA) began to investigate membranes that are permeable to water, but restrict the passage of dissolved salts.

Such membranes are common in nature. When there is a salty solution on one side of a semi-permeable membrane (such as a cell wall), and a less salty solution on the other, water diffuses through the membrane from the less concentrated side to the more concentrated side. This process, which tends to equalise the saltiness of the two solutions, is called osmosis. Researchers wondered whether osmosis could be reversed by applying pressure to the more concentrated solution, causing water molecules to diffuse through the membrane and leave behind even more highly concentrated brine.

Initial efforts showed only limited success, producing tiny amounts of fresh water. That changed in 1960, when Sidney Loeb and Srinivasa Sourirajan of UCLA hand-cast their own membranes from cellulose acetate, a polymer used in photographic film. Their new membranes boasted a dramatically improved flux (the rate at which water molecules diffuse through a membrane of a given size) leading, in 1965, to a small “reverse osmosis” plant for desalting brackish water in Coalinga, California.

The energy requirements for thermal desalination do not much depend on the saltiness of the source water, but the energy needed for reverse osmosis is directly related to the concentration of dissolved salts. The saltier the water, the higher the pressure it takes (and hence the more energy you need) to push water through a membrane in order to leave behind the salt. Seawater generally contains 33-37 grams of dissolved solids per litre. To turn it into drinking water, nearly 99% of these salts must be removed. Because brackish water contains less salt than seawater, it is less energy-intensive, and thus less expensive, to process. As a result, reverse osmosis first became established as a way to treat brackish water.

Another important distinction is that reverse osmosis, unlike thermal desalination, calls for extensive pre-treatment of the feed water. Reverse-osmosis plants use filters and chemicals to remove particles that could clog up the membranes, and the membranes must also be washed periodically to reduce scaling and fouling.

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