Scratch beneath that salty surface of desalination, and the process reveals itself to be a bit of an adventure. There is a history to the removal of salt from seawater, and it makes for an interesting read.
Desalination in Nature
Before looking on the story of desalination, it should be noted that seabirds have employed the process for centuries. Ever wondered what migrating seabirds drink during the gruelling trips, which sometimes find them hundreds of miles from the land? As they can’t drink seawater, these birds have developed a counter current exchange process in their beaks. The highly concentrated brine is sneezed out through their nostrils. Pelicans, albatrosses, gulls, petrels and terns all have desalination systems within their beaks. Sea ice also removed salt from seawater, which is largely expelled during the freezing process. And mangrove trees are adept at purifying seawater, by filtering it into their leaves, which they then shed, or by trapping it in their roots system, which are then eaten by sea crustaceans.
How has man adapted this process to suit his needs? Desalination plants are in a state of flux, currently. Whilst they are globally widespread, their cost and high-energy needs makes them a mixed blessing, environmentally. Two methods have been used in modern plants since the 1950s – reverse osmosis and multistage flash, which have improved the cost and energy requirements, however not significantly enough.
Reverse osmosis is, simply, as form of filtration, but one that requires the saltwater to be put under high pressure in order for it to pass through the incredibly fine filtering membranes. Because the filter is so fine, hydraulic pumps are needed to force the water through. Hydraulic hose fittings come under extreme pressure during this process, as the larger salt crystals are left on one side of the membrane, and pure water is produced on the other. The process is high-maintenance, as the filters need frequent cleaning and de-clogging.
This method requires high levels of heat to convert saltwater into pure water, by boiling. This happens rapidly (hence ‘flash’) and during each boiling stage the water vapour produced is collected. The brine is left behind at the end of this multi-stage process.
The Future of Desalination
Clearly, desalination is labour and energy intensive. Currently, on two-tenths of a percent of the water consumed globally comes from desalinated salt water. With temperatures destined to rise, and hot countries most at risk, the hunt is on for a more efficient way of processing saltwater. The Massachusetts Institute of Technology is currently working on a way of purifying seawater at a nano level, by employing electrostatic ion-selective membranes. This removes the need for high pressure and heat associated with existing methods. However, ‘ion concentration polarization’, as they have named it, cannot be used to produce vast quantities of water at this stage. Current applications would be only for portable water purifying units, powered by solar energy. These could be lifesavers in disaster areas, when infrastructure has broken down. This exciting new phase in desalination heralds a new dawn, and one that we expect to hear more about in the future.
Leading the pack when it comes to desalination is Israel, who produce 40% of their domestic usage water via desalination. The largest desalinations plant in the world, however, is in the UAE. The Jebel Ali Desalination plant is a modern wonder, with eight multi-stage flash units. We tried to calculate the length of stainless steel piping, the number of stainless steel hose fittings and BSP Adaptors they must have used in its construction, but we gave up. Suffice it to say, the plant produces 17.5 million gallons of pure water a day…