Recent developments in desalination help to provide drinking water worldwide

Water is a commodity taken for granted by most in the developed world, but some countries are forced to use inventive means to provide it; Singapore is one such country. It is a miniscule archipelago nation, roughly half the area of London, with a population comparable to Scotland’s. Understandably, this places a huge strain on resources, including water supplies. Singapore already recycles its wastewater to make up 30% of its drinking water demand and also imports a considerable amount from Malaysia, but with the water demand expected to double in the next 50 years, the country is looking for alternative methods of water production. Desalinated seawater accounts for 10% of drinking water and the country wants to treble this figure by 2060.

Desalination plants usually work by the principle of reverse osmosis. This involves forcing sea water through a semi-permeable membrane; water can pass through the membrane but larger ions and molecules, such as the constituents of salt, cannot. This produces pure water, which is suitable for drinking, and brine as a by-product. The water has to be forced against the osmotic gradient, since the water naturally wants to move towards the concentrated salt solution. This is a highly energy intensive process, and in a world of global warming and dwindling fossil fuel supplies, this is a problem which has led to advancements in forward osmosis.

Predictably, forward osmosis works on the opposite principle of reverse osmosis. Seawater is separated from a draw solution by a semi-permeable membrane, across which water flows; the draw solution is simply a solution, more concentrated than seawater, which establishes a concentration gradient. The water is flowing with the osmotic gradient, so there is no need to apply pressure. This is clearly less energetically demanding than reverse osmosis, but the water must be extracted from the draw solution, which can be varyingly simple depending on the chosen solute. Researchers at Yale University and the National University of Singapore used ammonium bicarbonate, which decomposes to gas at 60oC and can be regenerated easily, making it an environmentally viable option. However, the membrane used can prove troublesome if the seawater contains organic matter. As researcher William Phillip says, “When you try to desalinate the ocean, there is a whole bunch of other junk in there…that sticks to the membrane”; this means the seawater has to be processed to remove any organic matter before it meets the membrane. Currently, the main obstacle to overcome regarding both forward and reverse osmosis technology is the development of a less sensitive membrane, so that some of these energy costs can be reduced.

Another concern regarding desalination is the environmental impact of pumping seawater out of, and brine into the ocean. Although marine life may be sucked into pumps, researchers decided that intelligent siting of the inlets would all but eliminate this problem. The impact of brine on marine life can be reduced by releasing it in a region with high currents, to enable swift dispersion of the harmful brine into sea water.

This ingenious solution for producing safe drinking water is essential in desalination plants, but can also be employed in portable water purifiers. Countries, such as Singapore, Spain, Australia and the US are relying to a greater extent than ever before on desalination to provide fresh drinking water. There is even a water desalination plant in East London which purifies enough brackish Thames water for nearly a million people per day – clearly no drop in the ocean!

 

Ian Barnett

Image – Pemanducomm