In distillation saltwater is heated in one container to make the water evaporate, leaving the salt behind. The desalinated vapor is then condensed to form water in a separate container. Although long known, distillation has found limited application in water supply because of the fuel costs involved in converting saltwater to vapor. Representative of the early attempts in this direction were the solar distillation methods employed (c.49 B.C.) by the legions of Julius Caesar for using water from the Mediterranean. Modern technological advances led to the development of more efficient distillation units using solar energy; however, since these units have small capacities, their utility is restricted.
Distillation plants having high capacities and using combustible fuels employ various devices to conserve heat. In the most common system a vacuum is applied to reduce the boiling point of the water, or a spray or thin film of water is exposed to high heat, causing flash evaporation; the water is flashed repeatedly, yielding fresh distilled water. This multistage flash distillation method is used in more than 2,000 desalination plants, including one in Saudi Arabia that produces 250 million gallons of freshwater per day.
Another method of desalination is by electrodialysis. When salt dissolves in water, it splits up into charged particles called ions. Placed in a container with a negative electrode at one end and a positive electrode at the other, the ions are filtered by the membranes as they are attracted toward the electrodes; they become trapped between semipermeable membranes, leaving outside the membranes a supply of desalinated water that can be tapped. The first large installation using this process began operating in South Africa in 1958, but its electrical demands make it impractical except where such energy is abundant.
By far the most promising approach is the reverse osmosis process, in which pressure is applied to saltwater to force it through a special membrane. Only pure water passes, leaving concentrated seawater behind. Where multistage flash distillation costs about $4 per 1,000 gallons, reverse osmosis costs about half that amount. This process is used by a plant in the Tampa Bay area, Florida, that produces 25 million gallons of drinking water a day. Another type uses an empty hollow sphere of semipermeable material that is lowered into the sea. The water flowing into the sphere is fresh, since the salt is excluded by the membrane that covers the entire sphere and is its guard.
One final approach is under development in Hawaii, where different layers of seawater display a large temperature differential. Here an Ocean Thermal Energy Conversion plant is being built which will use steam produced by the flash method to produce energy, then condense the steam into freshwater. Three such plants could produce a hundred megawatts of power, as well as supply 30% of Hawaii's water needs.
For emergency use, i.e., in lifeboats, various systems are available in addition to solar or fuel-heated distillation devices. One device made of flexible plastic is worn around the waist of the user to employ body heat for evaporation.
Without natural freshwater rivers and lakes, the primary domestic source of water in Singapore is rainfall, collected in reservoirs or water catchment areas. Rainfall supplies approximately 50% of Singapore's water; the remainder is mainly imported from Malaysia. Presently, more catchment areas, facilities to recycle water (producing NEWater) or desalination plants are being built. This "four tap" strategy aims to reduce reliance on foreign supply and to diversify its water sources.
Singapore's first reservoir is MacRitchie reservoir, which was built by the British in 1867. Subsequently, as Singapore developed into a modern city over the years, more sources of water were needed to sustain the city’s growth. Thus two local reservoirs were added to the rapidly modernising colonial city.
After independence, the Public Utilities Board took over the control of basic supplies including water. From then on, there have been many developments and improvements to increase the water supply for Singapore.
The 1960s and 1970s saw great development for Singapore. In order to maintain that, many big water projects were embarked upon to increase the water supply. Examples of these projects included the Kranji-Pandan Scheme, the Upper Pierce Project and the Western Catchment Water Scheme.
Modern Singapore sources her water from a network of reservoirs and water catchment areas. By 2001, there were 19 raw water reservoirs, 9 treatment works and 14 storage or service reservoirs locally to serve domestic needs.
A dam is also being constructed around the estuary of three Singapore rivers, creating a huge freshwater reservoir, the Marina Bay reservoir, by 2009. When developed, this will increase the rainfall catchment to two-thirds of the country's surface area.
NEWater is the brand name given to reclaimed water produced by Singapore's public utilities. More specifically, it is treated wastewater (sewage) that has been purified using dual-membrane (via microfiltration and reverse osmosis) and ultraviolet technologies, in addition to conventional water treatment processes.
There are three NEWater factories, located at the Bedok, Kranji Water Reclamation Plants, and Seletar Water Reclamation Plant, producing about 20 million US gallons per day (0.9 m³/s). About 6% of this is used for indirect potable use, which contributes 1 % of Singapore's potable water requirements of 300 million US gallons per day (13 m³/s). The rest of the water is used at wafer fabrication plants and other non-potable applications in industries.
The fourth recycling plant, with a capacity of 32 million gallons per day opened in Ulu Pandan on 15 March 2007. With this new capacity coming on stream, NEWater can now meet 15 % of Singapore's water needs.
At the desalination plant, sea water is forced through plastic membranes with microscopic pores to extract dissolved salts. Silt is removed by dousing the seawater with chemicals that coagulate the particles.
Coinciding with the official opening of the desalination plant, the International Desalination Association (IDA) held its 6-day World Congress in Singapore. About six hundred experts and delegates attended the congress to discuss about desalination and water reuse. Several experts suggested that Singapore could become the world's water hub for water recycling and desalination technology and could export this technology to the world including China. Dr Masaru Kurihara, director of IDA, said that with the new technology in water reclamation, waste water would become the most important sustainable water resource in the future.