There are no major technical obstacles to desalination as a means of providing an unlimited supply of fresh water, but the high energy requirements of this process pose a major challenge. Theoretically, about 0.86 kWh of energy is needed to desalinate 1 m3 of salt water (34,500 ppm). This is equivalent to 3 kJ kg"1. The present day desalination plants use 5 to 26 times as much as this theoretical minimum depending on the type of process used. Clearly, it is necessary to make desalination processes as energy-efficient as possible through improvements in technology and economies of scale.

Desalination as currently practiced is driven almost entirely by the combustion of fossil fuels. These fuels are in finite supply; they also pollute the air and contribute to global climate change. The whole character of human society in the 20th century in terms of its history, economics and politics has been shaped by energy obtained mostly from oil. Almost all oil produced to date is what is called conventional oil, which can be made to flow freely from wells (i.e. excluding oil from tar sands and shale). Of this vast resource, about 1600 billion barrels have so far been discovered, and just over 800 billion barrels had been used by the end of 1997. It is estimated that there may be a further 400 billion barrels of conventional oil yet to be found. With current annual global consumption of oil being approximately 25 billion barrels, and rising at 2 per cent per annum, the "business as usual" scenario would suggest that the remaining oil will be exhausted by 2050. The supply of oil will undoubtedly be boosted by an increase of supplies from unconventional sources, notably the tar sands and shale of Canada and the "Orinoco sludge" of Venezuela. This oil can only be extracted using high energy inputs, and at very high environmental costs. There will be strong political and international pressure against development of these resources, but, when world oil prices are high enough, production will inevitably increase. In theory, unconventional oil could stretch the world's oil supply by another 30 years. In practice, of course, the rate of consumption of oil will be heavily influenced by economic and many other factors, so that prediction in this area is very difficult. The political situation of two of the world's largest potential producers, Iran and Iraq, could be highly relevant to supplies as well as to the global political economy. It is clear, however, that one of the most important of the influencing factors will be the relative cost of renewable energy and how quickly the world can switch to sustainable technologies. There is nothing to gain by deferring investment in this area, and everything to lose by postponing it any longer.

While salinity or salty water, is generally used to describe and measure seawater or certain industrial wastes, we use the term total dissolved solids ("TDS") to describe water high in various salt compounds and dissolved minerals. While one could have very high total dissolved solids, and very low salinity from a chemistry standpoint, here we are talking about high TDS. Total Dissolved Solids (TDS) refers to the amount of dissolved solids (typically various compounds of salts, minerals and metals) in a given volume of water. It is expressed in parts per million (also known as milligrams per liter) and is determined by evaporating a small amount of amount of water in the lab, and weighing the remaining solids. Another way to approximately determine TDS is by measuring the conductivity of a water sample and converting the resistance in micromhos to TDS. TDS in municipally-treated waters in our area range from 90 ppm to over 1000 ppm. The most common range on city water is 200 - 400 ppm. The maximum contaminant level set by USEPA is 500 ppm. California sets its standard as 1000 ppm, probably due to the high number of ground water sources in the state. The MCL is known as a Secondary Standard and in one sense, refers to the aesthetic quality of a a given water. The higher the TDS, the less palatable the water is thought to be. Sea water ranges from 30,000 to 40,000 ppm. Many brackish ground water supplies are used around California and we have many clients whose private well water has a TDS of 1500 - 2000 ppm. In some cases the levels exceed 7000 ppm. Generally, one wants a TDS of less than 500 for household use. In our experience, it appears that folks can tolerate for general household use, soft clean water with a TDS of up to 1500 ppm. When the levels start to exceed 1500 ppm, most people start to complain of dry skin, stiff laundry, and corrosion of fixtures. White spotting and films on surfaces and fixtures is also common at these levels and can be very difficult or impossible to remove.

TDS affects taste also, and waters over 500 - 600 ppm can taste poor. When the levels top 1500 ppm, most people will report the water tastes very similar to weak alka-seltzer. TDS is removed by distillation, reverse-osmosis or electrodialysis. In our area, most desalination projects, both large and small are accomplished with reverse-osmosis. Depending on the water chemistry, reverse osmosis systems are the most popular, given their low cost and ease of use. Distillers work very well also, and produce very high quality water, but require electricity and higher maintenance than reverse osmosis systems. For whole house treatment, commercial-sized reverse osmosis systems are usually the best approach. You will find a compilation of research and review articles at the end of this chapter that will provide you more in-depth information on each of the technologies covered.

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