Water is one of the most important resources to society. The availability of water has dictated the locations of some cities and limited the growth of others. It is also one of the major necessities for agriculture.
Many areas have limited supplies of water and others are prone to shortages or droughts. Still others do have sufficient water supplies, but in securing necessary water, huge quantities had to be diverted, resulting in ecological disasters such as the shrinking of the Aral Sea.
Of course, there is an effectively limitless supply of water in the world’s oceans, and many of the most arid regions are located near the coast. However, ocean water is far too salty for consumption by humans, for irrigation and for most other uses. Thus, it is not that the world lacks water – we have plenty of it, but that many areas lack fresh water.
Therefore, assuming it could be made economical, desalination would seem like an ideal solution to this persistent problem. Desalination is the only source of water that can be considered to be, for all intents and purposes, unlimited. After all, all nearly all water ends up back in the ocean anyway. With desalination, there are no concerns over droughts or of overdrawing an aquifer. There are no seasonal shortages or reduction in the availability of water.
It could also be argued that desalination, in and of itself, has virtually no negative ecological consequences. The need for water has lead to aquifers being depleted, rivers being diverted, lakes running dry and to the construction of massive dams and canals, sometimes with severe environmental consequences. Therefore, even in areas where adequate fresh water is available, using desalination for basic water needs could greatly reduce the impacts of water sourced from rivers, lakes and aquifers.
The only negative environmental consequence associated with desalination is the need to dispose of the highly concentrated brine that is produced. Separating the water from the sale of seawater means that salt must be disposed of. It is usually in the form of a highly concentrated brine, much more salty than the water that was taken in. This brine is not itself toxic, but the salinity levels are too high for most marine life. If it were to be discharged directly into the ocean, it would result in the area around the discharge becoming too salty for most marine life.
This is certainly not an unmanageable problem. The most obvious solution is to dilute and disperse the waste bring back into the ocean. This is possible, but it can be a major task for large facilities. Other options include recycling the brine into a useful product. For example, it can be used to produce saltcrete. Or, it can be further concentrated and then dried into salt, which can be sold commercially.
The one major downside of desalination is that it is energy intensive, far more energy intensive than more conventional means of obtaining freshwater. In addition to energy usage, desalination plants can be complicated, and the handling of saltwater requires the use of corrosion-resistant materials. The water produced often requires additives for PH adjustment and the addition of trace minerals. All of this adds to the expense of desalination as a water source.
For this reason, it is not generally used if other alternatives exist. Many parts of the world, including much of the middle east and numerous islands are dependent on desalination to provide for their basic water needs. While it does work for this, it remains the option of last resort, due to the economics.
That said, the economics of desalination have been improving steadily over the years. With increasing demand for water, a great deal has been invested in desalination research and development. New plants are constantly being built with ever-increasing efficiency and improved economics. In recent years, major improvements have been made to reverse osmosis-based water desalination systems, which are now being deployed on an industrial scale. The efficiency of distillation systems have also improved with the introduction of better heat recovery and multiple-effect distillation.
Modern desalination plants can now get a large portion of their energy requirements from the waste heat produced by power generation. The use of co-generation for desalination further improves economics and reduce energy requirements. Nuclear desalination is an especially appealing option, since nuclear reactors can produce ample process heat without emissions. The Soviet Union built a highly successful plant to produce water from the Caspian Sea and today, India and China are exploring the use of nuclear reactors to run large desalination plants.
So, desalination is a good thing and we would like to see it continue to improve and become more economical, so it could be put to greater use….right?
Read the rest of this entry »