Desalination

Desalination is a term applied to various processes for desalting seawater or other kinds of saline water. Desalination is of growing worldwide Importance because fresh water is not always available when and where it is needed. For example, seawater desalting is vital to the development of the Middle Eastern oil producing countries. Desalination is used to provide:

• Potable and domestic water
• Industrial process water
• Agricultural water (including that for Irrigation)

It is also used to purify wastewater. Although fresh water is usually the objective of desalination, the same techniques can be used for recovery of the dissolved salts. For example, food grade salt is produced from seawater by electrodialysis in Japan.

This report evaluates three desalting processes that are in widespread use and are commercially available in a wide range of capacities:

• Distillation
• Reverse osmosis (RO)
• Electrodialysis (ED)

All these processes can be used for seawater or brackish water. Reverse osmosis and electrodialysis, both membrane processes, are particularly suitable for brackish water, whereas distillation is mainly used for seawater, especially in large installations. Most of the published cost data on these processes are based on the assumption that they are used for municipal water supplies. In this report, we have assumed that the water is for a chemical facility; therefore, we have used Process Economics Program estimation forms and procedures. Our estimates are based on a production capacity of 1 million gallons per day (1 mgd).

Even though it might be possible to use any or all of the desalination processes for a potential application, each process has its own strengths and weaknesses and each application should be evaluated on an individual basis. Important factors include the feedwater characteristics, quantity and quality of product water required, energy costs, and capital costs. It is sometimes feasible to operate distillation plants on low pressure, low-cost exhaust steam from electric generating plants or waste heat from other sources. Because of the large number of site dependent variables in these arrangements, we have not attempted to evaluate such dual-purpose plants.

In addition to the three desalting processes already mentioned, others are technically feasible, but are not commercially available. One such process, freezing, has the advantage that energy requirements are low in comparison with those of the other desalination processes. Research and development are under way on several variations of the process. This report includes an evaluation of a conceptual design for a 1 mgd plant for desalting seawater by freezing.

Ion exchange is sometimes used to treat water from an RO or ED installation when very high water purity is required. It is not ordinarily considered as a prime desalination process because of the large amounts of regenerating chemicals required for the standard types of resins. However, certain kinds of ion exchange resins can be regenerated by hot water and these are being developed in Australia and elsewhere as a basis for the "Sirotherm" process for demineralizing brackish waters by thermally regenerated ion exchange resins.