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transport costs against the cost of electrodialysis. This consideration can further be extended by examining varying locations along an estuary. As the plant site is moved upstream away from the demand point, for example, salinity decreases and transportation length increases (Fig. 5). The computer models, thus, have several key variables from which the minimum cost for supplying drinking water from an estuarial source can be

posal or reuse, together with the recovery of pure water, makes several desalting processes likely contenders for ever-increasing application in the purification of contaminated water. In order to appreciate better the virtues and potential of this specialized technology, an overall perspective of the general pollution problem and its technical, economic, and social implications is desirable.

Greenfield, Iowa, has a population of about 2,200 people who depend upon water provided by a municipally owned plant. The usual water supply for the town is an impoundment of surface water. The quality of this water is very good, with a total dissolved solids content of about 200 ppm . But during periods of drought, the supply of good surface water decreases, and the town must supplement its supply with water from a 3,500-ft well that taps into the Jordan Aquifier. Unfortunately, the deepwell water is of a poor brackish quality, with a TDS of more than 2,200 ppm, including 600 ppm of hardness. To assure a good supply of highquality water regardless of rainfall, the Greenfield Utilities Superintendent and Board of Trustees decided to buy a demineralization plant. Their objective was to procure a system that could produce 150,000 gpd of water meeting USPHS standards of a TDS of 500 ppm. The plant had to fit into a basement room about 25 ft X 25 ft X 12 ft in the town's exist