The countries and regions which suffer from the shortage of fresh water resources have developed sea and brackish water desalination plants to supply drinking water. The desalination process has usually been designed from the rejection ratio of salt to meet the level of soluble residue, chloride concentration for drinking purposes. The Japanese government revised the drinking water quality standards to enhance an appropriate drinking water quality management for many hazardous micro-pollutants in drinking water. Since there has not been sufficient studyies on the performance of R.O. desalination processes on micro hazardous constituents, the authors carried out a pilot plant study and field study of several desalination plants for the public water supply.

Most of the constituents of R.O. filtrate meets the drinking water quality standard under the design and operational condition that to attain 99% of the salt rejection. However, Langlier's index, boron and bromoform produced different figures from other inorganic constituents and organic micro pollutants. The rejection ratio of boron ranged only from 43 to 78%. Although the rejectio ratio of boron can be improved by increasing the driving pressure and by adjusting pH, the filtrate cannot meet the drinking water quality standard. Chlorination in the R.O. desalination process produces bromoform that can be removed by the R.O. membrane. However, bromide ion in the filtrate can enhance the formation of disinfection byproducts, if the filtrate is mixed with fresh water, in the distribution system of the public water supply.

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