Many large seawater desalination plants, using reverse osmosis (RO) membranes, are now running in the world, and large efforts have been conducted in order to reduce the production cost.As one of the solutions for cost reduction, a new scheme called a brine conversion two-stage RO process was developed. In this process, pre-treated seawater is fed to the first stage and approximately 40 % of permeate is obtained from seawater, and concentrate of the first stage is boosted up its pressure again and more permeate is obtained at the second stage RO, in which 60 % can be totally recovered. In this two-stage process, high pressure is applied in order to obtain the effective pressure for the concentrated seawater at the second stage.This process became possible through the development of a new composite membrane and a new element, which can be operated at high pressure and high concentration.In this study, we developed a method to analyze and design the brine conversion two-stage RO process, and it was verified for each stage by comparing simulated result with the actual plant data. Then, we found the relationship between the permeability of TDS (total dissolved salt) and that of boron, and established the technique to estimate the concentration of boron in the permeate.

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