Polishing Reverse Osmosis Permeate
By William F. Harfst
Reverse osmosis (RO) produces water of excellent purity for many applications, including semiconductor manufacturing, pharmaceutical, medical, laboratory, and power generation to name a few. The RO process removes more than 99% of the dissolved solids from the feedwater. But this means that 1% or so remain, which is unacceptable for many applications. RO permeate is not considered of equal quality to demineralized, or deionized water. Nor is it considered high-purity water. To achieve and maintain this level of purity, the RO permeate must be ﾓpolishedﾔ. This is commonly achieved using supplemental mixed-bed ion exchange (IX) polishers. A treatment design that uses RO followed by a mixed-bed IX polisher produces very high purity water. This process is capable of achieving 18 megohm-cm quality. An RO that processes feedwater having a specific conductance of 300 micromhos/cm, for example, would produce a permeate stream of 5 micromhos/cm or less. A conductivity of 5 is equivalent to a resistivity of 0.2 megohm-cm. Getting the permeate to within the 10 to 18 megohm-cm quality range, however, requires additional treatment. This is commonly accomplished by one of three methods
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