According to the raw water conditions and water production purposes, RO system generally adopt Pretreatment, Reverse Osmosis Devices and after-treatment system, and usually sectional process and separate-stage process are being applied. The product water of the Level 1 RO system is being used as the feed water of the Level 2 RO system, after secondary desalination process, the product water conductivity is usually<5μS/cm. And through further treatment, the product water of Level 2 RO system will be processed into high pure water, like the power plant boiler feed water, IC cleaning water and so on.