Patent ID: 7018529

Claim:
An ultrapure water producing system comprising: a subtank for storing primary water; an ultrapure water producing apparatus for treating primary water to produce ultrapure water comprising at least an ultraviolet oxidization device, an ion exchange device, and a membrane separation device which are arranged in this order; a supplying line for supplying ultrapure water produced by the ultrapure water producing apparatus to a point of use; a return line for returning residual ultrapure water passed through the point of use without being used therein to said subtank; means for passing bactericide water through said membrane separation device with bypassing the devices arranged at the upstream side of said membrane separation device; means for passing the bactericide water through said device arranged at the upstream side of said ion exchange device, and making the bactericide water flowing out of said device bypass said ion exchange device and said membrane separation device; means for feeding bactericide to water stored in said subtank to prepare the bactericide water whereby the bactericide water in the subtank is sent therefrom for cleaning the devices; and means for substituting the bactericide water in the subtank for primary water.