| 摘要 |
PROBLEM TO BE SOLVED: To stably produce a high-quality demineralized water a three-stage RO treatment even from a raw water with the carbonate concn. fluctuated. SOLUTION: When the raw water is subjected to a three-stage RO treatment without being decarbonated a scale preventive agent is added to the raw water, an alkali is further added to control the raw water to >=pH 7.5, and the raw water is passed through a first RO device 2. An RO membrane having a high salt blocking rate in a low-salt-concn. region is used as the RO membrane of a third RO device 4, and a membrane deaerator 1 for removing dissolved oxygen is provided on the upstream side of the first, second or third RO devices 2, 3 and 4.
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