Abstract: The present invention provides a method for operating a pure-water production system, which includes an ultraviolet oxidation device and an electrical de-ionization device and channels water to these devices in this order from an upstream side. The method includes: setting hydrogen peroxide concentrations of concentrated water in a concentration chamber and electrode water in an electrode chamber of the electrical de-ionization device less than a hydrogen peroxide concentration of treated water that permeates through a desalination chamber of the electrical de-ionization device.

Abstract: Provided are an electrodeionization system containing multiple electrodeionization devices installed in parallel and a control method for the system. The system contains a regulator varying a flow rate of supply water to the electrodeionization devices; an adjustor adjusting, for each electrodeionization device, a flow rate of concentrated water discharged from each electrodeionization device; and a maintainer maintaining the flow rate of the concentrated water at a certain level or higher. The maintainer is installed only in a main flow path where multiple sub-flow paths are integrated and a main path where multiple sub-paths are integrated. Desalted water produced in each electrodeionization device flows through each sub-flow path while concentrated water discharged from each electrodeionization device flows through each sub-path.

Abstract: The control method for an electrodeionization device (1) of the present invention includes stepwise decreasing the flow rate of supply water (W1) supplied to the electrodeionization device (1) while maintaining a constant flow rate of concentrated water (W5) discharged from the electrodeionization device (1). With such a control method for an electrodeionization device, even when the flow rate of the supply water supplied to the electrodeionization device is decreased, it is possible to prevent an increase in the electrical conductivity, thereby suppressing the occurrence of scale.