METHOD AND APPARATUS FOR PROVIDING ULTRAPURE WATER

A method of providing ultrapure water from feed water containing hardness forming ions, comprising at least the steps of passing the feed water successively through: (i) providing a feed water stream containing hardness forming ions: (ii) passing the feed water stream through a reverse osmosis (RO) unit to remove at least some ionic contamination to provide a RO permeate water stream: (iii) passing the RO permeate water stream though a capacitive de-ionisation (CDI) unit to remove at least some of the ions to provide a post-CDI stream: and (iv) passing the post-CDI stream through an electrodeionisation (EDI) unit to remove at least some of the ions and carbon dioxide and to provide an ultrapure water stream for dispense or storage. In this way, the capacitive deionisation provides a wholly or substantially continuous process for removing a proportion of unwanted ions, especially the hardness forming ions, from the feed water prior to the use of electrodeionisation.