Method and apparatus of providing ultrapure water

A method of providing ultrapure water from feed water 2 containing hardness forming ions, comprises the steps of: passing the feed water through a pre-treatment unit 6 to remove particulate matter from the feed water; passing the pre-treated water through a reverse osmosis (RO) unit 10 to remove at least some ionic contamination to provide RO permeate water; passing the RO permeate water through a degassing unit 40 to provide a degassed stream; passing the degassed stream though a capacitive deionisation (CDI) unit 16 to remove at least some of the ions to provide a post-CDI stream; passing the post-CDI stream through an electrodeionisation (EDI) unit 22 to remove at least some of the ions and carbon dioxide to provide a first ultrapure water stream; passing the first ultrapure water stream to a reservoir 32; passing ultrapure water in the reservoir through an UV irradiation unit 38, a second EDI unit 44, and an ultrafiltration unit 48 to provide further ultrapure water for dispense or storage in the reservoir; and recirculating any water in the reservoir around a recirculation circuit 30 comprising the UV irradiation unit; the second EDI unit and the ultrafiltration unit.