A method of manufacturing a water treatment apparatus having one or more chambers comprising the step of: filling at least one chamber with ion exchanger resin; wherein at least one ion exchanger resin is dry. Preferably, the dry ion exchange resin has a water content of 0-40 wt %, preferably 0-20 wt %, and more preferably 0-10 wt %, such that the volume of the dry ion exchange resin is 25-50 vol % less than the volume of the same ion exchange resin in a moist or wetted form. One form of suitable water treatment apparatus is an EDI apparatus comprising a plurality of chambers to form a stack, for the purification of the water. The present invention provides an easier method of filling of chambers compared with the use of wetted or moist resins, and `over filling` of the cells of water treatment apparatus.