A nanoporous insulating oxide deionization device, method of manufacture and method of use thereof for deionizing a water supply (such as a hard water supply), for desalinating a salt water supply, and for treating a bacteria-containing water supply. The device contains two composite electrodes each constructed from a conductive backing electrode and a composite oxide layer being an insulating oxide or a non-insulating oxide and an intermediate porous layer. The composite layer being substantially free of mixed oxidation states and nanoporous and having a median pore diameter of 0.5-500 nanometers and average surface area of 300-600 m.sup.2/g. The composite layer made from a stable sol-gel suspension containing particles of the insulating oxide, the median primary particle diameter being 1-50 nanometers. The difference in zeta potential, at a pH in the range of 6-9, being sufficient to suitably remove alkaline and alkaline earth cations (such as Ca.sup.2+ and Na.sup.1+), various organic and other inorganic cations and organic and inorganic anions from water, preferably household hard water. One composite layer being constructed from a mixture of Al.sub.2O.sub.3, MgAl.sub.2O.sub.4 and/or Mg-doped Al.sub.2O.sub.3 particles, and the other composite layer being constructed from SiO.sub.2 or TiO.sub.2.