US Patent 7871523 Purification of water

A process for providing water with a lowered salinity and which produces at least one recoverable waste stream containing a useful by-product, the process including treating raw water which includes an undesirably high concentration of cations, including Ca2+ and Na+, in a cation exchange stage which includes first and second cation exchangers. The first cation exchanger having a selectivity towards di-valent cations and the second cation exchanger being downstream of the first cation exchanger, to retain at least a portion of the cations of the raw water in or on the first and second cation exchangers, thereby providing upgraded water of a lowered salinity containing no or a low concentration of the cations of the raw water. The process further includes from time to time, regenerating the first cation exchanger with an acid to produce a first recoverable waste stream which includes a calcium salt and removing Na+ from the second cation exchanger to produce a second recoverable waste stream which includes a sodium compound, and then regenerating the second cation exchanger.