METHOD FOR PURIFYING WATER BY CYCLIC IONIC EXCHANGE

摘要

The present invention provides a method for purifying or softening water comprising: passing a specific volume of feedwater through at least one service column comprising a strong acid cationic exchange resin capable of binding divalent cations that are present in the feedwater, wherein the loading of the divalent cations on the resin is restricted to about 1 to 25% of the available ion exchange sites on the resin, and the total dissolved solids in the feedwater is greater than 100 mg/l; feeding the water exiting the service column to a reverse osmosis membrane or a nanofiltration membrane to produce permeate water stream and a reject water stream; and passing all or some of the volume of the reject stream corresponding the specific volume of feedwater through at least one off-line column capable of binding monovalent cations; wherein the chemical equivalent ratio of monovalent to divalent cations in the water exiting the service column is greater than 20 to 1; wherein no external source of regenerant salt is used. The inventive method allows for multiple softening/regeneration cycles so that steady state hardness leakage is achieved that is lower than obtainable with conventional ion exchange softening systems.