Ion exchange resin development in the 1940’s allowed the efficient removal of ionic impurities from water by reversibly exchanging “acceptable” ions for undesirable ions. The system design incorporated for these early cocurrent systems
used downflow (in the direction of gravity) service and downflow regeneration. This design was simple and
it worked effectively from an ion exchange standpoint. However, ion exchange vessels are also effective
filters and will remove particulate matter that is present in the feedwater. To cleanse the bed of accumulated suspended solids, the resin was backwashed when it was taken out of service for regeneration. Incorporating a backwash step to
the ion exchange regeneration procedure was a logical approach. The designs of pressure sand filters and cocurrent ion exchange vessels have a number of similarities and a backwash step was already being used for pressure sand filters.
