As water passes through it, the resin's stronger attraction for the hardness ions causes it to take on the hardness ions and give up its sodium ions. Ion exchange cannot remove suspended matter. As water flows downward through the resin bed, the resin at the top of the bed gives up its sodium first. The regeneration cycle starts with backwash, an upward flow that loosens the resin bed and flushes out suspended particles which have been mechanically filtered out by the resin bed. Backwash usually lasts about 10 minutes.
Concentrated brine is drawn from a storage tank and diluted to the proper concentration.
The large excess of sodium ions causes the resin to release its hold on hardness ions picked during the previous service cycle and returns the resin to its sodium state. The brining step is followed by a slow downflow rinse to displace spent brine from the resin. It also carries the hardness removed from the resin to drain. The rinse rate is regulated to ensure correct contact time between the salt and the resin. Slow rinse usually lasts about 30 minutes.