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Regeneration is a process that takes ion exchange resin beads that are exhausted (fully loaded), and removes ions that have been picked up during the in-service cycle so the resin can continue to be used.
An ion exchange system consists of a bed of resin beads with the ability to pick up hardness or other elements by ion exchange. The resin beads can then be regenerated by a high concentration (10% brine) of salt or other regenerant chemical to restore the resin’s capacity, enabling the system to be used over and over for many years.
Regeneration of an ion exchange resin bed involves multiple processes, including:
The first step in regeneration is to backwash the system by running water backwards through the bottom of the bed. This lifts the bed and dislodges dirt, debris and other insoluble material that is in the bed. It also helps to remove air pockets in the resin bed and reclassifies the resin. Backwash is essential to minimize pressure drop and ensure even flow in the bed. Regular backwash is only carried out in co-flow systems. In counter-flow systems, backwash is done only when required.
The bed is then settled and regenerated with a brine solution or other regenerant chemical. This step drives off the hardness or other ions and restores the resin back to the required starting form for beginning a new service cycle.
After regeneration, the slow rinse step continues to push regenerant through the bed to continue conversion and remove the regenerant from the system. The fast rinse is a final rinse with raw water to ensure that water quality is being met after regeneration. In counter-flow systems, there can be a recycle step as an alternative to the fast rinse. Recycling between cation resin and anion resin will remove any remaining regenerant chemical.
After rinsing, the resin bed is returned to service. The total time to regenerate is < 2 hours and the total water used is about 7 x the resin volume: 15L bed = 105L (50 gal./ft3). The wastewater is discharged.
When a resin system regenerates, only about 60% – 80% of the total ion exchange resin capacity is restored. Some hardness or elemental ions are left on and in the resin. When next in service, some of these residual compounds will leach off the resin and pass into the effluent, and this is called “leakage.”