A sodium zeolite softener operates through two basis cycles: the service cycle, which produces soft water, and the regeneration cycle, which restores the exhausted resin to capacity. During the service cycle, raw water enters the softener through the inlet distributor, flows through the resin bed, is collected by the underdrain system and then transferred to the point of use.
When a softener is exhausted, it must be regenerated. A number of methods may be used to signal the need for regeneration. Many facilities rely on operator testing to determine when hardness breaks through. Another common method for determining when regeneration is needed is to measure the quantity of water treated between regenerations. A water meter in the service water line is used to sound an alarm or automatically initiate regeneration when a preset number of gallons has been softened.
As we have said, regeneration is the process by which the resin is exposed to a strong concentration of highly ionizable material, such as salt. While the resin has a greater affinity for calcium and magnesium than for sodium, the high concentration of the regenerant forces the ions on the bead into solution and substitutes the sodium ion. This process is called elution. The concentration of the regenerant is a critical factor in all ion exchange. Softener regeneration generally consists of four (4) steps:
- Slow Rinse
- Fast Rinse
Backwashing is an upward flow of water which lifts and expands the resin bed. It removes the accumulation of particulates (entering via raw water) and resin fines (broken pieces of resin).
The brine regenerant stream enters the softener, flows downward through the resin bed and then is discharged to waste.
A low flow rate of rinse water follows the regenerant to displace the brine downward through the resin bed while slowly rinsing the unit. The flow rate for this slow rinse step is the same as the flow rate for the brining step.
A high flow of rinse water follows the slow rinse procedure to remove residual brine from the resin bed. The flow rate for the fast rinse step is identical to the flow rate while the softener is in the service cycle.
Usually a unit can return to service as soon as the hardness value reaches the desired preset level, but some operators continue to rinse until chlorides are reduced to a value near that of the influent level.
Again, the frequency at which any resin must be regenerated is a function of:
- The volume of water treated.
- The concentration of exchangeable ions in the water.
- The type of resin.
- The amount of resin present.
- The type of regenerant.
- The amount of regenerant.