A water softener is used to remove hardness (excess of magnesium and calcium) from water. Hardness in water will form scale on heat transfer surfaces in cooling towers, heat exchangers and boilers. Hardness will also diminish the effectiveness of additives such as soap and detergent.
Water softeners have two major components: a mineral tank that contains ion exchange resin, and a sodium chloride tank for resin regeneration. The resin tanks are often duplexed or triplexed to deliver the required flow and to provide continuous operation.
The resin in a water softener removes hardness ions by a process called “ion exchange.” In this process, positive-charged (++) calcium and magnesium ions in the water stream are exchanged for positive-charged (+) sodium ions. The resin is capable of holding a limited amount of calcium and magnesium, and therefore needs to be regenerated periodically.
During the regeneration process, a high concentration of salt brine (sodium chloride) is used to remove the calcium and magnesium from the ion exchange resin. The high concentration of sodium in the brine solution forces the exchange process to reverse, and sodium replaces hardness ions on the resin. Once the resin is dominated by sodium ions, the bed is regenerated and prepared for normal (softening) service.
To size a water softener, the peak flow rate and maximum gallons per day of water usage will be used.
Deionization is an ion exchange process that is used for meeting high water purity standards. Deionization is the favored water purification method for applications such as high-pressure boilers, semi-conductor plants, plating manufacturers, and medical device manufacturing.
A deionizer uses both cation and anion exchange resin to remove dissolved solids from water. The process is similar to how a water softener works; in fact, the cation exchange resin is quite often the same variety that is used in a water softener.
The regeneration process uses (H) in an acid to regenerate the cation resin, and it uses (OH) in caustic soda to regenerate the anion resin. This process ultimately replaces the dissolved ions (such as Ca++ and CO3--) with H+ and OH- ; in this manner, both the cations and anions are ultimately exchanged for water (H 2 O).
To recommend a deionization process, several questions must be answered:
- What is the finished water quality specification?
- What is the quantity of water per day?
- What are the discharge limits?
- Is there a waste water treatment plant that can handle the pH swings that occur during acid and caustic regeneration process?
- What are the waste waterdischarge limits?
- Is the plant handling caustic and acid now? If not, is there concern for meeting OSHA requirements?
- Others specific to each application.