Ion exchangers use the principle of ion exchange to remove harmful minerals from water. Water softeners convert scale-forming salts, such as calcium sulfate and magnesium bicarbonate, into soluble, non-scale-forming salts that contain sodium. Most sodium compounds are non-scale-forming because they are highly soluble and therefore do not drop out of solution to form scale. Deionizers and demineralizers remove even more minerals.
A water softener is a piece of equipment that contains a granular, zeolite material called ion exchange resin that is impregnated with sodium cations (positively charged ions). When water passes through the resin, the zeolite material gives up the sodium cations in exchange for the calcium or magnesium ions.
When the ion exchange material has given up all its sodium cations in exchange for the calcium and magnesium cations, it has to be regenerated before it can resume the softening process.
To regenerate the resin, the softener is removed from service and backwashed so that raw water enters at the bottom and flows up through the bed. This water is sent out as waste. Backwashing expands the resin bed and removes particulate matter.
Next, a solution of brine (sodium chloride) is drawn into the resin bed. The resin absorbs sodium cations from the brine and discards the calcium and magnesium cations into the brine. The brine, now containing the calcium and magnesium cations, is flushed into the sewer, and the resin bed is rinsed out with water. The softener is returned to service until regeneration is again necessary.
Here are two examples of how cations are exchanged in a water softener:
- If water contains the scale-forming salt calcium sulfate, this salt is in the form of calcium cations and sulfate anions (negatively charged ions). If the calcium cations are exchanged for sodium cations, then the salt becomes sodium sulfate. This salt is extremely soluble, so it will not produce scale in a boiler.
- If water contains magnesium bicarbonate, this salt is in the form of magnesium cations and bicarbonate anions. If the magnesium cations are exchanged for sodium cations, the salt will become sodium bicarbonate. This salt is also extremely soluble, so it will not produce scale in a boiler.
The capacity of a water softener depends on the following five factors:
- The quantity, type, and condition of the exchange material
- The amount of dissolved minerals in the water
- The amount and strength of regenerant used
- Proper regeneration
- The mechanical condition of the softener and regenerating equipment
The four steps of water softener operation are
- Service (softening)
Service (Softening): The hard water enters the top of the softener and travels down through the bed of resin. The calcium and magnesium ions of the salts in the water are exchanged for the sodium ions held by the resin. The softened water leaves the softener at the bottom.
Backwash: When the resin becomes exhausted, the softener is taken out of service and backwashed by manipulating the valves so that raw water enters at the bottom and flows up through the bed to the wash water collector and then to waste. The backwashing expands the bed and removes particulates.
Regeneration: Untreated or raw water is admitted to an ejector or eductor. The water flowing through the eductor produces a vacuum that draws the brine up from the regenerant or brine tank, and the brine is then forced into the softener just above the surface of the beds. The sodium ions of the brine solution exchange places with the calcium and magnesium ions held by the resin.
Rinsing: Raw water entering the top of the softener flows through the resin bed and washes any remaining brine from the resin. When salimeter tests taken on the water leaving the softener show that all salts have been rinsed out, the softener is put back into service.
Problems in Water Softeners
Problems encountered in the operation of a softener are usually indicated by a reduction in capacity or test readings that indicate incomplete softening of the water.
The causes of these problems can be divided broadly into the following six groups:
- Change in water quality
- Improper flow rates
- Improper brine injection during regeneration
- Fouled exchange material
- Mechanical defects
- Loss of exchange material by attrition
Change in Water Quality: The hardness of surface waters usually changes each season. This change affects softener capacity. For example, if a sodium zeolite softener is rated to soften 3,000 gallons of water when the hardness of the raw or untreated water is 120 ppm and the hardness of the water increases to 160 ppm, the capacity of the softener is reduced to 2,250 gallons. On the other hand, a drop in hardness increases the softener’s capacity. For example, if the hardness drops from 120 ppm to 100 ppm, the capacity of the same softener increases to 3,600 gallons. Changes in capacity due to varying hardness in the raw water are to be expected.
Improper Flow Rates: The maximum flow rates during softening and backwash cycles should not exceed those recommended by the manufacturer. An excessive flow during softening results in insufficient softening; excess flow during backwash will cause resin material to be washed out of the softener.
Improper Brine Injection during Regeneration: The amount and strength of the brine injected into the softener during regeneration should be as recommended by the manufacturer. Erratic brine injection can be caused by an insufficient amount of salt for dissolution in the tank, a worn injector, a defective float valve in the tank, or a malfunctioning sequence timer.
Fouled Exchange Material: Contaminants in the supply water or microbiological growth in the resin bed interfere with the proper reaction in the softener, resulting in reduced output or improper softening.
Mechanical Defects: Broken or plugged baffles, distributors, or collecting headers can cause poor distribution of water or brine solution or channeling. Again, the result is reduced capacity or incomplete softening of the water.
Loss of Exchange Material by Attrition: There is a slow wearing down of the surface of the beads. The amount of fine particles produced and washed out of the softener is about 3 percent per year of the total amount of beads, resulting in a drop in capacity of 3 percent per year.
Deionizers and Demineralizers
The deionizer operates on the ion exchange principle much like the water softener. It is designed to produce water with almost all impurities removed, much like distilled water. Small deionizers are sometimes used in buildings to provide pure water for batteries, print shops, and mist-type humidifiers in computer rooms.
A demineralizer uses a hydrogen cation exchanger that removes the sodium, magnesium, and calcium cations. It also uses an anion exchanger to remove sulfate, chloride, and silica anions. The demineralizer that has the anion and cation exchangers mixed together in a container is called a mixed bed deionizer. A degassifier is used to remove any carbon dioxide.
This article is adapted from BOMI International’s course Air Handling, Water Treatment, and Plumbing Systems, part of the SMA and SMT designation programs. More information regarding this course or the new High-Performance certificate courses is available by calling 1-800-235-2664. Visit BOMI International’s website, www.bomi.org.