[center][left][center] Ion Exchange Softening
The other method commonly used for water softening is ion exchange softening, also known as zeolite softening. Ion exchange softening exchanges calcium and magnesium ions in water for sodium ions as the hard water passes through a softener. The softener is similar in design to a pressure filter, with resins in place of the filter media.
Ion exchange softener
During treatment, water enters the softener and is directed by a baffle. The water passes through a bed of resin underlain by a bed of gravel, then is collected by an underdrain and piped out of the softener.
Despite the superficial resemblance between softeners and filters, the two operate in very different manners. In a softener, the bed is made up of resins, which are insoluble solids with attached cations or anions capable of reversible exchange with mobile ions of the opposite sign in the solutions in which they are brought in contact.
In the case of the ion exchange resins used in softening, sodium ions are attached to the insoluble solids of the resins. When water passes through the softener, the sodium ions are exchanged for calcium and magnesium ions in the water. The calcium and magnesium ions are retained on the resin grains. The water leaving the softener has sodium ions in the place of calcium and magnesium ions in its compounds, as shown below.
Since sodium ions do not cause hardness, the treated water is no longer hard.
In a properly operating softener, the treated water will have a hardness of 0 mg/L. To prevent corrosion due to excessively soft water, some of the source water is bypassed and does not pass through the softener. This untreated water is blended back into with the water downstream of the softener using a valve and meter to ensure that the proper quantity of water bypasses the softener.
There are many types of resins which can be used in ion exchange units. One type of resin commonly used in water softening is zeolite resin, which is why ion exchange softening is often known as zeolite softening. When other types of resins are used, ion exchange units can be used to remove minerals in brackish water or to remove alkalinity from water.
A scientific understanding of ion exchange did not begin until about 1850 when two English agricultural chemists, Harris S. Thompson and John T. Way, observed an exchange between ammonium and calcium in the soil. Twenty-six years later, Lemberg identified the resins involved and showed that the exchange of cations was reversible. It was soon discovered that various natural minerals, called zeolites, had exchange characteristics suitable for the softening of water.
In 1904, a German chemist by the name of Gans produced the first artificial resins. These artificial zeolites had effective capacities two to three times those of the naturally occurring minerals. Synthesis of these zeolites stimulated the industrial development of water softening. Later developments of other types of resins expanded the use of ion exchange to include the removal of other substances from water.
When all of the sodium ions on the softener resin have been replaced by calcium and magnesium ions, the softener must be recharged. The recharge process involves backwash, followed by regeneration, and ending in rinse.
Backwashing the softener is very similar to backwashing a pressure filter and the purpose is the same. Although the purpose of a softener is not to filter out particulate matter, some particles inevitably get caught in the softener. By sending water backwards through the softener, this particulate matter is removed.
After backwashing, the softener is ready to be regenerated. This is the part of the process in which the magnesium and calcium ions on the resin become replaced with sodium so that the softener can be used to treat more hard water. In order to regenerate the resin, a salt solution, known as brine, is allowed to flow through the softener for about an hour.
The salt used to regenerate the resin is ordinary table salt (sodium chloride, NaCl), so it is easy to handle. When dissolved in water, the salt dissociates into its constituent ions - Na+ and Cl.- The sodium ions replace the calcium and magnesium ions on the resin in the following manner (where "R" preceding an ion means that the ion is bound to the resin.)
RCa + NaCl → RNa + CaCl2
RMg + NaCl → RNa + MgCl2
So the regeneration process can be summarized as follows:
During regeneration, calcium chloride, magnesium chloride, and excess sodium chloride flow to waste.
Rinse and Waste Disposal
After the brine has been given a sufficient contact time, it must be rinsed out of the softener. During the rinse cycle, fresh water is passed through the unit as it would be during treatment, but with the effluent going to waste. Rinse usually takes about 20 to 40 minutes.
Both the spent brine from regeneration and from the rinse must be disposed of carefully since the calcium, magnesium, and sodium salts are corrosive and toxic to the environment. Spent brine is sometimes discharged in sewers or into streams at very high dilutions. Alternatively, the brine can be disposed of in a landfill.
In the Treatment Process
Location in the Plant
Ion exchange softening requires influent water free of turbidity since particulate matter can clog the resin bed. As a result, it is often one of the last steps in the treatment process, following flash mixing, flocculation, sedimentation, and filtration. As with lime-soda ash softening, ion exchange softening can be problematic when dealing with surface water with changing water characteristics.
Ion exchange softening is used widely in small water treatment plants and in individual homes. The process has many advantages for these installations, including its compactness, simplicity, and low cost. Operation of the unit can be nearly automatic and the chemicals used are relatively safe and easy to handle.
Like any other treatment process, ion exchange softening requires a certain degree of water quality monitoring. The following water quality characteristics should be tested:
- Hardness of influent water influences the time between cleanings. The more calcium and magnesium compounds are present in the water, the more quickly the sodium bound to the resin becomes used up and the more often the softener must be regenerated.
- Iron and manganese in the influent water will be caught in the filter bed and can plug it up. Iron and manganese should always be removed before water is softened.
- Sodium in the influent water will react with the softener's resin as it would during regeneration, resulting in magnesium and calcium leaking through into the treated water.
- Chlorine residual of the influent water should be monitored since an excessive chlorine residual can damage the resin.
- Langelier Index should be monitored in the effluent to ensure that corrosive water is not being released to the distribution system.
In addition to raw water characteristics, the operation of the softener can influence efficiency. The most important factors are resin, flow, and cleaning.
Both the resin type and depth influence softening. There are many types of resins, each with different removal abilities, so the type of resin used will determine what can be removed from water. The depth of the resin bed will influence how much hardness can be removed from the water, with deeper beds removing more hardness.
Operation of the softener during the softening phase can also influence efficiency. Specifically, the flow rate of the water through the softener influences how much hardness is removed.
The efficiency of regeneration greatly affects operation of the softener during the softening phase. The salt dosage during regeneration, the brine concentration, and the brine contact time all influence how well sodium is regenerated on the resin surface. If regeneration is not complete, then the softener will not operate as long before it requires another round of cleaning.