By Peter Meyers

Summary: The concept of water softening has been in use longer than many of us may have been led to believe. Here, the author gives a brief historical lesson as well as a look into the future on where he sees the industry headed.

Water treatment has been important to mankind since biblical times. As he led the Israelites safely through the wilderness, Moses probably recorded the first published reference to water treatment.

“They could not drink of the waters of Marah, for they were bitter . . . And he cried unto Jehovah; and Jehovah showed him a tree, and he cast it into the waters, and the waters were made sweet.”

—Exodus 15: 23-25

This is most likely a very early example of water treatment by carbon filtration and/or ion exchange softening. Other historical records make reference to use of various clays and soil compounds to make seawater into drinking water. For instance, Aristotle stated “seawater loses part of its salt content by percolating through certain sands.” This is probably one of the earliest references to ion exchange.

First studies

The first published scientific studies of softening by ion exchange were apparently performed centuries later in the early 1800s by such notable scientists of the day as Sir Humphrey Davy, Lambuschini and Huxtable. They discovered that some types of clays could decolorize liquidmanure and adsorb ammonia from soil. Indeed, the term “base exchange” is still used in Europe today and refers to the early phenomenon of natural soils removing bases such as ammonia.

In the middle 1850s, the Royal Agriculture Society of London reached several conclusions:

1. Exchange of ions in soil involved equivalent exchanges,

2. Various ions were more readily exchanged than others,

3. Aluminum silicates present in soils were responsible for the exchange, and

4. Ion exchange materials could be synthesized from soluble silicate and alum 

Later, the materials responsible for the base exchange phenomenon were identified as clays, glauconites (greensands), zeolites and humic acids. Although the “holy grail” of Alchemy was to find a way to turn lead into gold, the research performed in the late 19th century paid off in other ways. The first synthetic (inorganic) ion exchangers were prepared in 1903 by two German scientists, Harm and Rümpler.

Industrial revolution

Although the concept of ion exchange stimulated many soil chemists as well as alchemists, it wasn’t until the beginning of the 20th century that ion exchange was used for water softening. This, of course, coincides with the beginning of the industrial revolution. The increasing use of steam created a need to minimize or completely prevent scale formation. Precipitation softeningwith lime and soda ash were partly successful to remove “temporary hardness.” Precipitation processes don’t result in complete removal of hardness. They’re also quite messy and not particularly user friendly, thus this method of softening boiler feed water was never really satisfactory. The alumino silicates, although vastly superior to soil and clays from the standpoint of capacity, had a very serious drawback in that they gradually dissolved during use.

In the late 1920s, Adams and Holmes discovered that coal could be soaked in concentrated sulfuric acid and thus sulfonated, forming a quite stable and relatively high-capacity cation exchange material. Sulfonated coal became the basis for a large industry that specialized in removal of hardness from boiler feed water. This was probably the birth of the modern water softener. With relatively few changes in process design, the water softener still exists today. Indeed, a few examples of sulfonated coal-base exchangers still remain as well.

In 1935, Adams and Holmes observed that certain synthetic vinyl compounds could also be sulfonated. This amazing discovery was the foundation of almost all organic-based ionexchangers in use today. The polystyrene-based sulfonic acid resins were found to have exceptionally high capacity as well as being versatile and quite stable. With very little change, we still use the sulfonated polystyrene cation exchange resins in softening today. Despite claims by various resin manufacturers of “new” or “revolutionary” improvements, the chemical structure of softening resins today is virtually identical to the first “vinyl” resins of 70 years ago.

Uses of softeners 

Following their initial commercial use to remove scale-forming compounds from boilers, softeners have been developed for a myriad of purposes, primarily centered on exchange of hardness ions for sodium ions. A second large commercial business grew from softening water used in laundro-mats. Soap reacts with hardness ions, lessening the ability of soap to form suds or lift dirt from clothing. The removal of hardness is beneficial to all laundry applications, because soft water requires substantially less soap (compared to hard water) to accomplish the same degree of cleaning. Today, the two leading industrial uses of water softeners are boiler feed water and laundry make-up applications. Other significant uses include car and truck washes, window washing operations, pretreatment to reverse osmosis (RO) and other membrane systems as well as many different chemical manufacturing processes.

In the residential softening arena, the demand generally has been driven by a need for scaleprevention in hot water heaters and plumbing fixtures. Soft water doesn’t form scale and leads to longer life of hot water heaters. In many parts of the country, a softener also will prevent the eventual plugging of water piping within a house. Many people feel bathing in soft water gets you cleaner because it allows the soap to work better, which is true. Softened water helps soap to work better because soap reacts with hardness ions to form a scum. Softeners also remove objectionable contaminants, such as iron and manganese, which cause staining of clothing and faucet fixtures.

Today, production of ion exchange resins and zeolites are well-established industries with sales of several billion dollars a year worldwide. Approximately 40 percent of all ion exchange resins sold in the United States are used in water softening with the majority for residential use. In addition to residential softening, there’s a huge business surrounding inorganic zeolites, primarily as additives to laundry detergents and for ammonia removal from disposable diapers. Those of us who make their living selling softeners may not realize that the market for inorganiczeolites is about 10 times as large as the market for synthetic ion exchange resins.

Water softening remains one of the best tools for removal of primary contaminants from drinking water and will continue to be widely used to purify water for the foreseeable future. Ordinary softening resins remove radium, strontium and other radioactive contaminants. They remove potentially harmful inorganic contaminants such as barium as well as lead and other heavy metals. When used in combination with other types of ion exchange medias, water treatment systems that include softeners can remove almost all objectionable inorganiccontaminants—although additional precautions may need to be employed by the designer or consultant to assure the different resins (cation and anion) function properly in conjunction with one another and the risk of “dumping” of a particular contaminant because of resin affinity or selectivity is significantly reduced. Softening is one of the least expensive methods of purifying water, and provides a higher degree of protection against trace levels of these inorganiccontaminants than most other available methods of treatment.

Recent developments

Although the basic chemistry hasn’t changed in many years, softener manufacturers are always looking for a better way to build a new mousetrap. The introduction of countercurrent regeneration technology is one such advance. Countercurrent softeners can operate at very low salt doses, significantly improving salt efficiency. Such designs allow the softening industry to flourish in some areas where legislative restrictions might otherwise prohibit their use. Discharge restrictions have also led to developments such as brine reclamation schemes and an upsurge in portable exchange softener services.

Other recent improvements include changes in the physical characteristics of the resin (such as smaller beads, inert cores and discrete porosity), changes in the relative percentage of various ingredients used to make the resin (such as reduced crosslink and partial sulfonation), or changes in the method of regeneration (such as packed beds and other countercurrent regenerationtechniques). Since softening is an inherently efficient chemical process anyway, these developments offer modest improvements in salt usage and/or hardness leakage. Often overlooked is the development of “demand regeneration” algorithms made possible by the micro-electronics revolution. Although this isn’t an improvement in softening per se, it has the potential to significantly improve softening efficiency by accurately monitoring and controlling softener operation.

Conclusion

Overall, the future of water softening looks bright. Despite concerns over discharge of salt to the environment in some areas, softening enjoys a very high chemical efficiency and produces a much smaller waste volume or average flow of wastewater than RO and other membraneprocesses. Softeners are easily transported, and lend themselves well to a service-oriented industry with central, bulk regeneration systems. Look for an increasing percentage of residential softeners to be regenerated off site, because bulk regeneration systems can be designed with even greater efficiency—and with little or no liquid discharge. As homeowners demand better quality water, softeners will continue to be part of larger, more comprehensive water treatment systems. In the industrial arena, softeners will find increasing use as pre-treatment for other processes such as electrodeionization. Softening will continue to grow in niche markets for removal of undesirable or potentially harmful contaminants from drinking water.