Of all the objectionable conditions to which water is subject, hardness is easily the most common. Fully 85 percent of the nation's water supplies are hard enough to require softening for maximum utility. Even in places like New York City, where the hardness is a relatively low 2.3 grains per gallon (gpg), many industrial and domestic users insist on having their water treated.
The hardness content of water in the united States ranges from a minimum of about one grain to a maximum of about 350. But few areas have water with the minimum hardness. For the most part, the range is 3-50 gpg. The problem of hard water is often compounded by the presence of other undesirable contaminants such as iron, sulfur, etc.
What level of hardness is considered objectionable? In domestic consumption, twenty grains or more will seem insufferable to most people. But householders accustomed to the satisfaction of fully soft water will find even 5 gpg intolerable. And industrial users often require much less hardness or even water that is completely mineral-free.
Effects of hardness. Why is hardness so objectionable? For the residential user, it creates a whole host of problems, of which the following are most serious:
The minerals in hard water combine with soap (and some detergents) to form an insoluble curd. This action results in higher soap consumption and less efficient cleansing. Moreover, the curd deposits on clothes, dulling colored fabrics and graying white ones. It also shortens the life of washables. This soap curd also affects grooming in that it leaves a dull film on hair.
The reaction with soap is also responsible for hard-to-remove rings on bathtubs, sinks, and lavatories.
Minerals in hard water spot--and eventually etch--silverware, dishes, and glasses.
Similarly, they build up scale in piping systems and all water using appliances. This can cause stoppages and adversely affect the efficiency of water heaters and hydronic systems.
In industrial uses, hardness can be equally troublesome. Scale build-up in boilers is one obvious headache. And hardness can also cause serious problems in air conditioning systems, cooling towers, refrigeration plants, process cooling equipment, etc.
The meaning of hardness. Just what is hardness? Broadly, it's the presence of dissolved minerals in water. But for domestic water treatment purposes hardness is considered to be the presence in water of dissolved salts of magnesium and calcium. The most common of these are as follows:
Calcium chloride, Magnesium chloride, Magnesium sulfate, Calcium bicarbonate, Magnesium bicarbonate.
All of these minerals are highly soluble in water. In addition, calcium sulfate (which is only moderately soluble) may be present.
The bicarbonates of calcium and magnesium are often referred to as "temporary hardness." Reason is that they can exist only when dissolved in water-never in the form of solid minerals. If the water is heated-or evaporated-these bicarbonates decompose into carbonates, which are essentially insoluble in water.
The bicarbonates of calcium and magnesium, although termed "temporary," are nevertheless often the principal hardness compounds found in water. They are formed by the action of water and carbon dioxide on minerals containing the insoluble calcium and magnesium carbonates.
Whatever the chemical makeup of hardness, it is customary in domestic water treatment work to express it in terms of the equivalent amount of calcium carbonate. Thus, for example, water from the Great Lakes contains bicarbonates, chlorides, sulfates, etc. of calcium, magnesium, and sodium. But the hardness is reported as, say, "9 gpg (or simply 9 grains) of calcium carbonate."
Water softening process. Water softening is the removal of these dissolved hardness-causing minerals from the water. To understand how this is done, it's first necessary to get an idea of how these minerals behave when in solution. That's where the word ion comes in.
When any mineral is dissolved in water, it breaks down into two or more components known as ions. These are individual atoms or groups of atoms which can carry an electrical charge. In any solution the number of positively charged ions (also known as cations) must equal the number of negatively charged ions (anions). Dissolved calcium and magnesium-the objectionable ingredients in hard water-are both positive ions. These are the particles that react with soap to form curd and scum.
The basic procedure for getting rid of them in a domestic water supply system is known as ion exchange. This is simply a process of "swapping" the "hard" calcium and magnesium ions for "soft" sodium ions.
The process takes place on the surface of a material known as an ion exchange medium. This may be a synthetic resin, gel zeolite, or greensand. Such materials have the property of being able to take hardness-causing ions out of solution and to replace them with sodium ions. This interaction takes place on the granular particles of the exchange medium in the softener.
In actual practice, the ion exchange medium is precharged with sodium ions. As the hard water flows through the softener, the medium pulls the calcium and magnesium ions out of the solution, and replaces them with sodium ions. This process occurs literally billions of times every second when a softener is in use.
After a while, the surface of the exchange medium becomes relatively saturated with the "hard" ions and relatively free of "soft" sodium ions. As this action occurs, the softening process gradually grinds to a halt and the water emerging from the unit remains hard.
In order to restore the softener's effectiveness, it must be (a) stripped of its hard calcium and magnesium ions and (b) given a new charge of exchangeable sodium ions. This process is known as regeneration.
Regeneration is nothing more than operating the softener in reverse. It's done by running a fairly concentrated solution of sodium chloride (ordinary table salt) through the ion exchange medium. The salt solution-rich in sodium ions-forces the calcium and magnesium off the exchange medium surface, and forces the sodium ions onto the same surface. The unwanted calcium and magnesium ions, together with the excess salt solution, are flushed down the drain.
The frequency with which a softener must be regenerated depends on a number of factors: hardness of the incoming water, the type of equipment involved, and the level of water consumption.
Industrial demineralization. The ion exchange process outlined above is entirely adequate for softening domestic water. For some industrial purposes, however, more thorough treatment may be required. It is often necessary, for example, to remove virtually all of the mineral content. This type of treatment is known as "demineralization".
One form of the process calls for the use of two ion exchange media. In the first, the positive calcium and magnesium ions are taken out of solution. But they are not replaced with sodium ions as in the domestic softener. Rather, the exchange media gives up hydrogen ions as it pulls the calcium and magnesium onto its surface.
At this point, the remaining mineral matter is present in the form of negative ions of chloride, sulfate, bicarbonate, etc. These are removed by passing the water through a second exchanger, where the negative ions are pulled out of solution. The resulting water is essentially mineral-free.
This double-exchange process is widely used in the manufacture of chemicals and pharmaceuticals, food processing, electroplating, tannery operations, etc.
Lime-Soda Ash Treatment. No discussion of the hardness headache would be complete without mentioning the lime and soda ash treatment. This particular method is sometimes used in municipal water works to lessen the hardness level. It's not a complete treatment, however. In a city system where the raw water is 35-40 grains hard, for example, the lime-soda ash treatment will cut the level to 5-1- grains. This is a big improvement, but it does not represent fully softened water.
Treatment with lime and soda ash is especially effective if the water contains bicarbonate (temporary) hardness. Where calcium and magnesium are present primarily as chlorides or sulfates, the process is noticeably less efficient.
The lime-soda ash process consists simply of treating the hard water with slaked lime or a mixture of slaked lime and soda ash. These chemicals cause the magnesium and calcium ions to precipitate out of solution so that they may be filtered or allowed to settle. Ordinarily, the lime-soda ash process is not economically feasible when hardness is to be reduced to less than five grains.
Nor is this process suitable for use in residential systems. For one thing, there are difficulties in feeding lime and soda ash into raw water in the small quantities consumed in the home. Furthermore, close control of the operation is required as the mineral matter settles and is filtered out.
Most important of all, however, there is far more efficient equipment on the market in the form of domestic water softeners. Rapid strides have been made in this field in recent years, so that high-efficiency equipment is now within the reach of most home owners. And with the public becoming increasingly aware of the benefits of hardness-free water, the softener is becoming one of the most important "growth products" available for sale by the plumbing and water systems contractor.