the post or after deposition problem in water softening is associated with
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Answer:For example, "hard water is what causes the white scale buildup on my pots," or "soft water doesn't leave a detergent film on my freshwashed clothes or fixtures." Some might even contend that soft water makes their skin smoother and hair more silky and manageable. While these observations may be true, they may not be substantial reasons to purchase a water softening device. It is also important to note that water softeners will not necessarily remove any of the more serious drinking water contamination problems. An understanding of the chemistry of hard and soft water and the treatment process used to produce softer water can help you answer the question, "Do I need to soften my water?"
Hard Water/Soft Water
Whether a water supply is labelled "soft" or "hard" is dependent on the presence of two highly soluble minerals, calcium and magnesium. From a health standpoint, these minerals have no adverse effects and are, in fact, essential daily nutrients. It is minerals that give water the refreshing flavor many people find desirable. However, when calcium and magnesium permeate water, they buildup on contact surfaces, possibly plug pipes and damage water heaters, and decrease the effectiveness of soaps and detergents. At this point the water is said to be hard.
Water hardness is expressed in one of two units of measurement. The first unit is parts per million (ppm) of calcium carbonate, a term equivalent to the concentration of dissolved calcium and magnesium. Using this equivalent simplifies hardness calculations. One ppm means that one unit of calcium carbonate is dissolved in one million units of water. Parts per million is also equal to milligrams/liter (mg/l). A second expression of hardness is grains per gallon (gpg) of calcium carbonate. A gpg is used exclusively as a hardness unit and equals approximately 17 mg/l or ppm.
If you have your water tested, the report will use one or both of these units to tell you how hard your water is. Since the level of calcium carbonate means little to water consumers, water specialists have classified levels of hardness. Table 1 shows these classifications.
Table 1. Water Hardness Classification.
Classification Grains per gallon
(gpg) Parts per million or
milligrams per
liter (ppm) or mg/l)
Soft Less than 1.0 Less than 17
Slightly hard 1.0 to 3.5 17 to 60
Moderately hard 3.5 to 7.0 60 to 120
Hard 7.0 to 10.5 120 to 180
Very hard Greater than 10.5 Greater than 180
The Water Softening Process
Once water hardness is known, you have two options. You can live with the hardness level, recognizing that levels below 7.0 gpg will probably not cause major scaling and soap film, or treat the water to reduce the calcium and magnesium present. A water softener, also called an ion exchange unit, will effectively accomplish the latter option.
Ion Exchange
Because water softening devices have long been available in the water treatment industry, the technology is highly developed and in most cases works well to reduce the hardness level.
How does ion exchange work? A physical and chemical process filters the water through an exchange media known as resin or zeolite. Typically, the resin is a synthetic or natural, sand-like material coated with positively charged sodium ions. As the calcium and magnesium dissolves into positively charged ions, an ion exchange environment is created. The water flows through the unit while the resin releases its sodium ions and readily trades them for the calcium and magnesium ions. The water flowing out of the device is now considered soft.
Regeneration
Clearly the resin is not an inexhaustible exchange site. When all the sodium exchange sites are replaced with hardness minerals, the resin is spent and will no longer soften water. At this point, the water softener will need to be run on an alternate cycle called regeneration. During this cycle, resin is backwashed with a salt solution. The brine is reverse flushed through the system taking with it the calcium and magnesium ions that had been adsorbed on the resin. Once backwashing is complete, the softener can be returned to use. Some water softeners will automatically switch to the operation cycle. Others have a manual switch. Figure 1 illustrates both cycles of the water softening process--ion exchange and regeneration.
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