ION EXCHANGE – THE THEORY
As rain falls through the atmosphere, it often dissolves traces of acidic gases from the air (carbon dioxide, sulfur dioxide, etc.). When the water reaches the ground, it percolates through the soil and dissolves certain components of the soil; the greater the acidity of the rain water, the more soil material it dissolves. Ground waters (wells, springs) as well as surface waters (rivers, lakes, oceans) both contain a certain amount of dissolved matter. Most of these substances are electrolytes, which dissolve in water to form electrically charged particles called ions.
Two of the most commonly occurring ions in natural waters are calcium and magnesium. Both are positively charged ions, called cations, and each carries two unit charges. The presence of these two minerals in natural water causes hardness, which produces a scum or curd with soap, and forms a hard scale in piping, water heaters, steam irons, and even pots and pans. Other cations present in natural waters may include sodium, potassium, iron and other metallic components, but only calcium and magnesium make hard water.
Softening of water by ion exchange consists essentially of passing hard water through a bed of ion exchanger. The exchange rate is so rapid that water can be easily softened at normal flow rates regardless of hardness. However, reasonably clear water must be applied to the exchanger to avoid coating its particles with colloidal or suspended material from the water. Water containing very high amounts of chlorides, sulphates, or iron should be pretreated before applied to a softener.
The most common method used is the sodium cycle operation, in which the calcium and magnesium ions are removed and replaced in solution by sodium. Since the ions involved are all positively charged, the process is known as positive ion exchange or cation exchange. Negatively charged ions remain in the softened water, and softening does not reduce the total dissolved solids.
ION EXCHANGE – THE PROCESS
The cation exchange material used most often is polystyrene resin, which takes the form of spheres or beads; they are insoluble in water and have a negative electrical charge. The standard resin bead is less than 1/32 of an inch in diameter, and is the site where the exchange of ions actually occurs. Cation resin beads attract positively charged ions and hold them until the beads encounter some other cations for which they have a greater affinity.
In the beginning of the water softening cycle, the resin beads are covered with soft sodium (Na+) ions by washing them in a rich sodium chloride brine solution. These beads are contained in a pressurized vessel called a resin tank. The untreated hard water enters the resin tank and passes through the bed of resin.
The negatively charged resin beads have a greater attraction for the two positive charges in each ion of calcium (Ca++) and magnesium (Mg++) than they do for the single positive charge of the sodium (Na+) ion. Therefore, sodium ions on the resin beads will be displaced by the calcium and magnesium ions. In effect, the resin beads exchange the sodium ions for the “hard water” ions, allowing “soft” water to flow from the resin tank.
A softener will continue to give soft water only as long as there are sufficient sodium ions remaining on the resin beads. After a vast number of calcium and magnesium ions from the water have become attached to the resin beads, the exchanger will become exhausted. Before the regeneration cycle begins, the resin bed is backwashed.
In backwashing, water is passed through the resin bed in the opposite direction of normal flow, which flushes suspended matter out of the tank, reducing the possibility of fouling the bed. Backwashing also loosens the resin bed which becomes compacted during the softening cycle.
In the regeneration process, the resin beads are washed with a strong solution of salt water, known as brine solution. Although the resin beads prefer calcium and magnesium ions, the overwhelming concentration of sodium ions overcomes this affinity. the sodium ions in the brine solution force the calcium and magnesium ions off the beads to be discharged as waste. The resin bed is then rinsed to remove the excess brine solution from the tank, and the resin beads are ready to produce soft water again. The frequency of this regeneration process is determined by the capacity of the softener, the hardness of the water, and the water usage.
SOFT WATER QUALITY
The definition of “soft water” allows for trace elements of hardness (less than one grain per gallon of dissolved calcium and magnesium salts). The quality of the softened water, then, refers to the amount of hardness actually remaining.
The amount of salt used to regenerate the exchange material governs both its hardness removal capacity and the water quality. Generally, a salt dosage of 15 pounds per cubic foot of softening material will fully regenerate the mineral. Lesser salt dosages provide only partial regeneration, leaving some hardness in the mineral bed. As the unit is used, the water softened in the upper part of bed exhibits a regenerating effect on the bed below by displacing the calcium and magnesium ions from the beads. The resulting trace of hardness is called “hardness leakage.” Since the residual hardness in the bed increases as the salting level decreases, hardness leakage occurs more readily with low salt dosages.
The total dissolved salts (TDS) content of the water also influences the effective softening capacity of a water softener and the hardness leakage. TDS content is the sum of the calcium, magnesium, and sodium salts present in the water, and varies for each water supply. Highly mineralized waters (high TDS) tend to reduce the efficiency of a softener and, therefore should be considered when selecting the salt setting. If the TDS limitations are not observed, excessive hardness leakage can occur.
In situations where the conditioned water is used for general purposes, soft water quality is not critical. Slight traces of hardness don’t influence the overall operation and are ignored. When the TDS level is low, lower salt dosages are recommended for general use to provide the greatest operating economy. Although more frequent regenerations would be required, this is of little consequence when the operation is fully automatic.
COMMON CONCERNS REGARDING WATER SOFTENING
Perhaps the most common misconception regarding water conditioning in general is that water is naturally good and is best in an untreated form as nature provides it. While a bubbling spring emitting cool, clear water evokes an image of purity and health, such water often harbors millions of disease-causing bacteria. In fact, only a very small percentage of the earth’s fresh water supply is drinkable without some sort of treatment.
Water supports our environment and water treatment aids in providing a healthful, efficient supply. The following questions and answers address some common concerns about the use of conditioned water.
Will drinking water softened by ion exchange be lacking in minerals necessary to good health? No. Primarily, the human body gains the minerals necessary to good health from foods, not from drinking water. The body may absorb the minerals in water, but in most cases, the amount is insignificant. In order to obtain sufficient minerals from water, it would be necessary to drink many gallons a day. In general, water with a high mineral content can’t be considered a significant source of minerals.
Is the sodium in softened water harmful to people on restrictive salt diets? This depends on the strictness of the diet itself. In normal dietary considerations, the additional sodium produced by softening is insignificant. In an extremely restrictive diet, one should drink neither hard nor soft water, but demineralized or distilled water. Ultimately, if a patient is on a very restricted regimen, he should follow the physician’s instructions regarding water intake.
Does a water softener have any harmful effect in the operation of a septic tank? The backwash and brining effluents from a softener can be discharged into a septic tank that is properly installed and of adequate size. Many states recommend units with a minimum of 500-gallon capacity, but even smaller units may provide adequate in relation to the amount of salt in the water. Tests show that mixtures of sodium, calcium, and magnesium salts as found in regeneration wastes do not hamper the digestion of sludge. Bacteria in the septic tank can become acclimated to the salt environment, with mixed salt concentrations up to 1 having no adverse effects. Even at concentrations of 1.2%, the digestive process slackens but returns to normal in a short time.
Is soft water safe for tropical fish? Yes, soft water is satisfactory for most tropical fish. According to several authorities, both fully soft water and municipally softened water have no toxic effect in an aquarium. When making the change from hard to soft water, it’s necessary to make the substitution on a gradual basis. Drastic changes constitute a shock to the delicate systems of tropical fish and would result in fatalities. By replacing about one-fourth of the water in the aquarium at weekly intervals, the fish will suffer no ill effects as a result of the change.
Should softened water be used for watering houseplants or sprinkling the garden or lawn? In watering houseplants, distilled water or rainwater is the best choice. While softened water is not the best for houseplants, it can still be used safely. It is important to water heavily each time so that the minerals deposited in the soil during the previous watering are washed away. This means that containers should be equipped with holes or bottom drains.
For outside sprinkling, the use of softened water is, first and foremost, wasteful. It could also be harmful, especially where the concentration of hardness minerals is heavy. The sodium salts replacing them when the water is softened could retard the growth or even kill the grass.
Water softening by ion exchange is the process of removing hard calcium and magnesium ions from water and replacing them with soft sodium ions. This is done by passing the hard water by a resin known as an ion exchanger, which is originally covered with the sodium ions. After a period of use, these sodium ions are depleted, being replaced by calcium and magnesium. The ion exchange material regains its water softening capacity by regeneration, a process in which the extracted hardness ions are replaced by sodium ions.