Water softening is the removal of calcium, magnesium, and certain other metal cations in hard water. The resulting soft water requires less soap for the same cleaning effort, as soap is not wasted bonding with calcium ions. Soft water also extends the lifetime of plumbing by reducing or eliminating scale build-up in pipes and fittings. Water softening is usually achieved using lime softening or ion-exchange resins but is increasingly being accomplished using nanofiltration or reverse osmosis membranes.
Hard water leads to the buildup of limescale, which can foul plumbing, and promote galvanic corrosion. In industrial scale water softening plants, the effluent flow from the re-generation process can precipitate scale that can interfere with sewage systems.
The slippery feeling associated with washing in soft water is caused by the weaker attraction of the soap to the water ions when the water has been stripped of its mineral content. The surface of human skin has a light charge that the soap tends to bind with, requiring more effort and a greater volume of water to remove. Hard water contains calcium or magnesium ions that form insoluble salts upon reacting with soap, leaving a coating of insoluble stearates on tub and shower surfaces, commonly called soap scum.
Ion-exchange resin method
Conventional water-softening appliances intended for household use depend on an ion-exchange resin in which "hardness ions"—mainly Ca2+ and Mg2+—are exchanged for sodium ions. As described by NSF/ANSI Standard 44, ion-exchange devices reduce the hardness by replacing magnesium and calcium (Mg2+ and Ca2+) with sodium or potassium ions (Na+ and K+)."
Ion-exchange resins are organic polymers containing anionic functional groups to which the divalent cations (Ca2+) bind more strongly than monovalent cations (Na+). Inorganic materials called zeolites also exhibit ion-exchange properties. These minerals are widely used in laundry detergents. Resins are also available to remove the carbonate, bicarbonate, and sulfate ions that are absorbed and hydroxide ions that are released from the resin.
When all the available Na+ ions have been replaced with calcium or magnesium ions, the resin must be recharged by eluting the Ca2+ and Mg2+ ions using a solution of sodium chloride or sodium hydroxide, depending on the type of resin used. For anionic resins, regeneration typically uses a solution of sodium hydroxide (lye) or potassium hydroxide. The waste waters eluted from the ion-exchange column containing the unwanted calcium and magnesium salts are typically discharged to the sewage system.
Recharge typically takes the following steps:
Water is directed through the resin in the opposite direction as normal flow, and the output is sent to a drain for disposal. This 10 minute process flushes out solids, and expands the resin bed.
Water is directed through a jet pump, which pulls salt water from the brine tank, before the water and brine pass through the resin bed in the normal direction, if co-current, or in the reverse direction, if counter-current. The output of this typical 30 minute process is discarded through the drain hose.
Brine draw stops, but water continues to flow from the inlet to the outlet, gradually flushing the brine out of the resin bed. The flushing water flows slowly for several minutes, then at a faster rate for as long as an hour. At some point, the brine reservoir is refilled with fresh water.
Lime softening is the process in which lime is added to hard water to make it softer. It has several advantages over the ion-exchange method but is mainly suited to commercial treatment applications.
Chelators are used in chemical analysis, as water softeners, and are ingredients in many commercial products such as shampoos and food preservatives. Citric acid is used to soften water in soaps, personal care products and laundry detergents. A commonly used synthetic chelator is ethylenediaminetetraacetic acid (EDTA), which may exist as a tetrasodium or disodium salt. Due to environmental and aquatic toxicity concerns regarding widespread use of EDTA in household and personal care products, alternatives such as sodium phytate/phytic acid, tetrasodium glutamate diacetate and trisodium ethylenediamine disuccinate are finding more prevalent usage.
Washing soda method
In this method, water is treated with a calculated amount of washing soda (Na2CO3), which converts the chlorides and sulphates of calcium and magnesium into their respective carbonates, which get precipitated.
CaCl2 + Na2CO3 --> CaCO3 + 2NaCl
MgSO4 + Na2CO3 --> MgCO3 + Na2SO4
Distillation and rain water
Since Ca2+ and Mg2+ exist as nonvolatile salts, they can be removed by distilling the water. Distillation is too expensive in most cases. Rainwater is soft because it is naturally distilled during the water cycle of evaporation, condensation and precipitation.
Reverse osmosis uses an applied pressure gradient across a semipermeable membrane to overcome osmotic pressure and remove water molecules from the solution with hardness ions. The membrane has pores large enough to admit water molecules for passage; hardness ions such as Ca2+ and Mg2+ will not fit through the pores. The resulting soft water supply is free of hardness ions without any other ions being added. Membranes are a type of water filter requiring regular cleaning or replacement maintenance.
Distillation and reverse osmosis are the most widely used two non-chemical methods of water softening.
Some manufacturers claim that the electrical devices they produce can affect the interaction of minerals with water so that the minerals do not bind to surfaces. Since these systems do not work by exchanging ions, like traditional water softeners do, one benefit claimed for the user is the elimination of the need to add salt to the system. Such systems do not remove minerals from the water itself. Rather, they can only alter the downstream effects that the mineral-bearing water would otherwise have. These systems do not fall within the term "water softening" but rather "water conditioning".
Alternatives to ion-exchange water softeners
Removing or replacing minerals in hard water is called water softening. An alternative water treatment is called water conditioning, in which minerals remain in the water, but are altered so they do not form scale. Although the United States has standards for measuring the minerals in water, it does not have standards for measuring scale forming ability of water. Instead researchers use the German DVGW-W512 protocol.
Rain water contains dissolved carbon dioxide taken from the atmosphere. Some of the dissolved carbon dioxide reacts with the water to form carbonic acid, which remains in solution. Minerals containing calcium and magnesium form soluble bicarbonates when exposed to carbonic acid. Water containing these minerals is known as "hard water."
When hard water is heated in a plumbing system, carbon dioxide goes out of solution, and bicarbonates become carbonates, which are much less soluble. The carbonates bind to plumbing surfaces providing seed crystals for further crystal growth, which build up as hard scale.
Physical water treatment (PWT) devices cause microscopic mineral crystals to form and remain suspended as they flow with the water, while also acting as seeds for further crystal growth. As water is heated, minerals will crystallize on these seeds, instead of the plumbing system. The dissolved minerals become insoluble solid particles in suspension, passing through the system without binding to plumbing surfaces.
The WateReuse Association has sponsored research into alternatives to ion-exchange water softeners. The work was done by Arizona State University and HDR Engineering. They tested four new technologies, and all resulted in a reduction of scaling, as summarized in the table:
|Treatment||Normalized scale formation|
|Electromagnetic Water Treatment||0.57|
|Electrically Induced Precipitation||0.50|
|Template Assisted Crystallization||0.04|
Template Assisted Crystallization
Cold water passes through a tank containing tiny polymeric beads with surfaces exposing seed crystals for dissolved minerals. Crystals grow on these seeds, and break off while still of microscopic size. As these tiny particles travel through a water heater, further crystal growth occurs on the particles, rather than on the water heater. This process is either called template assisted crystallization (TAC) or nucleation assisted crystallization (NAC). The polymeric beads are polyphosphates ranging in size from 0.5 to 2.0 μm.
The CDC recommends limiting daily total sodium intake to 2,300 mg per day, though the average American consumes 3,500 mg per day. Because the amount of sodium present in drinking water—even after softening—does not represent a significant percentage of a person's daily sodium intake, the EPA considers sodium in drinking water to be unlikely to cause adverse health effects.
For those who are on sodium-restricted diets, the use of a reverse osmosis system for drinking water and cooking water will remove sodium along with any other impurities that may be present. Potassium chloride can also be used as a regenerant instead of sodium chloride, although it is more costly. For people with impaired kidney function, however, elevated potassium levels, or hyperkalemia, can lead to complications such as cardiac arrhythmia.
Softened water (measured as residual sodium carbonate index) in which calcium and magnesium have been partly replaced by sodium is not suitable for irrigation use, as it tends to cause the development of alkali soils. Non-chemical devices are often used in place of traditional water softening for this application.
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