Water softening

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.

Idealized image of water softening process involving replacement of calcium ions in water with sodium ions donated by a cation-exchange resin.

Rationale

Main article: Hard water

The presence of certain metal ions like calcium and magnesium principally as bicarbonates, chlorides, and sulfates in water causes a variety of problems.[1]

Hard water leads to the buildup of limescale, which can foul plumbing, and promote galvanic corrosion.[2] In industrial scale water softening plants, the effluent flow from the re-generation process can precipitate scale that can interfere with sewage systems.[3]

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.[4]. 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.[4][5]

Limescale in a PVC pipe

Methods

The most common means for removing water hardness rely on ion-exchange resin or reverse osmosis. Other approaches include precipitation methods and sequestration by the addition of chelating agents.

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.[6] As described by NSF/ANSI Standard 44,[7] 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, in the form of beads, are a functional component of domestic water softening units.

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.[8] 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.[3]

Lime softening

Main article: Lime softening

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.[9]

Chelating agents

Main article: Chelation

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.[10]

Reverse osmosis

Main article: Reverse osmosis

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.

Non-chemical devices

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".

Similar claims for magnetic water treatment are not considered to be valid. For instance, no reduction of scale formation was found when such a magnet device was scientifically tested.[11]

Health effects

The CDC recommends limiting daily total sodium intake to 2,300 mg per day,[12] though the average American consumes 3,500 mg per day.[13] 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.[14]

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.

Compared to reverse osmosis and distilled methods of producing soft water, hard water conveys some benefits to health by reducing the solubility of potentially toxic metal ions such as lead and copper, which are more soluble in soft water than in hard water.[15]

High levels of water hardness in the home may also be linked to the development of eczema early in life.[16]

Environmental impact

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.[17] Non-chemical devices are often used in place of traditional water softening for this application.

See also

• Desalination
• Ion exchange
• Lime softening
• Water purification

References

1. The Editors of Encyclopædia Britannica (20 July 1998). Hard water. Encyclopædia Britannica. ISBN 9781593392925. Retrieved 4 March 2015.
2. Stephen Lower (July 2007). "Hard water and water softening". Retrieved 2007-10-08.
3. Rowe, Gary (1988). "Well Contamination By Water Softener Regeneration Discharge Water". Journal of Environmental Health. 50 (5): 272–276. JSTOR 44541189.
4. "Why can't I rinse the soap off my hands?". USGS. Retrieved 7 October 2019.
5. "Archived copy". Archived from the original on 2011-08-17. Retrieved 2011-08-16.CS1 maint: archived copy as title (link)
6. "Water Softeners". Canadian Mortgage and Housing Corporation. Archived from the original on October 10, 2006. Retrieved 2010-01-29.
7. Filtration Facts, September 2005, U.S. Environmental Protection Administration, pp. 6-7. Accessed 6 January 2013.
8. "Ion Exchange Treatment of Drinking Water" (PDF). Des.nh.gov. 2009. Retrieved 2016-07-23.
9. Ion Exchange vs. Lime Softening, Nancrede Engineering
10. Bartram, edited by Jamie; Ballance, Richard (1996). Water quality monitoring : a practical guide to the design and implementation of freshwater quality studies and monitoring programmes (1st ed.). London: E & FN Spon. ISBN 0419223207.CS1 maint: extra text: authors list (link)
11. Krauter, P. W.; Harrar, J. E.; Orloff, S. P.; Bahowick, S. M. (1 December 1996). "Test of a Magnetic Device for the Amelioration of Scale Formation at Treatment Facility D". Osti.gov. OSTI 567404. Cite journal requires |journal= (help)
12. "Salt Home — DHDSP". Cdc.gov. Retrieved 2016-07-23.
13. Layton, Lyndsey (20 April 2010). "FDA plans to limit amount of salt allowed in processed foods for health reasons". Washingtonpost.com.
14. "Drinking Water Contaminant Candidate List (CCL) and Regulatory Determination | US EPA". Water.epa.gov. 2016-05-09. Retrieved 2016-07-23.
15. "Common Water Quality Problems And Their Treatment" (PDF). Retrieved 2013-06-23.
16. Perkin, Michael (2016-05-18). "Hard water linked to risk of eczema in infants".
17. "Managing irrigation water quality" (PDF). Oregon State University. p. 12. Retrieved 2012-10-04.