Water softening

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Idealized image of cation-exchange resin performing water softening. It shows sodium ions being released by the resin, and replaced with calcium ions captured from water. CationExchCartoon.png
Idealized image of cation-exchange resin performing water softening. It shows sodium ions being released by the resin, and replaced with calcium ions captured from water.

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.

Contents

Rationale

Limescale buildup in a PVC pipe TuberiaCalcio.jpg
Limescale buildup in a PVC pipe

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]

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. Distillation and reverse osmosis are the most widely used two non-chemical methods of water softening.

Ion-exchange resin method

Ion exchange resins, in the form of beads, are a functional component of domestic water softening units Ion exchange resin beads.jpg
Ion exchange resins, in the form of beads, are a functional component of domestic water softening units

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 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. [8]

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. [9] 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]

Recharge typically takes the following steps: [10]

Lime softening

Lime softening is the process in which lime is added to hard water to make it softer. It has several advantages[ further explanation needed ] over the ion-exchange method but is mainly suited to commercial treatment applications. [12]

Chelating agents

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 expensive and energy-inefficient compared to other methods of water softening. Rainwater is soft because it is naturally distilled during the water cycle of evaporation, condensation and precipitation. [13]

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.

Nanofiltration

Nanofiltration is a process similar to reverse osmosis in that it involves the use of a semipermeable membrane, though the filter membrane is distinct in that its pores are ≤ 10 nanometers in diameter. The process is often used in conjunction with reverse osmosis filtration, as nanofiltration on its own is not as effective and more expensive than chemical water treatment methods. [14]

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".[ citation needed ]

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. [15]

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, US researchers use the German DVGW-W512 protocol. [16]

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".[ citation needed ]

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.[ citation needed ]

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. [17]

Alternatives to ion-exchange water softeners exist, see table below.

Conditioning of hard water from Tempe, Arizona with different types of treatment methods [18] [19]
TreatmentNormalized scale formation
No treatment1.00
Electromagnetic Water Treatment0.57
Electrically Induced Precipitation0.50
Capacitive Deionization0.17
Ion exchange0.06
Template Assisted Crystallization0.04

Template assisted crystallization

Cold hard water passes through a tank containing tiny polymeric beads with surfaces that allow nucleation of tiny bubbles of carbon dioxide gas. The initial nucleation of the gas bubbles can occur due to depressurization of the hard water as it flows up a water well just like when the top comes off of a beer bottle. Once carbon dioxide leaves the liquid a chemical reaction immediately drives formation of calcium carbonate crystals on the surface of the bubbles. As crystals grow on these seeds they break off in the flow while still of microscopic size. If these tiny particles travel through a water heater, further exsolution of carbon dioxide occurs due to increased temperature and new crystal growth occurs on the particles, rather than on the water heater. Once calcite occurs in the water, new calcite will prefer to form on the old calcite due to the available bonds on the crystals and the proximity and number of calcite surfaces in the water.[ citation needed ]

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.[ citation needed ] and some have a ceramic coating. Testing at the University of Arizona found TAC to be the most effective at reducing scale formation, followed closely by ion exchange (see chart above). They are more effective than approaches that attempt to sequester ions through application of magnetic or electric fields. The advantages of TAC tanks include simplicity, low maintenance, lack of toxic effluent (like chlorine), and the availability of calcium as a nutrient in drinking water. The disadvantages include that the calcite crystals are not avoided or removed from the water such that areas where water evaporates will still show deposits. It is claimed by manufacturers that these deposits are easier to clean since the calcite forms on seed crystals instead of on the surfaces.[ citation needed ]

Health effects

The UK's National Health Service recommends a maximum salt intake of 6g, against an actual current intake of 8.1g. The US CDC recommends limiting daily total sodium intake to 2,300 mg per day, [20] though the average US American consumes 3,500 mg per day. [21] 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 US EPA considers sodium in drinking water to be unlikely to cause adverse health effects. [22]

A study found the mean concentration of sodium in softened water to be 278 mg/L. [23] In 2 liters of water—the amount of drinking water typically suggested for an average adult, this constitutes about 22% of the recommended sodium intake by the US CDC and may make a difference to those who need to significantly limit their sodium consumption.[ citation needed ] 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.[ citation needed ] 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.[ citation needed ]

High levels of water hardness in the home may also be linked to the development of atopic dermatitis (eczema) early in life, [24] although the actual relationship is correlational at the present and further research is indicated to establish a causal one. However, using water softeners when atopic dermatitis is already established does not reduce the severity of the symptoms. [25] [26]

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

See also

Related Research Articles

<span class="mw-page-title-main">Carbonate</span> Salt or ester of carbonic acid

A carbonate is a salt of carbonic acid, H2CO3, characterized by the presence of the carbonate ion, a polyatomic ion with the formula CO2−3. The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate groupO=C(−O−)2.

<span class="mw-page-title-main">Calcium carbonate</span> Chemical compound

Calcium carbonate is a chemical compound with the chemical formula CaCO3. It is a common substance found in rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skeletons and pearls. Materials containing much calcium carbonate or resembling it are described as calcareous. Calcium carbonate is the active ingredient in agricultural lime and is produced when calcium ions in hard water react with carbonate ions to form limescale. It has medical use as a calcium supplement or as an antacid, but excessive consumption can be hazardous and cause hypercalcemia and digestive issues.

<span class="mw-page-title-main">Brine</span> Concentrated solution of salt in water

Brine is water with a high-concentration solution of salt. In diverse contexts, brine may refer to the salt solutions ranging from about 3.5% up to about 26%. Brine forms naturally due to evaporation of ground saline water but it is also generated in the mining of sodium chloride. Brine is used for food processing and cooking, for de-icing of roads and other structures, and in a number of technological processes. It is also a by-product of many industrial processes, such as desalination, so it requires wastewater treatment for proper disposal or further utilization.

<span class="mw-page-title-main">Sodium chloride</span> Chemical compound with formula NaCl

Sodium chloride, commonly known as edible salt, is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chlorine ions. It is transparent or translucent, brittle, hygroscopic, and occurs as the mineral halite. In its edible form, it is commonly used as a condiment and food preservative. Large quantities of sodium chloride are used in many industrial processes, and it is a major source of sodium and chlorine compounds used as feedstocks for further chemical syntheses. Another major application of sodium chloride is deicing of roadways in sub-freezing weather.

<span class="mw-page-title-main">Sodium carbonate</span> Chemical compound

Sodium carbonate is the inorganic compound with the formula Na2CO3 and its various hydrates. All forms are white, odourless, water-soluble salts that yield alkaline solutions in water. Historically, it was extracted from the ashes of plants grown in sodium-rich soils, and because the ashes of these sodium-rich plants were noticeably different from ashes of wood, sodium carbonate became known as "soda ash". It is produced in large quantities from sodium chloride and limestone by the Solvay process, as well as by carbonating sodium hydroxide which is made using the chloralkali process.

<span class="mw-page-title-main">Magnesium hydroxide</span> Inorganic compound of formula Mg(OH)2

Magnesium hydroxide is an inorganic compound with the chemical formula Mg(OH)2. It occurs in nature as the mineral brucite. It is a white solid with low solubility in water (Ksp = 5.61×10−12). Magnesium hydroxide is a common component of antacids, such as milk of magnesia.

Water purification is the process of removing undesirable chemicals, biological contaminants, suspended solids, and gases from water. The goal is to produce water that is fit for specific purposes. Most water is purified and disinfected for human consumption, but water purification may also be carried out for a variety of other purposes, including medical, pharmacological, chemical, and industrial applications. The history of water purification includes a wide variety of methods. The methods used include physical processes such as filtration, sedimentation, and distillation; biological processes such as slow sand filters or biologically active carbon; chemical processes such as flocculation and chlorination; and the use of electromagnetic radiation such as ultraviolet light.

<span class="mw-page-title-main">Hard water</span> Water that has a high mineral content

Hard water is water that has a high mineral content. Hard water is formed when water percolates through deposits of limestone, chalk or gypsum, which are largely made up of calcium and magnesium carbonates, bicarbonates and sulfates.

<span class="mw-page-title-main">Dolomite (rock)</span> Sedimentary carbonate rock that contains a high percentage of the mineral dolomite

Dolomite (also known as dolomite rock, dolostone or dolomitic rock) is a sedimentary carbonate rock that contains a high percentage of the mineral dolomite, CaMg(CO3)2. It occurs widely, often in association with limestone and evaporites, though it is less abundant than limestone and rare in Cenozoic rock beds (beds less than about 66 million years in age). One of the first geologists to distinguish dolomite from limestone was Déodat Gratet de Dolomieu; a French mineralogist and geologist whom it is named after. He recognized and described the distinct characteristics of dolomite in the late 18th century, differentiating it from limestone.

<span class="mw-page-title-main">Limescale</span> Hard, chalky deposit of calcium carbonate

Limescale is a hard, chalky deposit, consisting mainly of calcium carbonate (CaCO3). It often builds up inside kettles, boilers, and pipework, especially that for hot water. It is also often found as a similar deposit on the inner surfaces of old pipes and other surfaces where hard water has flowed. Limescale also forms as travertine or tufa in hard water springs.

<span class="mw-page-title-main">Purified water</span> Water treated to remove all impurities

Purified water is water that has been mechanically filtered or processed to remove impurities and make it suitable for use. Distilled water was, formerly, the most common form of purified water, but, in recent years, water is more frequently purified by other processes including capacitive deionization, reverse osmosis, carbon filtering, microfiltration, ultrafiltration, ultraviolet oxidation, or electrodeionization. Combinations of a number of these processes have come into use to produce ultrapure water of such high purity that its trace contaminants are measured in parts per billion (ppb) or parts per trillion (ppt).

<span class="mw-page-title-main">Ion exchange</span> Exchange of ions between an electrolyte solution and a solid

Ion exchange is a reversible interchange of one species of ion present in an insoluble solid with another of like charge present in a solution surrounding the solid. Ion exchange is used in softening or demineralizing of water, purification of chemicals, and separation of substances.

<span class="mw-page-title-main">Boiler water</span>

Boiler water is liquid water within a boiler, or in associated piping, pumps and other equipment, that is intended for evaporation into steam. The term may also be applied to raw water intended for use in boilers, treated boiler feedwater, steam condensate being returned to a boiler, or boiler blowdown being removed from a boiler.

Dishwasher salt is a particular grade of granulated, crystalline sodium chloride intended for regenerating the water softener circuit of household or industrial dishwashers. Analogous to water softener salt, dishwasher salt regenerates ion exchange resins, expelling the therein trapped calcium and magnesium ions that characterize hard water. Dishwater salt granules are larger than those of table salt. The granule size ensures that the salt dissolves slowly, and that fine particles do not block the softener unit.

<span class="mw-page-title-main">Nanofiltration</span> Filtration method that uses nanometer sized pores in biological membranes

Nanofiltration is a membrane filtration process that uses nanometer sized pores through which particles smaller than about 1–10 nanometers pass through the membrane. Nanofiltration membranes have pore sizes of about 1–10 nanometers, smaller than those used in microfiltration and ultrafiltration, but a slightly bigger than those in reverse osmosis. Membranes used are predominantly polymer thin films. It is used to soften, disinfect, and remove impurities from water, and to purify or separate chemicals such as pharmaceuticals.

<span class="mw-page-title-main">Alkali soil</span> Soil type with pH > 8.5

Alkali, or Alkaline, soils are clay soils with high pH, a poor soil structure and a low infiltration capacity. Often they have a hard calcareous layer at 0.5 to 1 metre depth. Alkali soils owe their unfavorable physico-chemical properties mainly to the dominating presence of sodium carbonate, which causes the soil to swell and difficult to clarify/settle. They derive their name from the alkali metal group of elements, to which sodium belongs, and which can induce basicity. Sometimes these soils are also referred to as alkaline sodic soils. Alkaline soils are basic, but not all basic soils are alkaline.

Reverse osmosis (RO) is a water purification process that uses a semi-permeable membrane to separate water molecules from other substances. RO applies pressure to overcome osmotic pressure that favors even distributions. RO can remove dissolved or suspended chemical species as well as biological substances, and is used in industrial processes and the production of potable water. RO retains the solute on the pressurized side of the membrane and the purified solvent passes to the other side. The relative sizes of the various molecules determines what passes through. "Selective" membranes reject large molecules, while accepting smaller molecules.

The dealkalization of water refers to the removal of alkalinity ions from water. Chloride cycle anion ion-exchange dealkalizers remove alkalinity from water.

Lime softening is a type of water treatment used for water softening, which uses the addition of limewater to remove hardness by precipitation. The process is also effective at removing a variety of microorganisms and dissolved organic matter by flocculation.

<span class="mw-page-title-main">Calcium cycle</span>

The calcium cycle is a transfer of calcium between dissolved and solid phases. There is a continuous supply of calcium ions into waterways from rocks, organisms, and soils. Calcium ions are consumed and removed from aqueous environments as they react to form insoluble structures such as calcium carbonate and calcium silicate, which can deposit to form sediments or the exoskeletons of organisms. Calcium ions can also be utilized biologically, as calcium is essential to biological functions such as the production of bones and teeth or cellular function. The calcium cycle is a common thread between terrestrial, marine, geological, and biological processes. Calcium moves through these different media as it cycles throughout the Earth. The marine calcium cycle is affected by changing atmospheric carbon dioxide due to ocean acidification.

References

  1. ↑ Hard water. Encyclopædia Britannica. 20 July 1998. ISBN   9781593392925 . Retrieved 4 March 2015.
  2. ↑ Stephen Lower (July 2007). "Hard water and water softening" . Retrieved 2007-10-08.
  3. 1 2 Rowe, Gary (1988). "Well Contamination By Water Softener Regeneration Discharge Water". Journal of Environmental Health. 50 (5): 272–276. JSTOR   44541189.
  4. 1 2 "Why can't I rinse the soap off my hands?". USGS. Retrieved 7 October 2019.
  5. ↑ "Soap". Archived from the original on 2011-08-17. Retrieved 2011-08-16.
  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. ↑ "How does water softening work". Culturalistpress.com. 2022. Retrieved 2022-02-28.
  9. ↑ "Ion Exchange Treatment of Drinking Water" (PDF). Des.nh.gov. 2009. Archived from the original (PDF) on 2017-12-08. Retrieved 2016-07-23.
  10. ↑ "How to Achieve Optimal Softener Performance". Chem Aqua, Inc. 2020. Retrieved December 21, 2020.
  11. ↑ Jerome Kovach (March 26, 2007). "The Art of Countercurrent Regeneration". Water Conditioning & Purification. Retrieved February 16, 2021.
  12. ↑ Ion Exchange vs. Lime Softening, Nancrede Engineering
  13. ↑ Bartram, 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.
  14. ↑ Mohammed, A.W.; et al. (2007). "Modelling the Effects of Nanofiltration Membrane Properties on System Cost Assessment for Desalination Applications". Desalination. 206 (1): 215–225. doi:10.1016/j.desal.2006.02.068. S2CID   98373166.
  15. ↑ 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 (Report). Lawrence Livermore National Laboratory. doi: 10.2172/567404 . OSTI   567404.
  16. ↑ Rick Andrew (October 14, 2014). "A New Standard for Evaluation of Scale Control Equipment". Water Conditioning & Purification Magazine. Retrieved February 10, 2021.
  17. ↑ Tijing, Leonard D.; Pak, Bock Choon; Baek, Byung Joon; Lee, Dong Hwan; Cho, Young I. (2007). "An experimental study on the bulk precipitation mechanism of physical water treatment for the mitigation of mineral fouling". International Communications in Heat and Mass Transfer. 34 (6): 673–681. doi:10.1016/j.icheatmasstransfer.2007.03.009. ISSN   0735-1933.
  18. ↑ Fox, Peter (2014). Evaluation of alternatives to domestic ion exchange water softeners (PDF). Alexandria, Va: WateReuse Research Foundation. ISBN   978-1-941242-00-1. LCCN   2014934179 . Retrieved February 9, 2021.
  19. ↑ Gebauer, Denis; Völkel, Antje; Cölfen, Helmut (2008). "Stable Prenucleation Calcium Carbonate Clusters". Science. 322 (5909): 1819–1822. Bibcode:2008Sci...322.1819G. doi: 10.1126/science.1164271 . ISSN   0036-8075. PMID   19095936.
  20. ↑ "Salt Home — DHDSP". Cdc.gov. Retrieved 2016-07-23.
  21. ↑ Layton, Lyndsey (20 April 2010). "FDA plans to limit amount of salt allowed in processed foods for health reasons". Washingtonpost.com.
  22. ↑ "Drinking Water Contaminant Candidate List (CCL) and Regulatory Determination | US EPA". Water.epa.gov. 2016-05-09. Retrieved 2016-07-23.
  23. ↑ Yarows, S A (Jan 27, 1997). "Sodium concentration of water from softeners". Arch Intern Med. 157 (2): 218–222. doi:10.1001/archinte.1997.00440230096012. PMID   9009980 . Retrieved August 26, 2023.
  24. ↑ Perkin, Michael (2016-05-18). "Hard water linked to risk of eczema in infants".
  25. ↑ Jabbar-Lopez ZK, Ung CY, Alexander H, Gurung N, Chalmers J, Danby S, Cork MJ, Peacock JL, Flohr C (March 2021). "The effect of water hardness on atopic eczema, skin barrier function: A systematic review, meta-analysis". Clinical and Experimental Allergy. 51 (3): 430–451. doi:10.1111/cea.13797. PMID   33259122. S2CID   227245344.
  26. ↑ Eczema in children: uncertainties addressed (Report). NIHR Evidence. 2024-03-19. doi:10.3310/nihrevidence_62438.
  27. ↑ "Managing irrigation water quality" (PDF). Oregon State University. p. 12. Retrieved 2012-10-04.
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