Hard water
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Hard water is water that has a high mineral content (water with a low mineral content being known as soft water). This content usually consists of high levels of metal ions, mainly calcium (Ca) and magnesium (Mg) in the form of carbonates, but may include several other metals as well as bicarbonates and sulphates. While it is not generally dangerous to safety or health, it does generally cause potentially costly nuisance problems in the home and in industry. On the other hand, hard water is considered to be ideal for brewing certain styles of beer. Kentucky's distinctive Bourbon whiskey owes its flavor in part to the high calcium content of its groundwater (compared to the pilsener style of beer, the brewing of which requires very soft water). Some hard water chemicals, particularly silicates and calcium carbonate, are also effective corrosion inhibitors, and can prevent damage to pipes or contamination by potentially toxic corrosion products.
The simplest way to determine if water is hard or soft is the lather/froth test. If the water is very soft, soap will tend to lather up easily when agitated, whereas with hard water it will not. Toothpaste will also not froth well in hard water. More exact methods of hardness detection use a wet titration method to determine hardness.
For many uses, including drinking (aside from the taste), it does not matter whether water is hard or soft. For instance, to put out fires, water a lawn, or wash the mud off the streets, water would have to be very hard to cause a problem. But for bathing, washing dishes and clothes, shaving, washing a car and many other uses of water, hard water is not as efficient or convenient as "soft water." Hard water can cause limescale deposits in kettles, washing machines and pipes.
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What is hard water?
Earlier generations coined the phrase "hard water" because it made cleaning difficult. Hardness is caused by compounds of calcium and magnesium. All freshwater sources contain calcium and magnesium in varying quantities. Water dissolves, suspends, and/or exchanges certain trace elements and compounds from many things that it contacts on its travels. For example peat will soften water whereas limestone will harden it.
Total water hardness (including both Ca and Mg) is reported as ppm w/v (or mg/L) of CaCO3. Water hardness usually measures the total concentration of Ca and Mg, the two most prevalent divalent metal ions, although in some geographical locations iron, aluminium, and manganese may also be present at elevated levels. Calcium usually enters the water from either CaCO3, as limestone or from mineral deposits of CaSO4. The predominant source of magnesium is dolomite, CaMg(CO3)2.
Types of hard water
A common distinction is made between 'temporary' and 'permanent' hardness.
Temporary Hardness
Temporary hardness is hardness that can be removed by boiling or by the addition of lime (calcium hydroxide). It is caused by dissolved calcium bicarbonate in the water.
Calcium carbonate is less soluble in hot water than in cold water, so boiling (which promotes the formation of carbonate) will precipitate calcium carbonate out of solution, leaving water that is less hard on cooling.
Permanent hardness
Permanent hardness is hardness (mineral content) that cannot be removed by boiling. It is usually caused by the presence of calcium and magnesium sulfates and/or chlorides in the water, which are more soluble as the temperature rises.
Types of measurement
It is possible to measure the level of hard water by obtaining a free water testing kit. These are supplied by most water softening companies. There are several different scales used to describe the hardness of water in different contexts.
- mmol/L
- mg/L calcium carbonate equivalent
- grains/gallon (gpg)
1 gr/U.S. gal = 17.11 mg/L - parts per million weight/volume (ppm w/v or ppm m/v)
- Various obsolete "degrees":
- Clark degrees (°Clark)/English degrees (°E)
- conversion to mg/L calcium: divide by 0.175
One degree Clark corresponds to one grain of calcium carbonate in one Imperial gallon of water which is equivalent to 14.28 parts calcium carbonate in 1,000,000 parts water. - German degrees (°dH)
- conversion to mg/L calcium: divide by 0.14
One degree German corresponds to one part calcium oxide in 100,000 parts of water. - French degrees (°f)(shares symbol with degree Fahrenheit, but in lowercase)
- conversion to mg/L calcium: divide by 0.25
One degree French corresponds to one part calcium carbonate in 100,000 parts of water. - American degrees
One degree American corresponds to one part calcium carbonate in 1,000,000 parts water (1 mg/L or 1 ppm) - Degrees of general hardness (dGH)
One degree of general hardness corresponds to 10 mg of calcium oxide or magnesium oxide per litre of water
- Clark degrees (°Clark)/English degrees (°E)
The precise mixture of minerals dissolved in the water, together with the water's acidity or alkalinity (pH) and temperature will determine the behaviour of the hardness, so single number on a scale does not give a full description. Descriptions of hardness correspond roughly with ranges of mineral concentrations:
- Soft: 0 - 20 mg/L as calcium
- Moderately soft: 20 - 40 mg/L as calcium
- Slightly hard: 40 - 60 mg/L as calcium
- Moderately hard: 60 - 80 mg/L as calcium
- Hard: 80 - 120 mg/L as calcium
- Very Hard: >120 mg/L as calcium
Problems caused by hard water
While hard water is not generally unhealthy (see below), it can cause many potentially costly nuisance problems.
Hard water causes scaling, which is the precipitation of minerals to form a rock-hard deposit called limescale. Scale can clog pipes and can decrease the life of toilet flushing units by 70% and water taps by 40%. It can coat the inside of tea and coffee pots, and clog and ruin water heaters.
In the home environment, hard water requires more soap and synthetic detergents for laundry and washing. It takes half as much soap for cleaning with soft water. Hard water and soap combine to form "soap scum" that can't be rinsed off, forming a “bathtub ring” on all surfaces, and it dries leaving unsightly spots on dishes.
Using soap on the body in hard water can cause the formation of a scum often referred to as “curd.” The formation of scum and curd is caused when calcium and magnesium form insoluble salts with anions. This curd remains on the skin even after rinsing, clogging pores and coating body hair. This can serve as a medium for bacterial growth, causing nappy rash, minor skin irritation and skin that looks dry and continually itches.
Similarly, the insoluble salts that get left behind from using regular shampoo in hard water tend to leave hair rougher and harder to detangle. <ref>Body And Fitness Healthy Hair Tips</ref>
In industry, hard water contributes to scaling in boilers, cooling towers and other industrial equipment. In these industrial settings, water hardness must be constantly monitored to avoid costly breakdowns. Hardness is controlled by addition of chemicals and by large-scale softening with zeolite resins.
Health considerations
Some studies have shown a weak inverse relationship between water hardness and cardiovascular disease in men, up to a level of 170 mg calcium carbonate per litre of water. The World Health Organization has reviewed the evidence [1] and concluded the data were inadequate to allow for a recommendation for a level of hardness.
A later review [2] by František Kožíšek, M.D., Ph.D. National Institute of Public Health, Czech Republic gives a good overview of the topic, and conversely to the WHO, sets some recommendations for the maximum and minimum levels of calcium (40-80 mg/L) and magnesium (20-30 mg/L) in drinking water, and a total hardness expressed as the sum of the calcium and magnesium concentrations of 2-4 mmol/L.
Very soft water is more likely to corrode (i.e. react chemically with) metal pipes in which it is carried, and as a result it may have elevated levels of cadmium, copper, lead and zinc.
Softening
A water softener works on the principle of cation or ion exchange in which ions of the hardness minerals are exchanged for sodium or potassium ions, effectively reducing the concentration of hardness minerals to tolerable levels.<ref>How does a water softener work? at Howstuffworks.com</ref>
The most economical way to soften household water is with an ion exchange water softener. This unit uses sodium chloride (table salt) to recharge beads made of ion exchange resin that exchange hardness minerals for sodium. Artificial or natural zeolites can also be used. As the hard water passes through and around the beads, the hardness mineral ions attach themselves to it, dislodging the sodium ions. This process is called ion exchange. When the bead or sodium zeolite has no sodium ions left, it is exhausted, and can no longer soften water. The resin is recharged by flushing (often back-flushing) with saltwater. The high excess of sodium ions force the hardness ions off the resin beads. The excess sodium is rinsed away, and the resin is ready to start the process all over again. This cycle can be repeated many, many time before the resin loses its ability to react to these forces.
Some softening processes in industry use the same method, but on a much larger scale. These methods create an enormous amount of salty water that is costly to treat and dispose of.
Temporary hardness, caused by hydrogen carbonate (or bicarbonate) ions, can be removed by boiling. For example, calcium hydrogen carbonate, often present in temporary hard water, is boiled in a kettle to remove the hardness. In the process, a scale forms on the inside of the kettle in a process known as "furring of kettles". This scale is composed of insoluble calcium carbonate.
Ca(HCO3)2 → CaCO3 + CO2 + H2O
Hardness can also be reduced with a lime-soda ash treatment. This process, developed by Thomas Clark in 1841, involves the addition of slaked lime (calcium hydroxide — Ca(OH)2) to a hard water supply to convert the hydrogen carbonate hardness to carbonate, which precipitates and can be removed by filtration:
Ca(HCO3)2 + Ca(OH)2 → 2CaCO3 + 2H2O
The addition of sodium carbonate also softens permanently hard water containing calcium sulfate, as the calcium ions form calcium carbonate which is insoluble and sodium sulfate is formed which is soluble. The calcium carbonate formed sinks to the bottom. Sodium sulfate has no effect on the hardness of water.
Na2CO3 + CaSO4 → Na2SO4 + CaCO3
It is desirable to soften hard water, as the latter does not readily form lather with soap. Soap is wasted when trying to form lather, and in the process, scum forms.
Hard water may be treated in other ways to reduce the effects of scaling and to make it more suitable for laundery and bathing. See residential water treatment
Hard water in Australia
Analysis of water hardness in major Australian cities by the Australian Water Association shows a range from very soft (Melbourne) to very hard (Adelaide). Total Hardness as Calcium Carbonate mg/L are: Melbourne: 11.3 - 14.0; Sydney: 39.4 - 60.1; Perth: 30 - 198; Brisbane: 100; Adelaide: 101 - 216; Hobart: 6 - 50; Darwin: 22 - 38.
Hard water in Canada
The Laurentian shield does not leach many minerals into the water, resulting in very soft source and surface water. However, the moraine material on which the prairie provinces are located (mainly Saskatchewan and Manitoba) contains high quantities of calcium and magnesium, often as dolomite, which are readily soluble in the groundwater that contains high concentrations of trapped carbon dioxide from the last glaciation. In these parts of Canada, the total hardness in mg/L calcium carbonate equivalent frequently exceeds 200 mg/L, if groundwater is the only source of potable water.
Some typical values are: Calgary 165 mg/L, Saskatoon < 140 mg/L, Toronto 121 mg/L, Vancouver < 5 mg/L.
Hard water in England and Wales
Information from the British Drinking Water Inspectorate shows that drinking water in England is generally considered to be 'very hard', with most areas of England, particularly the East, exhibiting above 200 mg/L as calcium carbonate equivalent. Wales, Cornwall and parts of North-West England are softer water areas, and range from 0 to 200 mg/L.
Hard water in the US
According to the US Geologic Survey, 85% of US homes have hard water. The softest waters occur in parts of the New England, South Atlantic-Gulf, Pacific Northwest, and Hawaii regions. Moderately hard waters are common in many of the rivers of the Tennessee, Great Lakes, Pacific Northwest, and Alaska regions. Hard and very hard waters are found in some of the streams in most of the regions throughout the country. Hardest waters (greater than 1,000 mg/L) are in streams in Texas, New Mexico, Kansas, Arizona, and southern California.<ref>Briggs, J.C., and Ficke, J.F.; Quality of Rivers of the United States, 1975 Water Year -- Based on the National Stream Quality Accounting Network (NASQAN): U.S. Geological Survey Open-File Report 78-200, 436 p. (1977)</ref>
See also
Note: Hard water should not be confused with Heavy Water.
References
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External links
- HardWater.org
- "With soft water, why can't we rinse off all the soap?"
- "How Water Softeners Work"de:Wasserhärte
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