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The application of Appropriate Technology

An Introduction To Water Hardness

A document providing an introduction to the causes and treatment of hard water, including magnetic water softeners.

Some Simple Chemistry

When ionic substances dissolve in water they are split into their constituent ions. For example when NaCl (sodium chloride, common salt) dissolves in water it produces Na+ (the positive or cation) and Cl (the negative or anion). These ions stay in the solution unless something happens to make them precipitate out as a solid; for example if water is evaporated from a salt solution, the solution becomes saturated and salt crystals will precipitate. The process of dissolving is therefore not a chemical reaction and can be reversed by physical, rather than chemical, means.

The Causes Of Hardness

Most hardness in water is caused by the presence of dissolved calcium (Ca2+) and magnesium (Mg2+) ions. Other cations, such as Al3+ and Fe3+ can contribute to hardness, however their presence is less critical.

The most common, and troublesome, form of hardness is caused by the presence of calcium bicarbonate (Ca(HCO3)2) which is picked up by rain water passing through lime stone (CaCO3). As rain water falls it dissolves carbon dioxide (CO2) from the air and becomes slightly acidic because carbonic acid (H2CO3) is formed:

CO_2 (g)+H_{2}O (l)\rightarrow H_2CO_3(aq)

In the above equation g=gas, l=liquid and aq=aqueous (i.e. dissolved in water). In the following equation s= solid.

CaCO3 is not very soluble in water however, when the dilute acid runs through the lime stone a reaction occurs that creates calcium bicarbonate which is readily soluble:

CaCO_3 (s) + H_2CO_3 (aq)\rightarrow Ca^{2+} (aq) + 2HCO^{3-} (aq)

Thus the rain water has picked up Ca2+ and HCO3 (bicarbonate) ions and become hard. When hard water is heated the previous two reactions are reversed and calcium carbonate, water and carbon dioxide are formed:

Ca^{2+}(aq)+2HCO^{3-}(aq)\rightarrow CaCO_3(s)+H_2O(l)+CO_2(g)

Since calcium carbonate is much less soluble in water than calcium bicarbonate it precipitates out of solution as a solid known as scale or lime scale. Because this type of hardness is easily removed (i.e. by simple heating) it is known as temporary hardness. Scale normally appears around heating elements and hot water systems. However, if the water is exceptionally hard scaling may occur in cold water pipes [1].

Other types of temporary hardness are caused by the presence of Mg2+ ions and the precipitation of magnesium hydroxide (Mg(OH)2) can contribute to scaling problems.

Combinations of Ca2+ and Mg2+ ions with chloride (Cl), sulphate (SO42-) and nitrate (NO32-) ions are known as permanent hardness. For example in some areas CaSO4 may cause considerable hardness. Permanent hardness can not be removed by boiling.

The term hardness total hardness is used to describe the combination of calcium and magnesium hardness. However, hardness values are usually quoted in terms of CaCO3 because this is the most common cause of scaling. The standard classifications are given below [1]:

Hardness mg/L as CaCO3
moderate 60-120
hard 120-180
very hard more than 180

Problems Caused By Hardness

  • Excessive soap is needed for washing (i.e. soap will not lather). Some modern detergents work less efficiently because anions (also known as surfactants) which are meant to hold dirt particles in suspension react with Ca2+ and Mg2+ instead [2].
  • Soap based on animal fats can react with Ca2+ and Mg2+ forming a precipitation that can ruin cloths and irritate skin.
  • Some foods, particular dried beans and peas, become tough and rubbery when cooked in hard water. Calcium ions cause cross-linking to occur between certain molecules within the beans, the subsequent structure prevents water entering and the bean remains hard. A simple way to counteract this effect is to ad baking soda (sodium bicarbonate (NaHCO3)) to the cooking water [3] [4] [5].
  • Scale can clog pipes and fittings. Also heating elements can become 90% less efficient with a 25mm coating of CaCO3 [6].

Softening

There are three basic ways to soften water:

  • Force insoluble substances, such as CaCO3 and Mg(OH)2, to precipitate before water enters the system.
  • Remove the Ca2+ and Mg2+ ions from the water.
  • Prevent the Ca2+ ions from CaCO3 by complexing them.

A. Soda-Lime Softening

Lime (calcium hydroxide – Ca(OH)2) and soda ash (sodium carbonate – Na2CO3) is added to the water causing CaCO3 and Mg(HO)2 to precipitate out.

In large municipal systems Ca(OH)2 is added and the solid precipitates are removed though sedimentation however fine particles will remain, therefore carbon dioxide is bubbled through the water to turn these back into soluble bicarbonates. Because some of the hardness is retained this process is know as a partial softening process.

On a smaller scale Na2CO3 (also known as washing soda) can be added to hard water which is to be used for washing cloths.

B. Cation Exchange

This process replaces the Ca2+ and Mg2+ ions with other ions which do not contribute to such as Na+ and k+. This is achieved by running the water through a container filled with a resin that contains sodium or potassium ions. The ions in the resin are exchanged for the hardening ions in the water. Once all of the resin ions have been used up the resin can not remove any more Ca2+ and Mg2+ ions until it has been ‘regenerated’ using NaCl or KCl. This method is commonly used in small domestic supplies.

Cation exchange is a ‘total softening’ method because it will remove all of the hardness.

C. Complexing

Another method used in small domestic water softeners is to add polyphosphates (containing the P6O186-) to the water. These ions surround the Ca2+ ions in solution and prevents them from precipitating as CaCO3: a process known as complexing.

Electric And Magnetic Anti-scaling Devices

Using electrical and magnetic fields to prevent scaling is a very contentious issue. There is no consistent data to prove how such methods work or indeed if they work at all. However, they do seem to work to some degree under certain circumstances [7] [8] [9].

The idea behind anti-scaling devices is not to remove Ca2+ or CaCO3 but to precipitate CaCO3 in a form that prevents it from making a solid, stubborn scale. It seems that after passing through these devices the CaCO3 is supposed to either stay as very fine particles suspended in the water or to settle out as a flaky, easily dislodged scale. One theory as to how this happens is as follows:

The water is passed through an electric or magnetic field which causes a localised increase in pH (via an electro-chemical reaction involving an electron and an O2 molecule producing an OH ion). The high pH allows CaCO3crystal nuclei to form in the body of water. These minuscule crystals clump together to form colloids. As these colloids are carried though the water they will not stick to the pipe surface and any further precipitation that occurs will deposit CaCO3 on the colloids and not the pipe. Thus the CaCO3 is held in a suspension of fine particles and if sedimentation occurs a very loose scale results.

Since this method requires an electrical current, the water must be flowing through the magnetic field (so that a current is induced), therefore the magnet will have no effect on static water. This seems to have been verified experimentally [10].

These devices are usually fitted to heating and hot water systems. However, many domestic users seem disappointed that they apparently have little effect, particularly on soap lathering. In the absence of any sedimentation CaCO3 will not be removed from the water and the overall chemical composition of the water will not change, therefore the only way to determine the magnets’ effectiveness against scale build up is long term observations. Interestingly industrial users, who continually pump the same water through a heating system (and the magnet) seem the most satisfied with these devices [6].

Anecdotal evidence suggests that beans, which could not be cooked in extremely hard water, were successfully cooked in water from the same source that had been passed through a magnetic device. This may be explained by the fact that the device causes the Ca2+ ions, which prevent the beans softening [4] [5], being removed from solution as very fine CaCO3 particles.

Since the process by which these magnetic devices work is not widely understood and since the experiments have been largely unrepeatable (in different labs around the world) what works on one water system may not work on another. Experiments performed by Cranfield University (UK) suggest that the degree to which the magnetic devices work is greatly effected by the water’s hardness and alkalinity [6].

Over Softening

The over softening of water can have an adverse effect on the corrosion rate of cast iron and galvanised steel pipes. Very soft water can also hasten the decay of concrete tanks [2].

Normally, the inside of ferrous pipes are protected by a layer which builds up as water flows, this layer contains CaCO3 and Fe. If very soft (aggressive) water is passed down a galvanised pipe the zinc layer is slowly removed exposing the steel. Also if there is not enough Ca2+ ions in the water calcium compounds can leach out of concrete causing a deterioration in it’s structural integrity.

Therefore care should be taken when using total softening methods with water that is to pass through ferrous pipe work.

Common Chemical Names

Baking Soda – NaHCO3 Sodium Bicarbonate

Caustic Soda – NaOH Sodium Hydroxide

Chalk – A type of lime stone that is largely CaCO3 Calcium Carbonate

Lime* – CaOH calcium Hydroxide

Lime Stone – Compositions vary, largely CaCO3 and MgCO3 – Calcium Carbonate and Magnesium Carbonate

Soda Ash – Na2CO3 Anhydrous Sodium Carbonate

Washing Soda – Na2CO3.10H2O Hydrated Sodium Hydroxide

Quick Lime* – CaO Calcium Oxide

*Both CaOH and CaO are sometimes called lime. When CaO is added to water CaOH is formed and the resulting solution is called lime water (CaOH forms this way is also called slaked lime). CaO formed by heating calcium in oxygen is called quick lime.

  1. [1]Theory And Practice Of Water And Waste Water Treatment, Ronald L. Droste, John Wiley and Son Inc., 1997.
  2. [2]Prevension Of Corrosion And Scaling In Water Supply Systems, L. Legrand, P. Leroy, Ellis Harwood, 1990.
  3. [3]Rural Water Supply And Sanitation, 3rd edition, Forrest B. Wright, Robert E. Krieger Pub. Co., 1977.
  4. [4]The Effect Of Water Quality On Food, O. Peter. Snuder http://www.hitm.com/documents/chem-h2o.htm
  5. [5]The Effects Of The Calcium Ion On Cooked Dry Beans, Mark A. Uebersax Find the PFD file athttp://math.unl.edu/~jump/center/labs/
  6. [6]Antiscale Magnetic Treatment, The School Of Water Science, Cranfield University,http://www.cranfield.ac.uk/sims/water/magnet.htm
  7. [7]Magnetic Water And Fuel Treatment: Myth, Magic Or Mainstream Science? http://wwwcsicop.ore/si/9801powwel.html
  8. [8]Budget Water USA, http://budgetwater.com/magnets_dont_work.htm
  9. [9]Magnetic Water “Conditioning” Lime Fighters (faqs at the bottom of page)http://plumbingstore.com/limefighters.html
  10. [10]Softening Water – Magic Realism (first published in The Economist, 2/3/1996)http://www.mtnhigh.com/mag_econ.html