How The Softening Process Works...

It is often desirable to soften hard water, as it 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 to reduce the effects of scaling and to make it more suitable for laundry and bathing.

The Process

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.

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 mineral ions for sodium ions. Artificial or natural zeolites can also be used.


As the hard water passes through and around the beads, the hardness mineral ions are preferentially absorbed, displacing the sodium ions. This process is called ion exchange. When the bead or sodium zeolite has a low concentration of 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 concentration of sodium ions alter the equilibrium between the ions in solution and the ions held on the surface of the resin, resulting in replacement of the hardness mineral ions on the resin or zeolite with sodium ions. The resulting saltwater and mineral ion solution is then rinsed away, and the resin is ready to start the process all over again. This cycle can be repeated many times.


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 (see image on the left).




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", and can also cause hard water stains. This scale is composed of 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 precipitates out and sodium sulfate is formed which is soluble. The calcium carbonate (also known as limescale) formed sinks to the bottom. Sodium sulfate has no effect on the hardness of water.

Na2CO3 + CaSO4 → Na2SO4 + CaCO3

Health Issues of Hard Water

The World Health Organization says, "There does not appear to be any convincing evidence that water hardness causes adverse health effects in humans."

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 liter of water.

Other studies have shown weak correlations between cardiovascular health and water hardness. The World Health Organization has reviewed the evidence and concluded the data were inadequate to allow for a recommendation for a level of hardness.

In a review by František Kožíšek, M.D., Ph.D. National Institute of Public Health, Czech Republic gives a good overview of the topic, and unlike 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.

Also, the National Research Council (National Academy of Sciences) advises that hard drinking water generally contributes a small amount toward total calcium and magnesium human dietary needs. It further states that in some instances, where dissolved calcium and magnesium are very high, water could be a major contributor of calcium and magnesium to the diet.

For some good information on how hard water may "stop" heart attacks, see this article at the BBC website at http://news.bbc.co.uk/1/hi/health/3396141.stm.

Testing for Hard Water

If you are on a municipal water system, the water supplier can tell you the hardness level of the water they deliver. If you have a private water supply, you can have the water tested for hardness. There are several different scales used to describe the hardness of water in different contexts.

•mmol/L (millimoles per litre)
•mg/L calcium carbonate equivalent
•grains/gallon (gpg). 1 gr/US gal = 17.11 mg/L
•parts per million weight/volume (ppm w/v or ppm m/v)

Various obsolete "degrees":

English degrees (°E) Clark degrees (°Clark) - 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.

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

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

As a guide, 60% of the UK and 85% of the US population live in a hard water area.

Most water softener companies will be able to supply you with a free water testing kit. Once you've tested your water supply, the hardness of your water will be reported in grains per gallon, milligrams per liter (mg/l) or parts per million (ppm). One grain of hardness equals 17.1 mg/l or ppm of hardness.

Hard Water in Heaters and Industry

Problems in Water Boiler Systems and Pipework
Hard water also contributes to inefficient and costly operation of water-using appliances. Heated hard water forms a scale of calcium and magnesium minerals (limescale deposits) that can contribute to the inefficient operation or failure of water-using appliances. Pipes can become clogged with scale that reduces water flow and ultimately requires pipe replacement. Limescale has been known to increase energy bills by up to 25%

Limescale in Solar Heating Systems
Solar heating, often used for heating swimming pools is prone to limescale buildup, which can reduce the efficiency of the electronic pump and therefore the overall systems performance will deteriorate.

There are low cost solutions to this problem which prevents limescale build up and over time will remove existing deposits completely – more information on this can be found in Chapter 4.

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 and ion exchange resins.

Hard Water Problems...

Bathroom
Showerheads and spray-nozzles can become blocked; they can even clog the small holes completely and reduce their efficiency. The bathtub and sink seem to be the places where there is a visible soap scum build up. Without proper treatment his build up is very difficult to remove and may require a lot of cleaners and many applications. Scale can clog pipes and can decrease the life of toilet flushing units.

Bathing
Bathing with soap in hard water leaves a film of sticky soap curd on the skin. The film may prevent removal of soil and bacteria. Soap curd interferes with the return of skin to its normal, slightly acid condition, and may lead to irritation. Soap curd on hair may make it dull, lifeless and difficult to manage. Similarly, the insoluble salts that get left behind from using regular shampoo in hard water tend to leave hair rougher and harder to detangle.

Laundering
Clothes washed in hard water often look dingy and feel harsh and scratchy. The hardness minerals combine with some soils to form insoluble salts, making them difficult to remove. Soil on clothes can introduce even more hardness minerals into the wash water. Continuous laundering in hard water can damage fibers and shorten the life of clothes by up to 40 percent.

Dishwashers
When washing dishes, especially in a dishwasher, hard water may cause spotting and filming on your crockery. The minerals from hard water are released faster when it comes into contact with heat, causing an increase in the amount of spotting and filming that occurs. This problem is not a health risk, but it can be a nuisance to clean and reduce the quality of your crockery.