Hard Water

What is hard water?

Water is hard if soap does not want to foam or froth when used. Hardness is mainly caused by the divalent cations of calcium (Ca2+) and magnesium (Mg2+) present in water. Iron also contributes to water hardness. All water sources contain calcium and magnesium in various concentrations and hard water usually has a high mineral content (Total Dissolved Salts – TDS).

How is hard water formed?

Rainwater is naturally soft, i.e. it has a low mineral content (TDS). As rain falls through the atmosphere, it absorbs acidic gases such as carbon dioxide (CO2) and sulphur dioxide (SO2) from the air causing the rain water to become acidic. When reaching the ground, the acidic water percolates through the soil and comes into contact with calcium and magnesium bearing rocks. Calcium enters the water either as calcium carbonate (CaCO3) from limestone and chalk or as calcium sulphate (gypsum – CaSO4) from other mineral deposits. Magnesium ions mainly come from dolomite (Ca.Mg(CO3)2).

The effects of hard water

Hard water is a menace which industries, agriculture and households have to cope with. Much money is spent each year on extra soap, replacement of heater elements, cleaning of nozzles, cooling towers, etc. Soap and other detergents curd when they react with calcium and magnesium. These curds form a layer on fabrics, dulling colours and making whites look grey. It also makes individual fibres brittle, which shortens the life of the fabrics and garments. Insoluble salt residues remain in hair after shampooing with hard water, making hair rougher and harder to manage.

Another serious problem hard water causes is the forming of scale. Scale or lime is formed when hard water is heated. The scale precipitates out on warm water pipes, taps, geyser, boiler and kettle elements. If the scale is left to build up it can become hard as rock. Elements need to heat up the scale first before heating up the water, consuming much more electricity.

Test for hard water

A simple test can be done to determine if water is hard. When toothpaste or soap is agitated in hard water it will not lather as easy as in soft water. The presence of the hardness ions (mainly calcium and magnesium) produce a solid scum or curd in water, which causes the so called “bath tub ring”.

Kinds of Hard Water

  1. Temporally Hardness

Temporally hardness is caused by soluble calcium bicarbonate (Ca(HCO3)2) and/or magnesium bicarbonate (Mg(HCO3)2) present in groundwater.

  1. Permanent Hardness

When temporally hard water is heated, it forms calcium carbonate and/or magnesium carbonate (MgCO3) which precipitates out as scale or lime. Permanent hardness cannot be removed by boiling.

Total Hardness

Total hardness is the sum of all hardness constituents in water and is expressed as the equivalent concentration of calcium carbonate. It is primarily caused by calcium and magnesium, but may include small amounts of metals such as iron which can act like calcium.

Determining of Hard Water

By definition a litre of water weighs one million milligrams, which means that 1 milligram per litre is the equivalent to a part per million.

Total water hardness (calcium and magnesium included) is expressed in parts per million (ppm) or milligram per litre (mg/l) CaCO3 (calcium carbonate). Water hardness tests usually measure the total concentration of Ca and Mg, the dominant divalent metal ions. In certain regions iron, aluminium and manganese can also be present in high concentrations. It can be determined by using test strips or, hardness test kits, such as Hagen, Serra or, more accurately, by titration or ion specific photometers.

To determine how hard water will react, more information is needed, such as pH and temperature.

The average South African uses about 180 litres of water per day (cooking, washing, bathing, toilets, etc.).

Hardness Classification

Perfect Water uses the hardness classification table from the Water Quality Association (WQA).

mg/l or ppm Classification
<17 soft
17 – 60 slightly hard
60 – 120 moderately hard
120 – 180 hard
>180 very hard


Water of up to 60 mg/l or ppm does not need softening. Generally, people are not aware of hardness at readings <60 mg/L or ppm. Should you want to know more about hard water and how to address it, please contact Perfect Water Head Office on 011-907-8377 or email us at info@perfectwater.co.za.

Be wise, don’t compromise



Hard Water and Water Softening: www.chem1.com

Water Softening and Septic Tanks: www.uwsp.edu

Wilkes University Centre for Environmental Quality: www.water-research.net

SRC Smart Science Solutions: www.src.sk.ca

Free Drinking Water: www.freedrinkingwater.com

Water Quality Association: www.wqa.org

Lenntech: www.lenntech.com

Water Quality Association (1994): Water Treatment Fundamentals, USA.