Hardness is primarily considered to be the dissolved Calcium and Magnesium which is found in all naturally occurring waters.

Raw water used for drinking water or for industrial purposes is usually obtained from surface sources, such as rivers or from below ground such as wells, or sometimes it can be supplied as a mixture of both.

If the supply is from surface or ground water, that water was originally obtained from rain, sleet or snow. Water collected over the oceans forms clouds as the air cools, the water held within the clouds is released. As the water falls to earth and passes through the atmosphere gasses dissolve into the water. It picks up carbon dioxide (CO2) and other gases and reaches the Earth’s surface as a weakly acidic solution. For example, CO2 + H2O = H2CO3 (Carbonic acid). 

The rain that forms surface waters such as upland lakes, streams and rivers are normally low in dissolved solids and hardness. However, some water seeps into the ground and if the ground rock is porous it passes underground until it reaches impervious rock where it forms underground water sources (aquifer) from where it can be extracted. During the time the water passes through various strata containing minerals, these dissolve into the acid water. One such mineral is limestone (CaCO3) which neutralizes the acidic rainwater forming a soluble calcium bicarbonate salt (H2CO3 + CaCO3 = Ca(HCO3)2) which can dissociate into ions: Calcium ions (Ca++) which is a 2+ positive charged cation and two bicarbonate ions HCO3- each with a single negative charge.  The same also applies to magnesium. This is how calcium, magnesium and other salts dissolve and contribute ions into all the water supplies around the world. Where the calcium and magnesium content are high then these waters are normally described as “hard” waters and are usually obtained from groundwater sources.

Calcium and Magnesium is not always associated with bicarbonate. Hardness can enter the water by dissolving from other sources where the calcium and magnesium is associated with other anions such as chloride or nitrate or sulfate etc.

Temporary and Permanent Hardness
Often in water analyses you will see the hardness expressed as the Total Hardness or split into two terms “temporary hardness” or “permanent hardness”. The difference between these two types of hardness is determined by which anion was associated with the calcium or magnesium. Hardness associated with bicarbonate is called “temporary hardness”. That associated with any other anions is called the “permanent hardness”. 

In most cases, on any water analyses, these two types of hardness are usually expressed in common terms, such as mg/l or ppm as CaCO3 or in meq/l. When expressed in common terms, you will notice that the calcium and magneium is greater than the bicarbonate present in the water, so there is always both temporary and permanent hardness. In these circumstances the temporary and permanent hardness can be added together to give the “Total Hardness” of the water supply. 

Occasionally however you will notice that the bicarbonate is higher than the total hardness of the water and hence there is no permanent hardness and the water actually contains sodium alkalinity which does not contribute to the water hardness. Such supplies are somewhat unusual, but later when we look at dealkalization it will be an important factor.

Why do we need to remove hardness?
Hard water can cause many problems in many water applications. When heated, salts of calcium and magnesium are less soluble than salts of potassium and sodium and so on boilers, and cooling systems the increase in concentration can result is solids forming,

When hard water is heated, the Ca(HCO3)2 decomposes to CaCO3 and evolves CO2 gas and H2O (water). CaCO3 has a limited solubility and hence it can then be deposited on the surfaces of water heaters, often found in boilers, coffee makers, pipes and on to fabrics, creating a hard coating (This is often referred to as calcite, lime scale, boiler scale or hardness scale). This scale is a poor conductor of heat and so it can affect the heating system because it takes more energy to heat water when the heater element or boiler tube is covered with this scale. In addition, because the heating is uneven across the heating surface it can cause premature failures. The scale, if dislodged from the surface can form a suspended solids problem clogging the pipes and boiler and reducing the performance and leading to failure of both domestic and industrial appliances.

There are many processes which cannot accept hardwater, for example laundries. The washing of clothes in hard water can reduce the life of fabrics due to its abrasive nature and when washing yourself, at home, you will have noticed the soluble calcium and magnesium salts will react with soap causing the familiar bathtub ring and soap scum that many people see in hard water areas.

These are just simple examples we will all be familiar with, but there are many more industrial and chemical processes that cannot use hardwater. Needless to say, it is therefore often desirable to remove hardness ions before using the water in many residential as well as industrial applications, particularly if the local supply is from a hard water source.