Water hardness is encountered every day of our lives, but what do you know about water softening? We present the facts here!

Hardness of Water

The hardness of water was initially recognized by the ability of water to precipitate soap; that is, water requires large amounts of soap to produce foam. Another feature of utmost importance in the industry recognized later, is the production of scale in hot water pipes, heaters, boilers, and some other units in which the water temperature is high.

The soap consumption capacity is important from the economic point of view and because of the difficulty of obtaining appropriate conditions for optimal cleaning. However, with synthetic detergents, this problem has decreased; therefore, the public demand for softened water in municipal treatment plants has also reduced, and the trend is towards private and industrial softening facilities except in those places where the hardness is exceptionally high.

The fouling problem has not reduced and it is a critical consideration, mainly in industry, because fouling can clog pipes to such an extent that explosions occur or industrial process units become useless, making it more economical to give the waters a softening treatment, which replaces cost of pipes, equipment, etc.

Hardness in water is mainly caused by the presence of calcium and magnesium ions. Some other divalent cations also contribute to the hardness, such as strontium, iron, and manganese, but a lesser degree since they are generally contained in small amounts.

Hardness is acquired by water as it passes through rock formations that contain the elements that produce it. The solvent power is achieved by water due to the acidic conditions that develop as it passes through the soil layer. The action of bacteria generates CO 2, which exists in equilibrium with carbonic acid. Under these pH conditions underwater, it attacks rocks, particularly calcite (CaCO 3 ), the compounds entering into solution.

According to the degree of hardness, the waters are classified as follows:

• 0 - 75 mg / 1 CaCO 3—soft water
• 75 - 150 mg / 1 CaCO 3—semi-hard water
• 150 - 300 mg / 1 CaCO 3—hard water
• More than 300 mg / 1 CaCO 3—very hard water

When the hardness is numerically greater than the sum of the alkalinities of carbonates and bicarbonates, the amount of hardness that is equivalent to this sum is called carbonate hardness, also called temporary, since when the water temperature rises to the point of Boiling, calcium and magnesium precipitate as calcium carbonate and magnesium hydroxide respectively.

The amount of hardness above carbonate hardness is called non-carbonate hardness and is distinguished as permanent; that is, it cannot be removed by thermal agitation, but chemical processes are necessary to remove it from the water. These water softening processes include softening with lime, soda-lime, and ion exchangers such as certain resins.

Hard water does not cause problems for the human body and is as satisfying as soft water; however, public acceptance varies from place to place. Its sensitivity depends on the degree of harshness to which people are accustomed. Many consumers object when the hardness of the water exceeds 150 mg/1 CaCO 3.

Processes for Water Softening

Lime - Soda Method

The water softening process with cal - soda (Ca (OH) 2 - Na 2 CO 3 ) precipitates the hardness of the water. The following reactions are carried out in this process, which must be considered to estimate the amounts of lime and soda necessary for softening.

Ion Exchange Method

This water softening method is an application of an old process that has been used for years to soften domestic water. The system works by exchanging ions from a solution with ions of a similar charge from a resin. When ion exchange is used to recover silver, the negatively charged silver thiosulfate complex found in wash water or a waste wash water mixture is exchanged with the anion in the resin. This is called the depletion step, and it is done by flowing the solution through a column containing the resin.

Three common ion exchange systems are used: conventional ion exchange, in situ precipitation, and electrolytic ion exchange circuit (combination of the first two methods).

Electrolyte System and Ion Exchange Combined

This water softening method uses an electrolytic system for primary recovery and an ion exchange system with in situ precipitation to further displace the effluent.