Water is a universal solvent. Soft rainwater picks up naturally occurring dissolved minerals, including calcium and magnesium carbonates, as it passes over rocks and through soil, which begins to make it hard. Most source water contains some amount of hardness. Water treatment is often required to make source water suitable for use in manufacturing processes, boilers, cooling towers, and rinse water applications.
The primary purpose of hard water softening is to prevent the precipitation and buildup of hard water minerals in equipment and piping. Reduction or elimination of hard water scaling can be performed using physical water treatment equipment, or, in limited circumstances, using chemical additives.
DEFINITION AND SOURCE OF HARDNESS
Limestone dissolved in water is termed “hardness.” Ground water dissolves limestone from deposits formed eons ago through the following steps: a) carbon dioxide reacts with water to produce carbonic acid which in the environment exists primarily as the bicarbonate ion (HCO3-1); b) microscopic marine organisms consume this as carbonate and form calcite skeletons which were deposited over millions of years to form limestone deposits which are found extensively in many parts of the world; c) ground water is often slightly to moderately acidic due to anaerobic decomposition and action of bacteria in the soil. This acidic characteristic causes limestone to be dissolved, in the form of calcium and bicarbonate ions, thus becoming hard.
PROBLEMS ASSOCIATED WITH HARD WATER
Calcium carbonate is moderately soluble in water and will come out of solution (i.e., form a precipitate) in the form of a hard scale when its concentration in water exceeds its solubility constant. This tendency may cause build-up in hot and cold-water pipes, water heaters, boiler tubes, cooling towers and any other surfaces it contacts. It also reacts with soap and detergent forming a precipitate in the form of a “scum” which is evident as spotting on glasses and silverware and as “bathtub ring.” The buildup in boilers can interfere with the transfer of heat and can even lead to boiler tube failure.