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 softening is often required to make source water suitable for use in manufacturing processes, boilers, cooling towers, and rinse water applications.
There are two main types of diffused aeration systems, retrievable and fixed grid, that are designed to serve different purposes. In the case of a plant with a single tank, a retrievable system is desirable, in order to avoid stopping operation of the plant when maintenance is required on the aeration system. Fixed systems, on the other hand, are typically less costly, and often more efficient because it is easier to make full use of the floor.
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 treatment equipment, or, in limited circumstances, through the use of chemical additives.
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.
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.