**2.2 Importance of calcium**

Next to alkalinity, calcium hardness is the most important parameter for posttreatment process for a number of reasons. Firstly, it is the most suitable counterion to the anionic alkalinity species. Other potential alternatives can result in water that is toxic to plant life (e.g. sodium ions), or are not so readily available (e.g. potassium). Calcium carbonate on the other hand, is one of the most readily sourced minerals in the world, making up 4% of the earth's crust [10]. Secondly, it is the concentration of calcium ions in addition to alkalinity and pH that defines the calcocarbonic equilibrium of the water. The calco-carbonic equilibrium defines a water's propensity to dissolve or precipitate calcium carbonate and is the primary indication of water stability for a number of reasons. Water that is aggressive to calcium carbonate, will be aggressive to concrete or cement-lined pipe, along with asbestos cement pipe. Because of the amount of water distribution infrastructure that is either made from concrete (storage tanks) or cement-lined (mild-steel concretelined or ductile iron concrete-lined pipe), this represents one of the largest investment costs for utility owners and highlights the importance of protecting these from corrosion. Whilst on the other hand, the precipitation of a thin layer of calcium carbonate on the surfaces of water treatment infrastructure is considered a suitable strategy against corrosion for a wide range of materials.

Finally, calcium has a number of health benefits to the consumer, with increased calcium levels within drinking water being linked to decreased incidences of cardiovascular disease. This has been acknowledged and accepted by the World Health Organization, who have clearly stated in their background document for the development of Guidelines for Drinking Water Quality, that insufficient calcium intake is associated with increased risks of osteoporosis, kidney stones, hypertension, stroke, coronary artery disease, some cancers and even obesity [11]. Calcium in drinking water is not just important for the body, but also for the teeth. Demineralization and remineralization processes are constantly taking place on the surface of tooth enamel based largely on the surrounding fluid. Saturation of the surrounding fluid with respect to calcium is critical to promote the remineralization or repair of dental tissue [12].
