Purification: when water is made good enough to drink

Purification processes are carried out to ensure that wateris compliant with the established requisites for drinking, and to improve its organoleptic (colour, odour, taste), chemical (e.g. removal of iron and manganese) and microbiological (e.g. disinfection of water to eliminate Escherichia coli) characteristics.

The main purification treatments are, in sequence, the following.

• Sedimentation: a physical process that removes solids contained in water by gravity;
• Clariflocculation: several chemical compounds (e.g. aluminium salts) are added to the water as a coagulating agent. They bring small particles (that do not settle naturally)together to form larger particles which can then be removed via filtration;
• Oxidation: performed using suitable chemical agents (e.g. potassium permanganate, ozone) which interact with dissolved contaminants (organic and inorganic substances) and aid their removal;
• Filtration: used to eliminate particles still present after the preceding processes. Sand filtration is a physical process that separates out particles that escaped the sedimentation and clariflocculation stages; active carbon filtration removes mainly organic micropollutants;
• Disinfection: this final treatment stage acts on the residual microbiological component and ensures the absence of pathogenic micro-organisms; this is usually achieved using chlorine-based products (sodium hypochlorite, chlorine dioxide) that allow a residual quantity of disinfectant to be maintained throughout the distribution phase. Chlorine dioxide is produced in liquid solution from the reaction between hydrochloric acid and sodium chlorite at the plant; chlorites are its disinfection by-products. Ready-to-use sodium hypochlorite is, instead, associated with trihalomethane disinfection by-products. For both of these by-products, Italian Legislative Decree 31/2001 (see Annex B) establishes the parameters to be respected. This report indicates - also for these two components - the outcome of analyses carried out by both the operator and local health authorities.

Water that is of higher quality at source, such as those from springs or deep wells, generally require simple treatment (e.g. sedimentation, filtration and disinfection). This is because they benefit from natural “self-purification” processes that occur during filtration in the ground and subsoil layers. Surface waters and some aquifer waters, especially on the plains, require - on account of their characteristics and vulnerability to accidental pollutant spillage - complex treatment that generally includes many or all of the above-described stages. In some aquifers it's necessary to remove naturally occurring contaminants such as ammonium, iron and manganese or anthropic pollutants, the most common of which are organohalogenated compounds.