Reasons for the lack of chemical stability of treated water rich in magnesium

摘要

Chemical stability of water should be high enough to ensure that the water reaching the consumers would have the same composition as at the treatment plant. The drinking water supplied by one of the water treatment plants for the city of Poznan was observed to produce periodically white non-sedimenting precipitate on boiling, deteriorating its organoleptic properties. The phenomenon was found to be related to a high content of magnesium in the water taken for treatment and low content of other ions besides bicarbonates. XRD and SEM analyses have shown that a low ratio of calcium ions to magnesium ions leads to formation of calcite crystals on water boiling in which a fraction of cationic crystallographic sites are substituted with Mg~(2+) ions giving (Ca_(1-x)Mg_x)CO_3 crystallites. Such crystallites have smaller size than those of calcite formed on boiling water coming from other Poznan suppliers. The smaller size of the crystallites is responsible for their slower sedimentation and hence the observed increase in the water turbidity on its boiling. It has been proved that the appearance of precipitates in drinking water at the consumers can be achieved by reduction of the Mg/(Mg + Ca) ratio to below 3, which would inhibit peptisation of the precipitate and prevent water opacity and/or adjustment of pH of the raw water and removal of the carbon dioxide released to convert some carbonate hardness into non-carbonate one. These measures will limit the amount of the precipitate forming upon water boiling and change its microcrystalline type into an easier sedimenting one.