Calcium carbonate equilibria in water supply systems

Water corrosion causes substantial damage to ductile, cast iron and steel water distribution systems. The corrosiveness of water depends on many physical, chemical, and even microbiological parameters. Stability indexes in respect of calcium carbonate like the Langelier Saturation Index (LSI) are also considered. First, the roles of inorganic complexes of calcium and manganese are discussed with respect to the accuracy of the calcium carbonate equilibrium calculations. As long as the drinking water parameters were below the EC Directive permissible values, neglecting these complexes had a small effect on the results of LSI computations. A model for computing LSI in water mixtures closed to the atmosphere was presented and partially verified on a small laboratory scale. Waters produced by all water treatment plants serving Kraków were evaluated in respect of the LSI, buffer capacity, and other parameters characterizing corrosive properties of water - in respect to iron. In general, the waters differed significantly in buffer capacity but all were evaluated as not presenting serious problems in internal corrosion of iron and cast iron, or more recently ductile iron pipes. This conclusion was verified based on a survey of main pipelines transporting water from the largest intake to the town, on the computations of the pipe roughness coefficient and on the mass balance of iron along the main transit pipes.