Abstract
Abstract. The new German standard on the calculation of calcite saturation in drinking water, DIN 38404-10, 2012 (DIN), marks a change in drinking water standardization from using simplified equations applicable for nomographs and simple calculators to using extensive chemical modeling requiring computer programs. The standard outlines the chemical modeling and presents a dataset with 10 water samples for validating used computer programs. The DIN standard, as well as the Standard Methods 2330 (SM) and NEN 6533 (NEN) for calculation of calcium carbonate saturation in drinking water were translated into chemical databases for use in PHREEQC (USGS, 2013). This novel approach gave the possibility to compare the calculations as defined in the standards with calculations using widely used chemical databases provided with PHREEQC. From this research it is concluded that the computer program PHREEQC with the developed chemical database din38404-10_2012.dat complies with the DIN standard for calculating Saturation Index (SI) and Calcite Dissolution Capacity (Calcitlösekapazität) or Calcium Carbonate Precipitation Potential (CCPP). This compliance is achieved by assuming equal values for molarity as used in DIN (obsolete) and molality as used in PHREEQC. From comparison with widely used chemical databases it is concluded that the use of molarity limits the use of DIN to a maximum temperature of 45 °C. For current practical applications in water treatment and drinking water applications, the PHREEQC database stimela.dat was developed within the Stimela platform of Delft University of Technology. This database is an extension of the chemical database phreeqc.dat and thus in compliance with SM. The database stimela.dat is also applicable for hot and boiling water, which is important in drinking water supply with regard to scaling of calcium carbonate in in-house drinking water practices. SM and NEN proved to be not accurate enough to comply with DIN, because of their simplifications. The differences in calculation results for DIN, SM and NEN illustrate the need for international unification of the standard for calcium carbonate saturation in drinking water.
Highlights
Calculation of calcium carbonate saturation in drinking water is performed with a simplification of the processes as shown in Table 1, in which ion pairs are neglected and calcite is assumed to be the determining crystalline phase
3.1 Saturation Index (SI) for calcium carbonate In Fig. 2 the deviations are shown between the SIs calculated in PHREEQC according to the DIN, Standard Methods 2330 (SM) and NEN standards and the SIs according to the DIN validation data set, see Table 5
It should be noted that the DIN tolerance range is more stringent than the accuracy of the chemical analyses which serve as input for the computer calculations
Summary
Calculation of calcium carbonate saturation in drinking water is performed with a simplification of the processes as shown in Table 1, in which ion pairs are neglected and calcite is assumed to be the determining crystalline phase. Calculation of calcium carbonate saturation in drinking water is performed with a simplification of the processes as shown, in which ion pairs are neglected and calcite is assumed to be the determining crystalline phase. P. J. de Moel et al.: Assessment of calculation methods for calcium carbonate saturation. Part Dissolution/precipitation of CaCO3 Dissociation of carbon dioxide CO2 + H2O = H2CO3 Ionisation of water. Reaction equations CaCO3 (s) ↔ Ca2+ + CO23− CO2 + H2O ↔ HCO−3 + H+ HCO−3 ↔ CO23− + H+ H2O (l) ↔ OH− + H+. {X}= relative activity of dissolved species/ion X
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