Calculating chemical speciation, pH, saturation index and calcium carbonate precipitation potential (CCPP) based on alkalinity and acidity using OpenModelica

Abstract

Of the most commonly used calcium carbonate (CaCO₃) saturation indices, the Calcium Carbonate Precipitation Potential (CCPP) most accurately describes CaCO₃ precipitation and dissolution. This paper illustrates how to write an OpenModelica programme that calculates the CCPP and other key water parameters starting from total alkalinity and CO₂-acidity. It describes the calculation steps of all components of the developed OpenModelica package and explains how the components work together. Underlying chemical principles follow the German standard DIN 38404-10 and are presented along with the OpenModelica code. The developed programme considers 14 different dissolved species, including complex species of carbonate, calcium, magnesium and sulphate. Data of the six phosphate-free example waters from DIN 38404-10 is compared to the results of the developed programme and the results of a programme which neglects complexation. The developed programme reaches required accuracy for four of the six CCPP values; with the other programme, requirements cannot be met. By using OpenModelica, the developed package can be readily integrated into a Modelica model of, for example, a drinking water treatment plant. There, it can help to monitor legal and technical requirements, or to assess the dissolution capacity if neutralizing filters containing CaCO₃ are employed for pH adjustment.