A study of seasonal effects on metal-NOM interactions and the impact of CaCO3 precipitation potentials using Visual MINTEQ, in raw and cooling water

Abstract

This paper reports on a study of calcium carbonate (CaCO₃) precipitation potentials of cooling water (CW) at Eskom power stations. It is important to understand the complexation reaction between calcium and dissolved organic carbon (Ca-DOC) because the natural organic matter (NOM) affects the precipitation potentials of CaCO₃. Secondly, it is necessary to understand the nature of the organic matter in the cooling and raw water. This study was carried out in raw and cooling water samples collected from Lethabo and Kriel power stations. These power stations obtain their cooling water from the Vaal River and Usuthu schemes, respectively. The DOC concentration in both water systems was higher in winter (little/no rainfall) as compared to spring (more rainfall). Specific ultraviolet absorption (SUVA) was calculated for the raw and cooling water for both stations. The SUVA results obtained for the raw water (RW) indicated that the NOM had a high degree of aromaticity (humic substances), whereas for the recycled CW the NOM present was found to be less aromatic. The Visual MINTEQ data for the winter sampling indicated that both Lethabo and Kriel were supplied by raw water that was under-saturated with respect to CaCO₃, with saturation indices (SI) of −0.29 and −0.43, respectively. In addition, the % Ca-DOC in the raw water at Lethabo was 2.01 compared to 3.08 in the raw water at the Kriel plant. Therefore this study gives an indication that % Ca-DOC is one of the factors that affect the CaCO₃ precipitation potentials. The difference in SUVA-254 values for both stations has been identified and requires further investigation. The SUVA values help in identifying the type of fractions that make up the NOM. The interaction between calcium/magnesium and NOM determines the potential for CaCO₃ to form scale in condenser tubes carrying cooling water in the power generation plants at Eskom.

Keywords: cooling water, calcium-NOM complexation, precipitation potentials, SUVA, Visual MINTEQ