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

Authors

  • Heena Parshotam 1. Department of Applied Chemistry, University of Johannesburg, PO Box 17011, Doornfontein 2028, Johannesburg, South Africa; 2. Sustainability Department, Applied Chemistry and Microbiology, Eskom, Private Bag X40175, Cleveland, 2022, South Africa
  • Gerhard Gericke Sustainability Department, Applied Chemistry and Microbiology, Eskom, Private Bag X40175, Cleveland, 2022, South Africa
  • J Catherine Ngila Department of Applied Chemistry, University of Johannesburg, PO Box 17011, Doornfontein 2028, Johannesburg, South Africa
  • Shivani Mishra Nanotechnology and Water Sustainability Unit, College of Science, Engineering and Technology, University of South Africa, Florida Campus, Johannesburg, South Africa

DOI:

https://doi.org/10.4314/wsa.v42i1.18

Keywords:

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

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.

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Published

2016-01-27

Issue

Vol. 42 No. 1 January (2016): Water SA

Section

Research paper

License

Copyright (c) 2022 Heena Parshotam, Gerhard Gericke, J Catherine Ngila, Shivani Mishra

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This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

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How to Cite

Heena Parshotam (2016) “A study of seasonal effects on metal-NOM interactions and the impact of CaCO3 precipitation potentials using Visual MINTEQ, in raw and cooling water”, Water SA, 42(1 January). doi:10.4314/wsa.v42i1.18.