Limestone selection criteria for EDR water remineralization

Water scarcity in Mediterranean regions has led to projects that increase water resources by desalination. In this context, it is planned to expand and improve the drinking water treatment plant of the Llobregat River near Barcelona (Catalonia, NE Spain), building the largest electrodialysis reversal (EDR) plant in the world. Llobregat River water shows high salinity due to the occurrence and exploitation of several geological formations upstream with minerals having high contents of Na, K, Cl, and Br. EDR desalinated water is highly reactive and shows a low degree of mineralization. In consequence, remineralization is necessary to equilibrate the water in order to avoid aggressiveness and corrosion in pipelines and to re-introduce some essential ions for human health. Running outlet water through limestone (CaCO3) is a simple and cheap process for remineralization. Five commercially available limestones were characterized (mineralogical and chemical composition, BET surface area, SEM analysis) and compared experimentally at both laboratory and pilot-plant scales during the EDR remineralization process. The main criteria in selecting a limestone for EDR water remineralization are its compositional purity in terms of both mineralogy and chemistry, and its textural characteristics. Texture mainly influences the suspended particulate matter (PM) generation, i.e., consumption and system performance. To minimize the negative effects of these features, an assessment, using SEM analysis and batch tests contacting limestones and desalinated water, of the capacity of PM generation is recommended.

Experimental results at laboratory and pilotplant scales inferred that the main criteria to select a limestone for EDR water remineralization are: C Compositional purity in terms of both mineralogy and chemistry. In relation to mineralogical purity, the occurrence of minerals other than calcite must be avoided as far as possible; and in relation to chemical purity, compliance with the maximum specifications indicated in norm EN 1018:1998 is recommended. C Textural characteristics. Texture influences mainly the suspended PM generation, that is, consumption and system performance. To minimize the negative effects, we recommend the assessment of the capacity for PM generation by means of SEM analysis and simple batch tests, contacting limestones and EDR water.