Modeling remineralization of desalinated water by limestone dissolution

Desalted waters or highly soft waters produced by desalination plants cannot be directly used as they are unpalatable, corrosive and unhealthy. Remineralization is necessary in order to overcome these problems. A commonly used operation in the remineralization process is to contact CO2 acidified desalinated water with a bed of domestic limestone. Limestone dissolution provides two essential ingredients to the water—bicarbonate alkalinity and calcium content: CaCO3 + CO2 + H2O = Ca2+ + 2HCO3–. Limestone dissolution is a slow rate-controlling step. Prediction of the limestone rate of dissolution as a function of the water composition is essential for reliable design and operation of the limestone contactor. A critical comparison of various kinetic expressions proposed in the literature carried out in this study reveals major differences in results evaluated from different dissolution models. An experimental study was conducted in order to identify the most reliable kinetic dissolution model. Two series of experiments were carried out—one involving remineralization of distilled water containing low initial CO2 concentrations (0.5–2 mM) and the other, remineralization of soft water, having high initial CO2 concentrations (1.5–15 mM). The CO2 acidified water was contacted in a 2 m high vertical column (32 mm I.D.), packed with 2.85 mm calcite particles. The change in water composition along the column was monitored to provide both differential and integral dissolution data. Analysis of the data showed that none of the available models fitted the experimental results. The closest agreement was with the rather complex model of Plummer et al but this agreement was rather mediocre. In the high CO2 content range, the model predicted dissolution rates higher by a factor of 2–4 in the high CO2 range and by a factor of 10–20 in the low CO2 range. Based on the experimental results, two models were developed for the design of limestone dissolution column contactors. When the final composition of the remineralized water has a CO2 content above 2 mM, the limestone bed can be designed by a very simple integral expression. However, if the dissolution depletes the CO2 concentration to low values, well below 2 mM, the bed design requires numerical integration of the more general dissolution rate expression derived in this work.