Sand filters scaling by calcium carbonate precipitation during groundwater reverse osmosis desalination

To face water scarcity in the south, Tunisia is relying on the use of fossil water from wells as feed water in reverse osmosis desalination. This geothermal water reaches the surface with specific characteristics (50 °C < T < 70 °C, hardness ≥ 10 mM, PCO2 ≈ 5.10− 2 atm, salinity ≥ 3 g/L). During the cooling stage, an extensive scaling takes place in the cooling towers following the release of CO2. This phenomenon continues even in the piping system. The scaling reached the sand filter in the pre-treatment process. In the current work, the scaling in sand filters was investigated. Firstly, the nature of scaling was identified. X-Ray Diffraction (DRX) of the sand samples revealed the existence of significant quantities of aragonite form of calcium carbonate (CaCO3). DRX and scanning electron microscopy (SEM) analysis revealed the presence of aragonite crystals in the raw water. Investigations showed that these crystals trapped in the sand filter act as seeds inducing secondary nucleation of CaCO3. The study showed that the raw water is over saturated with respect to both calcite and aragonite, while it is under saturated relative to the CaCO3 monohydrate form. It was suggested that the feed water is at meta-stable state, and crystal nucleation can only be driven through seeding. Laboratory experiments using the raw water with simulated sand filter confirmed the inevitable secondary nucleation supported by aragonite crystals. Consequently, the suggested solution was a water acidification ahead of the sand filtration operation instead of past of it in the desalination process.