What chemical is typically used to remove calcium carbonate scale from RO membranes?

A weak acid solution is typically used to remove calcium carbonate scale from RO membranes. Specifically, hydrochloric acid (HCl) or citric acid (C6H8O7) are commonly used. Calcium carbonate scale (CaCO3) forms on the membrane surface when the concentration of calcium and carbonate ions exceeds the solubility limit, often due to concentration polarization at the membrane surface. Acid cleaning works by dissolving the calcium carbonate scale through an acid-base reaction. The acid provides hydrogen ions (H+) that react with the carbonate ions (CO3^2-) in the scale, converting them into bicarbonate ions (HCO3-) or carbonic acid (H2CO3), which are more soluble and can be flushed away. For example, the reaction with hydrochloric acid is: CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + H2O(l) + CO2(g). The choice of acid and its concentration depends on the severity of the scaling and the membrane material. Hydrochloric acid is a stronger acid and can be more effective for severe scaling, but it must be used with caution to avoid damaging the membrane. Citric acid is a weaker organic acid that is less corrosive and may be preferred for routine cleaning or for membranes that are sensitive to strong acids. The cleaning process typically involves circulating the acid solution through the RO system at a low pressure and flow rate for a specific period, followed by a thorough rinsing with permeate water to remove any residual acid.