For small-scale RO systems, operating at a lower recovery rate helps minimize the concentration of ions in the concentrate stream, reducing the risk of calcium carbonate precipitation. In contrast, large-scale RO systems typically require higher water recovery rates for economic efficiency. To prevent calcium carbonate scaling, acid is often added to the feedwater. The solubility of calcium carbonate is heavily influenced by bicarbonate levels and the pH of the water. By adding sufficient acid, the bicarbonate concentration decreases (as it converts to carbon dioxide), and the pH is lowered, which enhances the solubility of CaCO3 and prevents scale formation. If the acid dosing system fails, calcium carbonate can precipitate, but once acid is reintroduced, the scale will dissolve again.
In CA membrane RO systems, adjusting the pH with acid not only helps reduce membrane hydrolysis but also serves as a primary method to prevent calcium carbonate buildup. When the system is shut down, dissolved CO2 can escape from the water, causing the pH to rise. This is especially true in areas where the salt concentration is highest, such as near the membrane elements. As a result, calcium carbonate scaling may occur. However, when the system is restarted and the pH is lowered to an appropriate level, any CaCO3 that has formed will dissolve back into solution. Prolonged downtime, however, can lead to CaCO3 accumulation on the membrane surface, accelerating hydrolysis and degrading the performance of the CA membrane.
Besides calcium ions, other multivalent cations can also form insoluble carbonates. The methods used to control calcium carbonate scaling are generally effective against other types of carbonate scaling as well. Unless the concentrations of other multivalent cations significantly exceed that of calcium (which is rare in natural water), using calcium carbonate as a reference for scaling potential is usually sufficient to predict whether carbonate scaling will occur.
When using acid treatment to prevent CaCO3 fouling, it's important to maintain the LSI (Langelier Saturation Index) value below zero. With the use of reverse osmosis antiscalants, the system can tolerate higher LSI values without precipitation. For example, our company’s high-quality JC-A10 reverse osmosis scale inhibitor allows operation at LSI values up to 2–3 without causing calcium carbonate precipitation, significantly improving system efficiency and reliability.
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