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 prioritize high water recovery rates to maximize efficiency and economic viability. To prevent calcium 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 introducing sufficient acid, the bicarbonate concentration decreases (as it converts to carbon dioxide), and the pH drops, which enhances the solubility of CaCO3 and prevents scale formation. If the acid dosing system fails, calcium carbonate may precipitate, but once acid is reintroduced, the scale typically dissolves.
In CA membrane RO systems, adjusting the pH with acid not only minimizes membrane hydrolysis but also serves as a primary method for preventing calcium carbonate buildup. When the system is shut down, dissolved CO2 can escape, causing the pH to rise. Since the water near the end of the RO unit has the highest salt concentration, this pH increase can lead to calcium carbonate fouling. When the system restarts and the pH is lowered again, any CaCO3 formed will dissolve back into solution. However, prolonged downtime can result in calcium carbonate deposits on the membrane surface, leading to accelerated hydrolysis of the CA membrane and reduced performance.
Beyond calcium ions, other multivalent cations such as magnesium or barium can also form insoluble carbonates. The methods used to control calcium carbonate scaling are generally effective for these types of scales as well. Unless the concentrations of other multivalent cations significantly exceed that of calcium (which is uncommon in natural water), using calcium carbonate as a basis for scaling calculations is usually sufficient to predict carbonate precipitation.
When using acid treatment to prevent calcium carbonate fouling, it's common practice to maintain a negative LSI (Langelier Saturation Index) value. However, when antiscalants are used, the system can tolerate higher LSI values without precipitation. For example, our company’s JC-A10 antiscalant allows operation at an LSI of 2–3 without calcium carbonate scaling, offering improved efficiency and reliability in RO processes.
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