limesodaprosess

Limesodaprosess, also known as the lime-soda process, is a historical and still-used method for softening hard water by removing calcium and magnesium ions through precipitation. It has been employed in municipal and industrial water treatment where lime (Calcium hydroxide) and soda ash (sodium carbonate) are readily available and exchange-based methods are impractical or too costly.

The process relies on two linked chemical reactions. First, lime is added to remove temporary (bicarbonate) hardness: Ca(OH)2 + 2 HCO3− → CaCO3↓ + 2 H2O + CO2. This raises the water’s pH and alkalinity, causing calcium carbonate to precipitate and some magnesium as hydroxide to form. In a subsequent stage, soda ash is introduced to address non-bicarbonate (permanent) hardness by supplying carbonate ions that form insoluble carbonates with calcium and magnesium: Ca2+ + CO3^2− → CaCO3↓; Mg2+ + CO3^2− → MgCO3↓. The precipitated solids are removed by clarification and filtration, leaving softened water.

Typical operation involves careful dosing and mixing of lime, followed by settling of sludge, then dosing of soda ash, another round of clarification, and pH adjustment before distribution. The process increases water alkalinity and may elevate pH to levels that require subsequent neutralization.

Advantages of the lime-soda process include relatively low chemical costs and the ability to handle large volumes. Limitations include the production of substantial sludge and the need for skilled management of pH and alkalinity to prevent scaling or corrosion. In many regions, ion-exchange or membrane-based methods have largely supplanted lime-soda softening, but the process remains in use for specific applications and legacy systems.