What effect does the addition of lime have during the alkalinity softening process?

The addition of lime during the alkalinity softening process effectively increases the carbonate alkalinity in water. When lime (calcium hydroxide) is added, it reacts with bicarbonates (HCO3) present in the water. This reaction results in the conversion of bicarbonate alkalinity into carbonate alkalinity (CO3), thus raising the overall alkalinity of the water.

The chemistry behind this process involves the following reactions:

1. The addition of lime raises the pH of the water.
2. At elevated pH levels, bicarbonate ions deprotonate to form carbonate ions. This conversion is essential as it leads to the precipitation of calcium carbonate (CaCO3) when the concentration of carbonate ions is sufficient, which facilitates the removal of hardness from the water through sedimentation.

By enhancing the carbonate alkalinity, the water treatment process ensures that the resulting treated water is less corrosive and more stable, which is crucial for protecting distribution systems and ensuring safe drinking water standards.

Understanding this conversion is vital for operators as it directly affects the efficiency of the softening process and the final water quality. Knowing that bicarbonate alkalinity transforms into carbonate alkalinity with lime addition helps in managing treatment regimes effectively.