Question

Why is lime (calcium hydroxide) often used in post-treatment after RO desalination?

Accepted Answer

Lime (calcium hydroxide, Ca(OH)2) is often used in post-treatment after RO desalination primarily to increase the pH and alkalinity of the desalinated water. RO desalination produces water that is low in pH and has very little buffering capacity. This aggressive water can corrode distribution pipes, leading to the leaching of metals into the water supply. By adding lime, the pH of the water is raised to a more neutral or slightly alkaline level, typically between 7.0 and 8.5. Lime also increases the alkalinity of the water, which is a measure of its ability to neutralize acids. Higher alkalinity provides buffering capacity, preventing drastic pH changes that can occur due to the presence of dissolved carbon dioxide or other acidic substances in the distribution system. The calcium ions (Ca2+) from the lime also contribute to the re-mineralization process, increasing the calcium hardness of the water, which is beneficial for taste and can help to reduce the corrosivity of the water. The reaction of lime with water is: Ca(OH)2(s) -> Ca2+(aq) + 2OH-(aq). The hydroxide ions (OH-) increase the pH, and the calcium ions contribute to hardness. The lime dosage is carefully controlled to achieve the desired pH and alkalinity levels without causing excessive scaling. Other chemicals, such as carbon dioxide, may be added in conjunction with lime to further control the water chemistry and stabilize the water for distribution.

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Why is lime (calcium hydroxide) often used in post-treatment after RO desalination?