How is the water stabilized before filtration in lime softening?

• A. By adding phosphate
• B. By adjusting the temperature
• C. By correcting the pH with carbon dioxide
• D. By aeration

Answer: (C)

In lime softening, water stabilization before filtration is primarily achieved by correcting the pH with carbon dioxide. This process is crucial because the addition of lime (calcium hydroxide) to water increases its pH significantly. A higher pH can lead to the formation of calcium carbonate precipitates, which can lead to the issues associated with scaling and operational difficulties in downstream processes.

By introducing carbon dioxide, the pH is lowered, which helps to dissolve any excess calcium carbonate that may have precipitated, thereby preventing these particles from causing clogging issues during filtration. This adjustment helps ensure that the water is treated effectively, allowing for the removal of hardness while maintaining a stable system that can be filtered properly.

While adding phosphate may be beneficial in other contexts, it does not play a stabilizing role in lime softening. Adjusting the temperature and aeration can influence various aspects of water treatment, but they do not directly stabilize the water in the context of lime softening. Therefore, the correction of pH with carbon dioxide is vital for effective stabilization before the filtration process.