Introduction:
The lime-soda process is a commonly used method for water softening, which involves the removal of hardness-causing ions such as calcium and magnesium from water. One of the steps in this process is re-carbonation, which serves a specific purpose. In this response, we will explain the purpose of re-carbonation and why option 'D' is the correct answer.

Purpose of re-carbonation:
The purpose of re-carbonation after water softening by the lime-soda process is to convert precipitates formed during the softening process into soluble forms in the water. This step ensures that the treated water does not contain any visible or settled solids, which may cause aesthetic issues and potentially clog pipes and equipment.

Explanation:
During the lime-soda process, lime (calcium hydroxide) and soda ash (sodium carbonate) are added to water to precipitate calcium and magnesium ions, which are the main contributors to water hardness. The precipitates formed are typically calcium carbonate (CaCO3) and magnesium hydroxide (Mg(OH)2). These precipitates are not soluble in water and can cause turbidity and sedimentation.

Re-carbonation involves the addition of carbon dioxide (CO2) to the water after the lime-soda treatment. The CO2 reacts with the precipitates, converting them into more soluble forms. The main reactions that occur during re-carbonation are:

1. Carbon dioxide and calcium hydroxide:
CO2 + Ca(OH)2 → CaCO3 + H2O

2. Carbon dioxide and magnesium hydroxide:
CO2 + Mg(OH)2 → MgCO3 + H2O

As a result of these reactions, the precipitates formed during water softening are transformed into calcium carbonate and magnesium carbonate, which are more soluble in water. The soluble forms remain in solution and do not settle or cause turbidity.

Advantages of re-carbonation:
1. Improved water clarity: Re-carbonation helps to eliminate the visible precipitates and suspended solids, resulting in clearer and aesthetically pleasing water.

2. Prevention of sedimentation: By converting the precipitates into soluble forms, re-carbonation prevents the settling of solids in the water. This reduces the likelihood of sedimentation in pipes, tanks, and appliances, which could lead to clogging and reduced water flow.

3. Enhanced water quality: The removal of precipitates through re-carbonation improves the overall quality of the treated water, making it more suitable for various applications such as drinking, industrial processes, and irrigation.

Conclusion:
Re-carbonation is a crucial step in the lime-soda process for water softening. It converts the precipitates formed during the treatment into more soluble forms, ensuring that the treated water remains clear and free from settled solids. This step improves water quality, prevents sedimentation, and enhances the overall effectiveness of the water softening process. Therefore, option 'D' is the correct answer.