Which of the following is NOT a process used for lime-soda softeners?

The continuous synthetic process is not a recognized method for lime-soda softening. Lime-soda softening primarily involves the chemical reaction of lime (calcium hydroxide) and soda ash (sodium carbonate) to precipitate calcium and magnesium hardness from water. This process is typically carried out using methods such as the continuous hot process, which necessitates the application of heat to enhance the reaction rates, and the intermittent cold process, where water is treated in batches rather than continuously.

The continuous sedimentation cold process also effectively contributes to softening by allowing precipitates to settle, but it still relies on traditional lime-soda chemical reactions.

In contrast, the continuous synthetic process suggests an approach that is not part of standard lime-soda softening and does not typically form part of the nomenclature or practices associated with this water treatment technique. Thus, identifying this process as not being used in lime-soda softeners aligns with conventional understanding and practices in the field.