Caustic soda pH adjustment issues
It may depend on the raw water ph coming into the plant. During the winter when the water is cold, and add rain, snow etc. ph and alkalinity will decrease. This will need to be offset with more caustic soda. We generally at caustic during the winter to our raw as the ph and alkalinity are not sufficient for the Alum to react.
So yes you use more in the winter. During the summer when the treating water is easy you will use less.