Complex sentence: Both the water and natural carbon dioxide gas are captured independently and in the bottling process, the carbon dioxide gas is added so that the level of carbonation in bottled Perrier is the same as the water of the Vergèze spring.
Split into simpler sentences: Both the water and natural carbon dioxide gas are captured independently. Then in the bottling process, the carbon dioxide gas is added so that the level of carbonation in bottled Perrier is the same as the water of the Vergèze spring.