Today they use high pressure, and filter. This takes a lot of energy
A better idea, is to turn the pumps around, and the water boils into fresh water. When we vent the pressure, we get cold water we can bottle.
So the same water, for 80 times less energy – with no filter to change. Sop this is cheaper than mains water – if you are near a river or the seas. So we produce bottles water for 5% of the cost today.