The water that goes through the sewers does not go back to the water supply, unless you count evaporation, rain, and ending up back in a resevoir or spring.
The typically go to treatment plants and then into rivers or the ocean (which is not the best, but it does not get re-used for drinking water).
In some places, where water is more scarce, they use "grey water" for irrigation and such.
"Any water that has been used in the home, except water from toilets, is called graywater . Dish, shower, sink, and laundry water comprise 50-80% of residential "waste" water. This may be reused for other purposes, especially landscape irrigation."
But the filtering used for tap water is the same if not higher than that used for bottled water.
And the energy costs for bottled water are ridiculous:
According to a 2001 report of the World Wide Fund for Nature (WWF), roughly 1.5 million tons of plastic are expended in the bottling of 89 billion liters of water each year.
Besides the sheer number of plastic bottles produced each year, the energy required to manufacture and transport these bottles to market severely drains limited fossil fuels. Bottled water companies, due to their unregulated use of valuable resources and their production of billions of plastic bottles have presented a significant strain on the environment.
If you really like the taste of filtered water, use tap with a filter or Britta pitcher, a refillable bottle (like SIGG or Nalgene [yes, there are issues with some of the plastics, I know], and save lots of money and lots of plastic.
All that being said, if I am going on a flight and can't bring that with me, I like Evian or Fiji.
