If you're on a "city" water supply, there is probably enough residual chlorine/bromine or whatever else they put in it to pretty much "stabilize" the crud. Re-using plastic bottles is probably safer if you do use a "treated system" water, rather than a "natural" well or spring water.

To be really safe, I suppose one could do as the ancient Roman army supposedly did, and pack along vinegar instead of water as a defense against typhoid, dysentery, etc; but then they ran into typhus (body lice), malaria (mosquitos), plague (fleas), and cholera (other fleas) in spite of it all. (Incidentally, all of these are still major problems in many parts of the world. "Don't drink the water" isn't an idle platitude, and good sanitation is absolutely necessary everywhere.)

I feel safest drinking the same water my neighbors do. Then if they start to drop, I'm pretty sure I'll hear about it. "Imported" water, coming from an unknown source, doesn't impress me as "safer" just because there's a lot of advertising that says pretty girls all drink it. A problem with the source probably wouldn't be known in my area until there were enough major effects in many remote places to link it to the imported source - and NO water source can be considered "naturally safe" without continual monitoring.

John