Not all filters can remove all the impurities added by today's water channels and sources...

Bottled water which is single-source, in bottles which leach less, AND has a short travel distance is rare...

In most areas, well water and other natural bodies is best tested regularly by a lab which will tell you the results...

I've heard of a few other methods of water purification, such as multi-step macro-process, but I've yet to try them myself.