Before I began looking into drinking water treatment systems, because I didn't like the taste of chlorinated water, I would fill flagons from the tap and place them outside.
The chlorine would gradually dissipate, resulting in better tasting water.
That works well but does nothing to remove the remaining toxins.
How they got there is not subject of this article.
Let's just say we all had a hand in it.
The types of water treatment begins at the supply end.
There, the treatment consists of sand filtration and bleaching with chlorine.
This process takes out most of the sediment and kills most of the bugs--gardia being a notable exception.
What about the other 2000 or more chemicals in the water, many of which are considered cancer forming? I'm afraid, these have to be taken out at the user's end--at the tap.
Methods used in filtration: Ion exchange:--(An ion is an electrically charged substance formed by gaining or losing one or more of its electrons) Treats hard water (calcium and magnesium) Removes  radium, barium, dissolved iron and manganese.
Removes some bad odors, colors and tastes.
Anion exchange unit can remove nitrate and fluoride.
Distillation:  Removes lead, nitrate, sodium chloride, pesticides and organic compounds.
Reverse Osmosis: (A method of producing pure water by forcing saline or impure water through a semipermeable membrane across which salts or impurities cannot pass).
Removes calcium, radium, magnesium,sulfate, potassium,  fluoride,nitrate and phosphorous.
Also removes some pesticides and organic compounds.
(Organic in this case, means a compound containing carbon).
Disinfection: (a) Chlorination Kills bacteria and some viruses (not gardia though) Removes some bad odors, tastes and colors.
(b) UV Radiation Kills bacteria and some viruses.
(c) Ozonation Kills bacteria and some viruses.
Removes some pesticides, can remove iron, sulfur, manganese.
Carbon: Carbon such as charcoal adsorbs (to gather on the surface) gases.
Activated carbon: (Carbon such as charcoal is 'activated' by increasing its porosity allowing a greater surface area to come in contact by the unwanted substance).
  Removes pesticides & Organic chemicals.
  The above isn't a complete list of filtering methods, but are the main ones.
Systems which employ a combination of methods are the most effective.
Here is a chart showing drinking water treatment systems of the more popular brands and their comparative effectiveness and cost.
It is possible to selectively remove unwanted substances while leaving in beneficial elements.
For example removing chlorine, leaving in calcium and magnesium, which are required by your body for healthy growth.
That is why the better types of filtration systems employ a two or even three stage process.
  A filter designed to filter all of the water entering your house, will be different to one that takes care of the tap or shower water only.
A shower filter is quite important by the way, as toxins are absorbed by the skin.
I hope this article gets you a little bit further with your understanding of water filtration and will prompt you to go buy one.
Any type is better than none at all, but there are thoroughbreds available at no extra cost.