What I've heard, and it's worth checking better is as follows:

You need two pots and a pipe. You fill one of the pots with water and connect the pipe such that it goes between the two pots.

You put the full pot to the boil, and the steam should go through the pipe to the other pot, this will slowly fill with clean water.

Of course this is an energy intensive proccess.
If you are worried about infections simply boiling water helps, if you are however worried about heavy metals, such as those in our tap water, boiling them alone is worst than simply drinking them, beacuse it just concentrate them more.

I would really seek better advice, these are just things I've picket up in random places, I don't want to be responsible for any risk.

Rain water will generally be the safest when boiled. As pointed out, any water on the ground can be toxic- irrespective of flooding (industrial dumping, pesticides, heavy metals etc.)

In a flood situation, it's best not even to come into contact with any ground water. In floods, the control over industrial waste, human excrement and agricultural products is right out the window. Everything gets churned up and washed everywhere.

So basically, unless you know for sure that the water isn't toxic, boiling or chemical purification will be totally insufficient.

To my knowledge the only way to clean groundwater to 100% safety is to use a series of filters including a reverse osmosis membrane... though in many cases an evaporation system would suffice.

None of which helps you!

Get a load of jerry cans from Millets and fill them from a chlorinated source (a proper mains tap).

If you are collecting rain water for drinking, I'd recommend using water purification tabs (also from Millets and the like) to minimise the risk of any nasty organisms in the water if it's left to stagnate. It's also ecology sounder than boiling the kettle.

Anyone who has had dysentery will tell you anything else just isn't worth the risk.

Here's some info on how to treat water for drinking & washing purposes:

 http://www.epa.gov/safewater/faq/emerg.html

If you decide to drink rain water :

Collect it only after it had rained for some time as it should contain way less of potentially toxic atmospheric dusts, etc...

Collect with material that are clean and not likely to generate potentially toxic dissolved materials.

Try to add minutes amount of salts to it, table salt, sodium bicarbonate etc... for example as to at least match the hardness of the less hard bottled spring waters. It is very likely that rainwater contains extremely little salts, especially if it is cleanly gathered during the end phase of a rainfall, and as such may strongly ressemble deionized water which is toxic and can even be lethal to drink in relatively little amounts due to it's total lack of dissolved salts.

people are drinking rain water.But no mainstream advice about the dangers of doing so.re the post above about salts.