Preparing a container for water storage?

[piglet]

Hello

Recently we have had our water switched off a number of times, for several hours, without warning, because of various local area problems in the last few months, and since we run directly off the mains the water stops running just about instantly they turn it off at the main.
Now I keep enough stored water for my own needs, but 2 of my neighbours/friends knowing that I am somewhat ‘self reliant’ have been coming to me when they need water.

So I decided to invest in a 100 litre rain water butt to store indoors in a small dark store room.
There is no heating in there, no windows, and the container is black plastic with a secure lid, so should hopefully not have any problems with algae or dust etc contaminating, and I will of course change it out regularly.

It is only meant as a temporary measure until I eventually move home and then I may make other arrangements, and it can probably revert to a garden rain butt. But in the meantime, it just might need to be used for drinking water...even if only, for first boiling, to make tea, or also used for washing.

So my question is, apart from first checking it does not leak.....what might people suggest, I do to make it as safe as possible for storing water.
I keep Bleach and Milton’s for cleaning out my smaller water containers. But what would be a good measurement for sterilising before first time use?
Would it be unwise to 'also' use boiling water to clean it first time or might that damage the seals of the tap, or even the plastic the container is made from?

Or more importantly, is there any risk of chemicals from the plastic it is manufactured from leeching into the water? I read somewhere that plastic water containers, if left in direct contact with concrete could cause problems from a chemical reaction between the two, but I have a plastic base/stand for it, so that should not cause a problem.

I really do not want to do something that might cause tummy upsets.

[tigs]

its always a good idea to clean and sterilize it if storing for a long time, warm water and miltons should do the job just let if stand for a few hours then rinse it out