Hi Annker
Looks like no-one knows the answer! (so far, at least). I don’t either. What we need is a chemist, with a lab who can test it out.
However, I suspect that many water butts might hold diseases because people don’t always empty them out completely. Even if they are emptied via the tap, there is still a reservoir of, usually slightly mucky water, left in the bottom, which could then re-infect the new water that goes in. Ideally people should empty their water butts fairly often by tipping them upside down once the bulk of the water has been used. Not so easy if you only have one water butt, but if you have a few, quite easy just to use one up first and then ‘rotate’ which one is used, so that each gets completely emptied in turn.
By the sound of the way you are collecting the rainwater, I don’t imagine that it lasts very long, so hopefully, it will be o.k.
