Coffee Appreciation

Here’s a question. I’m running out of “‘coffee water’” that my local coffee shop would give me (just tap water thats been run through R.O with something like 20% bypass). I could buy some bottled water but I don’t really like doing that. Or I could buy a filter, which I might do at some point. But I have water butts in my garden that have been collecting rainwater. Obviously…this water is not drinkable in its current form, but I have some time on my hands (…) what would I need to do to it to render it drinkable? Boil it for a few minutes? A google suggests this would work but am I overlooking something? (yes, I am overlooking something; the kitchen tap, but aside from that)