Now am also thinking about utilizing rainwater to some extent. We have some barrels but usually it's enough two for the summer to water the plants. With a good downpour all can be filled so why just let it stand? Dirt can be filtered out with an easy process (pouring it through gravel-sand-coal layers) and maybe even bacteria and parasites. But those really need boiling. The biggest problem however is certain chemicals which can't be filtered or boiled out, even distillation can't help. I dunno if there's any but wouldn't risk it.
There are some other problems. How to get 200 l water out of a barrel, standing on the ground without a tap on it, through a filter into some other container, then maybe boil it, then transport it to it's utilizing destination without making it too labour-intensive and inconvenient.
Eh, I'll ponder on it a little more.