I have one system. I bought it from a fundraiser event basically. It's a 55 gal. food grade drum with lid and screened hole for downspout to empty into. I just drop a bit of bleach into it periodically, though it's primary use is for watering the yard, mixing weed killer etc. In a pinch it could be boiled and / or filtered. If nothing else water for the dogs, washing items, toilet etc.
I don't ever empty it I just occasionally use the water and just just keep some bleach in there to keep the inside from getting slimey.
I have a separate 55 gal. drum with spout that I keep filled with fresh water and a bit of bleach. I partially drain it about once a year and ad in fresh water etc. So between that drum, the downspot drum and 5 Military Water Cans I'm pretty set for H2O