I remember back when I was a kid, we were under a boil order due to a loss of power in a lift station, and back flow into the water main... Mom was not happy having to produce the amount of water needed (don't think Clorox treatment was popular then)as at the time she didn't have enough large pots to do it efficiently... that's why I went with the filter that could be backflushed and produce 5 gallon amounts..

good luck with you rainwater collection system... you might look into a biological sand filter... several videos of them in tandem with rainwater collection units on YouTube