Here's an idea for a cannister rainwater filter. Let me know your suggestions.
screen > settling > take water from middle of barrel (perhaps 2nd barrel)
three stage filter > sediment > carbon block > ceramic (berkey etc) > unpressurized tank
The filters can be connected with 3 standard size RO housings, taken apart and arranged in a triangle, perhaps upside down, with right-angle fittings, with 1/4 inch tubing looped down from one right angle fitting and back up to the next.
There can be a 1/4 ball valve to turn the water on and off before it goes into the housings, which can be set in the bottom of a 5 gallon bucket, or just left connected together in a row. In the latter cause the ball value would be inserted into the side of the barrel, with the housings on the outside of the barrel, either inside of a bucket or not.
The water from the housings can go to an unpressurized tank, which can be a 5 gallon bucket, with a float valve
to turn the housings on and off automatically as the water is used.
This works similarly to an RO system except 1) works by gravity instead of high water pressure, and 2) has a ceramic cannister instead of an RO membrane. Rainwater already has a low TDS, so there is no reason for an RO membrane in this case. It would be better if the ceramic filter had a carbon filter before it instead of granulated carbon in the middle, but I think a system like this would work well. What do you think?