A first flush water diverter allows the
first few gallons of rainwater to flush the roof of dirt and not be
caught in your water containment system. Its hard to desribe with
words but it is a length of 3 inch pvc with a drain hole in bottom
that attaches to the gutter down spout that fills up with first flow
of rain and then diverts rest into your containers.

Also reccomend a 1 micron filter with carbon element to protect from
ghiardia which is present in bird poop!

I do not use my rainwater for drinking unless its an emergency and I
always filter and boil it if I do.