I'm interested to know what it takes to make rainwater drinkable. Is it a matter of filtering and boiling or is it more complicated than that?
It is probably as cleaner source of water as your gonna get without chemical treatments, I don't treat water from transpiration bags either, just make sure what ever I'm using to collect is clean, if from a tarp then let the rain wash it before putting a can under the line, if from a guttering butt the collect from the top with a ladle rather then the bottom( just always done it that way, probably makes no diff) but then i have no issues with drinking water from mountain streams untreated, below the habitation line or from stagnant water I always filter with a milbank then bring to the boil. but rain water is IMO cool.
I've used water, rough filtered straight off my tarp many times, it tends to taste a bit smoky at times but that's never bothered me.