echoing what others have said, it dosn't seem to be a big problem since so many people in the world( must be millions) drink catchment water.
When we were In Kehena Beaches we drank the water. Seemed a bit bland to me but, then , I am used to water with a very high mineral content, our local water. I was abit leery of the county water but only because I didn't know where it came from and I was aware that sanitary systems in Hawaii are behind the times. Sorry, but it's true. Cesspools have been outlawed for years because they contaminate groundwater.
When we build we will happily have catchment. I've read the books and my rules will be.
Don't have trees and stuff overhanging the house.
Try to keep animals off the roof
keep vegetation away from the tank
have a secure cover so that critters can't get into tank.
Just basic sanitary techniques
For those interested in further steps, the survivalist, back to the land sites had oxygen pills and other water treatment strategies if you are really stuck.
BTW, in Kehena we saw a property with small trees and rubbish growing in the hollow of the cover of their tank.