Here is some interesting information about slow sand filtration and rainwater harvesting:
In Washington state, King county officials have considered “allowing” people to harvest rainwater for drinking purposes. (this is info from last year)
The first link is new information just published, however the second link is about information from last year. The connections here to slow sand water filtration and rainwater harvesting may not be blindingly obvious to some at first glance but read through the articles completely and then decide.
The first link points to information about another civilization from several thousand years ago using sand to filter water. These sand filters did undoubtedly acquire some biological action as there were likely no electric motors and petroleum powered high pressure pumps to “backwash” the filters they built.
Regarding the second link, rain water harvesting will need some kind of filtration to go along with just capturing water from a roof or other impervious surface. Small slow sand filters are well suited to this task, particularly for individual water systems supplying one household. This is the focus of the information at the second link – rain water harvesting for individuals.