Catchment tank filter question

[VancouverIslander]

If you have a large tank it's probably a good idea to add chlorine once in a while to kill off it's resident population of bacteria and viri. It is short lived, but knocks down the resident population and is probably why the recommendation is to add chlorine monthly. U of H had a good guide on catchments. One interesting thing they say is that household bleach has not been approved for use so the safety conscious among us should probably consider an alternate source of chlorine (though perversely they don't say what source to consider)

In the end, though, filters are a very important part of the whole system.