how important is activated charcoal (AC) in a filter if main tank is
filled from municiple water supply : does AC (or other _______ (?)/
finer micron ratings) rank higher in importance when improvising water
supply ?
Activated carbon is used for decolorization, odor control, chlorine
reduction, etc. ab/adsorbtion. Its best to use such filters when
*filling* the tank but you need to have sufficient 'contact time' of the
fluid within the filter matrix so the absorbtion process can take place.
Unless the filter manufacturer specifies the flow rate, use such filters
as s-l-o-w as possible to gain maximum residence/contact time of the
fluid inside the filter. Use of a carbon packed filter as on onboard
installation is the least effective: 1. the carbon will strip (by
chemical equilibrium) most of any chlorine you add to the water as a
sanitization agent. 2. activated carbon is very good nutrient source for
bacteria and other microorganisms and will promote a large bloom of bio
growth.


My Irwin 37cc is the perfect boat for catching rain off the deck, the
previous owner bought but never installed the valves, figuring the
water we catch on deck will not be of the quality delivered from our
municiple supply and when cruising captured rain will be our greatest
supplement;
Rain water is probably better than most municipal supply (from surface
water) as the zoocysts, bacterial and viral load, heavy metals, etc. is
greatly reduced - just filter out the ambient/airborne molds/mildews and
a few airborne bacteria, airborne particulate and all should be OK. Just
remember that just about every rain drop forms on a microscopic piece of
dirt, etc.

aside from treating tank/captured water with bleach; if I
added a circulation path to the water system what guidelines would you
suggest in filter selection?
For a recirc. system that has a high flow pump, 5 to 10 micrometers
would probably be OK, then into a single pass filter of 1 micrometer
rating of at least 98-99% efficiency before charging the main storage tank.