i would use 2 different tanks, because you would need a filtration system for the one that you want to use for potable water, and the other for toilets does not need the filtering system, and if u do only one with a "bypass" for going thru the filter in one side and toilets the other will need at the end other kind of deposit after the deposit for the potable one.....

so i would go for 2 diff tanks from the beginning

im no expert but couple questions

what is the estimated annual precipiation for the site
what is the peak demand on the building
is there a public water supply or well water?
is stormwater collection roof only, or inclusive of parking lot runoff? (need to know because parking lot runoff requires different treatment method due to petroleum contaminants)

option A: Public Water supply/Harvested Rainwater Hybrid
option B: Well Water supply/Harvested Rainwater Hybrid
option C: All harvested rainwater

if public/well supply, water will enter building into pressurized holding tank prior to dispersal into heating element/cold distribution

if well water that water will pass through a whole house filter and a softener (depending on quality of water) and perhaps a conditioner

if public water, most likely water is sanitized prior to delivery at local treatment facility

harvested rainwater will have own tank, will need to have valve/pump connection to public/well supply in order to feed that system

that water must be filtered prior to entering that system, so that system will need its own filter, can soften off public/well system though, you just dont want to contaminate clean public/well water

unless you are to treat the harvested water to same standards as local treatment center

I'm designing a cistern as well, although my location is in the Carribean.

I guess the first thing you need to ask is does the water even need to be filtered in the first place? I know in my situation, the water that falls out of the sky at my site is practically distilled water, it's extremely clean seeing how it passes over no urban areas between the west coast of Africa and the site in the Carribean.

If the rainwater in Ghana does contain pollutants, I would imagine you would want all the rain water to collect in one large cistern and then have a small amount of that branch off into a filtration system and another smaller cistern for filtered water.

aseid..the peak months for precipitation is in the month of july through september. part of the challenge is the fact that the building is being built in a community without water supplies which is a given in these quasi-urban communities that are sprouting in most developing countries. Hence, the reason why my building itself has to be self sustaining.
Apurimac..i am not sure the level of contamination the rainwater contains, but i know for a fact that as kids we used to drink it.
MaDianito-the answer im looking of. But two systems then is awfully expensive, but in consideration of the long term payoffs. Thanks everyone.

need to weigh that estimated yield + any additional from well water i assume against the total demand from the occupants + facility operations (irrigation/maintenance), fire protection discharge/capacity, most likely need a pressurized water storage tank for that as well

for the well, need to determine
ground water yield/seasonal water table information
any interference with other wells in the area
the geological conditions of the site
flow rate/size/static level

hope that all helps, I use well water at home so I have some experience with wells

haha Mr. Well thanks for the information.