We are working with a building in an urban core area that wants to upsize the stormwater retention tank to capture all rainwater on site.  They then plan to use the water for irrigation, cooling tower makeup, and toilet flushing.  Is there any reason those three end uses cannot come from the same retention tank?  In other words, if the irrigation load is 10,000 gallons, and the CT makeup load (20%) is 50,000 gallons, and toilet flushing is 20,000 gallons, can they have one single 80,000 gallon tank?  Or would they need to segregate the rainwater storage into three separate tanks?

We can assume that the filtration system of the one large tank would be to the strictest standard of the three end uses.