Rainwater in most cases is ...

Rainwater in most cases is far cleaner than treated wastewater. I strongly recommend that you do not mix them. With a well designed rainwater catchment system you should have much cleaner water. Test a prototype of the proposed system design in the location you intend to collect, to confirm this (use potable materials). You reduce your exposure to a plethora of emerging contaminants including fire retardants, pharmaceuticals etc. that many wastewater treatment systems struggle to completely remove. A well designed rainwater catchment system can catch and safely store large volumes of rainwater if you protect it from heat. "Treated" wastewater can continue to be problematic and may not store for extended periods of time without serious complications. Rainwater alone should  be much more cost effective.