I'm considering the construction of a mikveh that will conserve on tap water and the heat needed for the tap water to be comfortable. The rainwater section would have the necessary legal volume. It's not practical to have a tap water section of the requisite volume, be cause the water is metered. I'd pour tap water at the top of the rainwater tank from plumbing. I'd use gravity (siphon) at the bottom of the tank to deliver the mixed (now kosher) water via shower head.
My main question deals with the process of immersion. Is it valid to immerse by a thorough soaking by a shower head, rather than a dunking into a tank, in order to conserve water?