Simplest use case would just to have a container in the basement / garage through which the water flows, assuming you are heating it at least a little. You like

Simplest use case would just to have a container in the basement / garage through which the water flows, assuming you are heating it at least a little. You likely keep lower temp there so it is much more efficient. Although in practice drain from the shower or bath quickly connects with the one from the toilet so it's not that easy to implement. In your case efficiency is also slightly lower due to evaporation. If anybody has any idea how significant it is I'd love to learn. I was surprised one day in winter to find that my evaporative humidifier has temp below 10C on it (room temp 23C). I think ground heat pumps could have an exchanger around the main drain pipe which should work nicely both in winter and during summer.