https://www.sunyit.edu/apps/weblog/?mode=login&reqmode=savepost&blog=sanyshs

Interesting concept - the idea of using waste heat to heat cold water.  In a residential situation this means installing a section of copper pipe in a wastewater drain that outputs hot water, such as a shower or dishwasher.  Wrapped around this copper pipe is thin copper tubing through which cold water is pumped.  The heat transfers from the hot waste water, through the copper pipes by conductivity and then eventually to the colder water circulating through the copper tubing.  Pretty neat.