Heat recuperation, or the recovery of heat from things that we normally throw away can be a significant part of energy savings. Most energy efficient houses today have Heat Recovery Ventilators that steal the heat out of stale air before throwing it out of the house. Finally some Canadians have perfected how to steel the heat from the shower water to pre-heat the cold water coming back into the house.
Grey Water heat recovery is recovering heat from waste water. The first diagram shows the idea. The other photos show the actual gadget. Simply you replace a section of your vertical drain pipe with this special piping. Then you run your cold water supply into the spiral that wraps around this copper drain pipe section. As the hot water drains out and swirls around the outside of the large drain pipe, the heat is transferred to the cold water pipe. There is a significant increase in the temperature of the incoming water. There are many different sized units and many different ways to use the warmed up water. General payback time is 2 to 5 years based on 4 showers a day.
One of the present limitations is that it only heats water while it drains water, that is both the drain water and the cold water have to be flowing at the same time. So it works for showers but not baths. But since I wrote this about the very first commercial product, Drain Water Heat Recovery units have become rather standard in highly energy efficient houses, with lots of manufacturers and models. Google it to locate current models.