Waste Water Heat Recovery
When you take a shower, you throw a lot of heat and energy away in the warm, dirty water going down the drain. The water still contains substantial heat.
Waste water heat recovery systems transfer the heat in the waste water to the cold water going to the mixing valve that combines it with hot water to give you the temperature you’ve selected. The fresh cold water doesn’t touch or mix with the dirty water; they are separated by a metal barrier. However, the heat in the waste water flows through the metal to warm the cold water. That allows you to use less hot water in the mix with the warmed-up (cold) water to achieve the shower temperature you desire.
The same is true of waste water from your clothes washer and dishwasher. There are whole-house waste water heat recovery systems for these as well.