[yungchin]

You could do some heat recovery from the sewer, but it would be a yucky affair - they already tend to clog even without heat sink fins in them :)

Here are some calculations suggesting that the numbers simply don't add up for ground-source heat pumps at urban population densities: https://www.withouthotair.com/c21/page_152.shtml

[henrikschroder]

A similar idea that's somewhat popular in Sweden is to add a heat-exchanger between outgoing sewer water and incoming tap water. You don't need to put fins into the outgoing water, it's simply a piece of regular straight copper pipe, but it's thicker, and the incoming water pipe coils around it a bit.

No need to muck around with fins in the sewage, or trying to convert to electricty, simply heating the incoming water a degree or two saves energy for your water heater.

[yungchin]

Yes, we've got this on our shower drain. In this configuration it does better than a degree or two, recovering some 50% of the heat, in fact.