It consumes a lot of water, so it'd be expensive if you send the outflow down the drain. If you pump the outflow back into the fresh water supply, you have to worry about contamination from everyone's heat exchanger.

In a dense urban setting, you can heat homes with water heated at the local power plant, either fossil or nuclear. https://en.wikipedia.org/wiki/Cogeneration

There's closed loop ground source heat pumps. It's all very situational as to whether they are practical to install. The vertical ones are more expensive but don't need a whole lot of ground area to be installed.

In my city, about 10% of the district heating load is covered by heat pumps where the heat source is municipal waste water.

Toronto does this for district air conditioning with incoming city water.

I don’t think they’ve bothered to calculate the effects of pumping warmer water into everyone’s air conditioned home...

If we think insulating a single home is bad, image trying to build the infrastructure for that throughout a city.

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

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.

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.

You normally have to drill one of more well holes. If you can't go horizontally you have to go vertically.