It might reduce the buildup of sediment inside pipes. It also might increase pipe erosion. It would probably have a comparable ground-warming effect to geothermal sinking. I have no idea what effect it would have on wastewater treatment plants, but it would cause significant warming local to sewage outflows similar to how water-cooled power plants can disturb river ecosystems. It would probably make sewage maintenance much more difficult since it would be so hot and humid. It might drive out animals living in the sewers, or just make it impossible for them to survive. I have no idea what that would mean. Mainly I was thinking that it would be a great deal easier to simply deal with the heat at the wastewater plant, but there would also be an immense heat sinking effect because of all the distance traveled inside sewers.

I was only thinking of it in context of using the ground to sink heat, not as a real implementable plan. I think it goes without saying that the heat-pump-as-a-water-heater idea is far more pragmatic.