Helen, a Helsinki-based energy company, has unveiled plans for a large-scale district heating plant complex. Germany's MAN Energy Solutions is supplying a 33 MW air-to-water heat pump, the largest ever used for a district-heating plant, with ambient air and renewable electricity for heat generation.
Helen runs your wastewater through a heat exchanger before this step. I guess the actual heat is from the water treatment when the solids are being nommed on in a big bubbly pool of bacteria that give off heat. But outgoing water is warmer than incoming by itself, too.
There’s just not a whole lot of industry close enough to an urban center like Helsinki, but paper mills and burning sorted trash is usually the source for these networks.