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.
Okay, district heating with a heat pump makes more sense to me if there are processes that require cooling and can act as the source, like lowering the temperature of treated wastewater before adding it back to a waterway. However, the heat supply for the water treatment plant should still probably come from cogeneration. District cooling with a central heat pump system also makes sense, especially if it eliminates noisy condensers on the sides of buildings.
