Heat is extracted from the wastewater for wastewater heat recycling, and depending on the size of the plant, cooling is around 0.5 to 1 k. In exceptional cases, it can also be higher. However, this is ultimately negligible. The wastewater is cooled down, but it recovers very quickly too, through the inflow of new wastewater and because it absorbs ambient heat through the sewer. The rule of thumb is: A system for recovering energy from wastewater needs a recovery line afterwards around two to three times the length of the plant itself, and then the temperature of the wastewater has recovered again. If a system is 100 m long, the wastewater has regenerated itself in terms of energy after 200 to 300 m at the most. Consequently, for the sewage treatment systems, which generally assume a minimum temperature of the wastewater, the only deciding factor is how much heat the last system withdraws from the wastewater before the sewage treatment plant and how far this is from the sewage treatment plant. The removal of energy in the network beforehand is irrelevant for the sewage treatment plants. The wastewater cooling is thus not a restriction on the extraction of energy from wastewater.
