Beneath our streets we find an energy source that we have so far ignored. Municipal sewage contains thermal energy; its temperature is generally between 10 and 20°C, all year round. This temperature permits economical operation of heat pumps for the heating of nearby buildings (e.g. nursery homes, schools, gymnasiums, swimming pools). The only challenge is how best to extract heat from sewage.
Manufacturers are busily developing sewer pipes with integrated heat exchangers; but then we would need new sewer construction. There are special heat exchangers that can be installed on the inverts of existing sewers; but this is only possible in large diameter sewers, and heat transfer is impeded by fouling layers on the surface of such heat exchangers; not only is installation of such heat exchangers difficult and expensive, but also their service and maintenance.