Inside the sewer there is a hidden and rarely used, stable temperature energy source to be found: the communal, household and industrial wastewater. A heat pump based technological solution has been developed to harness and utilize energy resided in wastewater to heat and cool buildings. This means for a city like Paris the potential of min. 200 MW energy-generation! The challenges are to use this valuable resource on site amongst city circumstances, to make it upsizeable to several MWs requirements and to achieve a highly efficient system (COP above 6!) in order to make it financeable. Driven from the technology solution’s “independence” from the sewers (nothing installed inside the pipeline) the system’s implementation is flexible, after ten years of development its possible size only depends on the wastewater flow. The technology developed has been monitored during operation for over 8 years at different locations (sizes: 1 - 3.8 MW). Learning from this unrivalled operational experience and the monitored performance data the technology solution has been improved through minimizing energy consumption, evolving efficiency of heat exchange, optimizing maintenance requirements and SCADA system.