AEM (Azienda Elettrica di Massagno SA) operates the hydropower plant Cassarate which exploits the flow of the rivers Cassarate and Franscinone in Canton Ticino (CH). The plant comprises a headrace tunnel at an elevation of 598 m. a.s.l. (maximum headpond water level) and a powerhouse at an elevation of 346 m. a.s.l. located in the Lugano suburbs. The hydropower plant has a gross head of 252 m and an installed capacity of 3.8 MW. The total length of the buried penstock, built in the year 1975, is about 1’520 m with a diameter of 1’000 mm. The rehabilitation works consist in the renewal of the internal corrosion protection coating of the steel penstock and the decommissioning of the existing headpond, by means of directly connecting the headrace tunnel to the penstock. The work phases of the renewal of the steel pipe are the following: - Preparation of temporary openings in the penstock, cutting the pipe for a length of 4 m in three points at equal distance, given that the diameter of the existing manholes is not suitable for a good access into the pipe (450 mm); - Removal of the existing internal protective coating byHDW (high pressure water jetting) and sand blasting; - Expansive cleaning and restoration of the existing corrosion pits inside the pipe, by means of surface refining (grinding); - Application of the new protective coating in 4 layers for a total thickness of 500 µm. The existing Sonvico headpond will be decommissioned and bypassed by a PRFV pipe with a diameter of 1100 mm linking the headrace tunnel directly to the penstock. In fact, the headrace tunnel has a compensation volume of 16'000 m3, higher than that of the decommissioned basin, and the gross head of the HPP is slightly increased in the new configuration.