In order to improve the energy footprint of Psyttalia wastewater treatment plant (WWTP) in Athens, the application of a thermal hydrolysis process (THP) was preferred to the option of constructing additional digesters. Since August 2015, approximately half of the generated waste activated sludge (WAS) has been treated by a Cambi B6-4 system, while the thickened primary sludge (PS) is by-passing the THP and is mixed with the hydrolysed WAS before entering the 4 digesters. The 4 other conventional digesters have treated the remaining sludge. The dewaterability of the mixed digested sludge has significantly been improved from 22% dry solids (DS) before THP installation, up to 31% DS after THP installation. This is providing substantial reduction of energy use and cost savings at the sludge drying plant of Psyttalia. In addition, biogas generation and digester efficiency (VSR) have been increased.