Conventional activated sludge (CAS) technology has been the most commonly applied technology for treatment of municipal wastewater for more than a century; however, a significant portion of energy content of the wastewater cannot be recovered by this technology. Therefore, different modifications can be applied to the CAS technology in order to increase the energy harvesting from wastewaters. In this paper, physically treated wastewater from a municipal preliminary wastewater treatment plant (WWTP) was treated at a pilot-scale high-rate activated sludge (HRAS) system. The HRAS system was operated under three different hydraulic retention times (HRTs) and the treatment performances were evaluated. Within this concept, HRTs of 130, 95, and 60 min were tested. The results revealed that total chemical oxygen demand (tCOD) removal of 59% was achievable and the effluent total suspended solids (TSS) concentration was 90 mg/L at the HRT of 60 min. Effluent COD and TSS concentrations decreased with the decrease in HRT by means of enhanced flocculation and sedimentation. This is confirmed by the improvement of sludge volume index (SVI) values at decreased HRTs. The calculated mass balance showed that more COD was diverted to sludge stream with decrease in HRT. Diversion of more COD to sludge can improve the energy balance of wastewater treatment by the application of anaerobic sludge digestion. By this way, HRAS process would be a promising technology in the concept of energy efficient wastewater treatment systems.