Incorporation of anaerobic processes into wastewater treatment trains minimizes amount and enhances stability of excess sludge, but lowers energy consumption. Objectives of this study were to develop a pilot-scale anaerobic fixed-bed loop (AFBL) reactor, to select critical parameters and to investigate feasibility and performance when treating municipal wastewater under real field conditions. Hydraulic residence tirnes (HRTs) of 0.23, 0.28 and 0.36 days were implemented at temperatures of 25 degrees C, 30 degrees C and 35 degrees C, respectively. The AFBL reactor was fed with raw wastewater and operated for 165 days. Results clearly show that the proposed pilot-scale AFBL reactor removes particulate COD (CODp), soluble COD (CODs), total suspended solids (TSS), volatile suspended solids (VSS) and total Kjedahl nitrogen (TKN), and produces biogas under real field conditions, which means that its application is feasible. Average COD and TSS removal was about 52% and 57%, respectively, whereas biogas production increased with temperature increase and HRT decrease. The higher biogas production was recorded at HRT of 0.23 days and temperature of 35 degrees C. Application of the AFBL reactor is suggested as a pre-treatment step to the conventional municipal wastewater treatment, especially in decentralized areas experiencing population fluctuations, such as Greek islands during the warm, high tourist season. At this time, heating would not be necessary and more energy would be saved.