Direct a naerobic treatment of domestic wastewater is becoming attractive as it can change a wastewater treatment plant from energy consuming to energy producing. A pilot scale UASB-digester was studied to treat domestic wastewater at temperatures of 10-20 degrees C and an HRT of 6 h. The results show a stable chemical oxygen demand (COD) removal efficiency of 60 +/- 4.6% during the operation at 12.5-20 degrees C. COD removal efficiency decreased to 51.5 +/- 5.5% at 10 degrees C as a result of insufficient methanogenic capacity caused by low temperature and increased influent COD load (from 2.0 g/(L.d) to 3.0 g/(L.d)). Suspended COD removal was 76.0 +/- 9.1% at 10-20 degrees C. Soluble COD removal fluctuated due to variation of the influent COD concentration, but the effluent COD concentration remained 90 +/- 23 mg/L at temperatures between 12.5 and 20 degrees C. The methane production was 39.7 +/- 4.4% of the influent COD, which was 80% of influent biological methane potential. The specific methanogenic activity of the UASB sludge and the digester sludge was 0.26 +/- 0.03 and 0.29 +/- 0.03 g CH4 COD/(g VSS d), respectively. The methanogenic community analysis revealed an overall dominance of the acetoclastic Methanosaetaceae and the hydrogenotrophic Methanomicrobiales during the operation between 10-20 degrees C. The results of the UASB-digester treating domestic wastewater at 10-20 degrees C as reported in this paper provide support for application of anaerobic domestic wastewater treatment in moderate climate zones.