The scum accumulation inside gas-solid-liquid separators (GSL) is one of the main limitations of upflow anaerobic sequencing batch (UASB) reactors during treatment of domestic sewage. Although this type of reactor can be equipped with devices that periodically remove scum, this solution has been proved to be very expensive in addition to being inefficient when discharging procedures are not correctly performed. The main goal of this study was to investigate the performance of a modified UASB reactor concept with a GSL separator which promotes continuous scum discharge to the settling compartment. Furthermore, this proposal was compared with a conventional UASB reactor which was used as control. Both reactors in demo-scale were fed with domestic wastewater and scum production was measured. The results demonstrated volumetric reduction of 50%, and 75% reduction in the mass of total solids in the modified reactor. Additionally, the amount of biogas recovered from the modified reactor was higher than the amount that the control reactor recovered. Therefore, the proposed modification has been proved to be effective, bringing new possibilities to the GSL project.