An anaerobic sequencing batch bioreactor with external circulation of the liquid phase wherein the biomass was immobilized on a polyurethane foam matrix was analyzed, focussing on the influence of the liquid superficial velocity on the reactor's stability and efficiency. Eight-hour cycles were carried out at 30ºC treating glucose-based synthetic wastewater around 500 mgDQO/L. The performance of the reactor was assessed without circulation and with circulating liquid superficial velocity between 0.034 and 0.188 cm/s. The reactor attained operating stability and a high organic matter removal was achieved when liquid was circulated. A first order model was used to evaluate the influence of the liquid superficial velocity (vS), resulting in an increase in the apparent first order parameter when vS increased from 0.034 to 0.094 cm/s. The parameter value remained unchangeable when 0.188 cm/s was applied, indicating that beyond this value no improvement on liquid mass transfer was observed. Moreover, the necessary time to reach the final removal efficiency decreased when liquid circulation was applied, indicating that a 3-hour cycle could be enough.