This paper presents and discusses data obtained from 125 days of operation of an anaerobic sequencing batch biofilm reactor (ASBBR) containing biomass immobilized in inert support (polyurethane foam cubes) applied for the treatment of domestic sewage. The reactor, with a total volume of 1.2 m3, was operated in 8-hour cycles treating 0.65 m3 per batch. Each batch cycle comprised feeding (2 h), reaction (5 h), discharge (0.5 h) and idle (0.5 h). The reactor took approximately 20 days to reach operational stability, confirming that anaerobic fixed film reactors require a shorter start-up period than anaerobic suspended growth reactors. Under stable operating conditions after the startup period, the mean values of COD removal efficiency and effluent total COD achieved 66% and 133 (±39) mg l−1, respectively, demonstrating the potential applicability of this reactor configuration for treating domestic sewage. Even so, a post-treatment unit is required as occurs generally when anaerobic reactors are applied for domestic sewage treatment.