This paper describes a new type of anaerobic bioreactor with a fixed-structure bed (ABFSB) in which the support for the biomass consists of polyurethane foam strips placed along the length of the bioreactor. This configuration prevents the accumulation of biomass or solids in the bed as well as clogging and channeling effects. In this study, complex synthetic wastewater with a chemical oxygen demand of 404.4 mg O2 L-1 is treated by the reactor. The ABFSB, which has a working volume of 4.77 L, was inoculated with anaerobic sludge obtained from an upflow anaerobic sludge blanket bioreactor. A removal efficiency of 78 % for organic matter and an effluent pH of 6.97 were achieved. An analysis of the organic volatile acids produced by the ABFSB indicated that it operated under stable conditions during an experimental run of 36 days. The stable and efficient operation of the bioreactor was compared with the configurations of other anaerobic bioreactors used for complex wastewater treatment. The results of the study indicate that the ABFSB is a technological alternative to packed-bed bioreactors. © 2013 Springer-Verlag Berlin Heidelberg. ; peer reviewed: yes ; system details: This record was machine loaded using metadata from Scopus ; NRC Pub: yes ; submission instructions: article in press