An aerobic co-culture, prepared by combining Ralstonia sp. LD35 and Pseudomonas putida DSM1868, was recently found to be capable of extensively degrading many of the hydroxylated and/or methoxylated benzoic, phenylacetic and 3-phenyl-2-propenoic acids occurring in the olive mill wastewaters (OMWs). In the perspective of developing a biotechnological process for the degradation of low-molecular weight (MW) aromatic compounds occurring in the effluents of anaerobic digestors treating OMWs, the capability of this bacterial co-culture of biodegrading a synthetic mix of the above mentioned compounds and the aromatic compounds of an anaerobic OMW-treatment plant effluent in the physiological state of immobilised cells was investigated. Two aerobic fixed-bed biofilm reactors were developed by immobilising the co-culture cells on Manville silica beads and on polyurethane foam cubes. Both supports were found to give rise to a microbiologically stable and biologically active biofilm. The two biofilm reactors were found to be similarly capable of rapidly and completely biodegrading the components of a synthetic mix of nine monocyclic aromatic acids typically present in OMWs and the low-MW aromatic compounds occurring in the anaerobic effluent in batch conditions. However, in the same conditions, the silica bead-packed reactor was found to be more effective in the removal of high-MW phenolic compounds from the anaerobic effluent with respect to the polyurethane cube-packed reactor. These results are encouraging in the perspective of using the co-culture as immobilized cells for developing a continuous biotechnological process for the post-treatment of effluents with low-MW aromatic compounds produced by anaerobic digestors treating OMWs.