The purpose of this work was to evaluate and compare two continuous systems of posttreatment of anaerobically pretreated weak black liquor (WBL). The first system consisted of a packed bed reactor (PBR) with Trametes versicolor (Tv) immobilized on wood cubes of holm oak (biocubes). The second system was a fluidized bed reactor (FBR) with Lentinus edodes (Le) immobilized on wood cubes of holm oak. The reactors operated for 65 days at a hydraulic retention time (HRT) of 5 days, at 28 degrees C, with continuous aeration. Response variables monitored were conventional and specific, unit, net removal efficiency (eta and eta(sun,) respectively) of chemical oxygen demand (COD), color, and ligninoids, and enzymatic activities of manganese peroxidase (MnP), lignin peroxidase (LiP), laccase (Lac) and proteases. The PBR showed an average color 17 superior to that of the FBR (52.42 +/-21.78% and 25.34 +/- 14.38% for PBR and FBR, respectively); removals of COD and ligninoids presented a similar pattern to that of color. Lac activity was significantly larger in PBR than in FBR. Activity of MnP in PBR was higher than that of the FBR (0.004 and 0.002 U MnP/mL, respectively). This difference could be ascribed to the different fungi present in each bioreactor. Lip activity was very low in both reactors. Average value of proteases was almost double in the FBR as compared with PBR (0.472 and 0.209 U Proteases/mL, respectively). During the last 2 weeks of operation, biocubes in the FBR experienced a significant loss of the attached Le biomass, probably by attrition. This and higher protease activity in the FBR could explain the lower pollutant removals achieved in the FBR. Overall, PBR with immobilized Tv showed a better performance than the FBR with Le for the posttreatment of the recalcitrant anaerobic effluent. Extended and sustained pollutant removal (65 days) was achieved in the PBR, although more research is needed to evaluate bioreactor performance at shorter hydraulic retention times. (c) 2006 Wiley Periodicals, Inc.