An anaerobic baffled reactor together with an immobilized cell system has been proposed for methanogenesis of the black liquor from pulp and paper mills in a continuous system. A maximum chemical oxygen demand reduction of 50%, and biogas generation of 10Ld(-1), having methane content of 66% (viv) at an organic loading rate (OLR) of 7kgm(-3)d(-1) with hydraulic retention time of 2days, were recorded. OLR values higher than 7 kg m(-3) d(-1) were toxic to methanogenesis and destabilized the reactor system. (c) 2006 Society of Chemical Industry.