A column reactor, in which the bottom two-thirds were occupied by a sludge blanket and the upper one-third by submerged clay rings, was evaluated using slaughterhouse wastewater as substrate. The reactor was operated at 35 degrees C at loading rates varying from 5 g to 45 g chemical oxygen demand (GOD) l(-1) x day(-1) at an influent concentration of 2450 mg COD l(-1). A maximum substrate removal rate of 32 g COD l(-1) x day(-1), coupled with a methane production rate of 6.9 l x l(-1) x day(-1) (STP), was obtained. This removal rate is significantly higher than those previously reported. The rate of substrate utilization by the biomass was 1.22 g COD (g volatile suspended solids)(-1) day(-1) COD removal was over 96% with loading rates up to 25 g COD l(-1) x day(-1), at higher loading rates performance decreased rapidly. It was found that the filter element of the reactor was highly efficient in retaining biomass, leading to a biomass accumulation yield coefficient of 0.029 g volatile suspended solids g(-1) GOD, higher than reported previously for either upflow anaerobic sludge-blanket reactors or anaerobic filters operating independently.