This paper evaluates the performance of a laboratory-scale anaerobic fixed-film reactor (AFFR) with arranged media treating slaughterhouse wastewater. The reactor was operated at 20 degrees C, its organic loading rate was increased from 1.8 to 9.2 kg COD m(-3) d(-1), and it had a short hydraulic residence time (5-9h). The influence of wastewater concentrations on its performance was studied by artificially increasing the blood content of the wastewater. The efficiency of the removal of organic matter decreased from 70% to 54% as the superficial velocity increased from 0.12 to 0.97 m h(-1), due mainly to distribution defects, as had been confirmed experimentally by tracer tests. The kinetics of the anaerobic processes was limited by substrate availability, even at high COD concentrations (between 700 and 1100 mg dm(-3)) due to a high content of slowly biodegradable and inert compounds present in the wastewater from the slaughterhouse. It was observed that a large amount of the organic matter had accumulated inside the reactor instead of being removed by methanogenic digestion. Furthermore, the fraction of organic matter held inside the reactor varied significantly in relation to the blood content of the wastewater. (c) 2005 Society of Chemical Industry