A combined system designed by converting the flow mixing chamber of an anaerobic filter into an UASB resulted in an increased efficiency of removal of organic matter of 92% and in a gas production of 4.64 l . l(-1). d(-1), at the highest organic loading rate tested compared with that of the unmodified anaerobic fiber. Both reactors were tested using dairy industry wastewater at identical operating conditions at 30 degrees C and organic loading rate between 1 to 8 g COD . l(-1). d(-1).