In this work the performance of an anaerobic UASB reactor coupled with an activated sludge reactor for carbon and nitrogen removal in slaughterhouse wastewater is investigated. Periods with and without recirculation of aerobic effluent over 165 days are analysed. Working with a recirculation ratio of 2, removal efficiencies up to 90% and 65% are obtained for DQO and total nitrogen (TN), respectively. Higher recirculation ratios caused severe washout of active biomass in both reactors due to the high hydraulic loading rates applied. Denitrification in the UASB reactor was complete, with no nitrite accumulation and mainly to nitrogen gas. Significant decreases in COD removal efficiencies in the UASB reactor were observed at recirculation ratio of 2. Sudden decreases in total nitrogen efficiencies were related to inhibition process of nitrifying microorganisms, especially at high recirculation ratios.