This laboratory research was carried out to evaluate the performance of a multistage anaerobic biofilm reactor, with six compartments and a working volume of 70 L, for the treatment of a strong synthetic nitrogenous and high-strength wastewater at an operational temperature of 26+/-0.5 degrees C. Initially, the performance of the reactor was studied when subjected to an increase in the hydraulic retention time (HRT) at a constant influent COD concentration of 10,000 mg/L. Five different HRTs were studied: 0.25, 0.67, 1, 3 and 5 days, which were equivalent to 6, 16, 24, 72 and 120 h, respectively. By increasing the HRTs from 6 h to 1 day, COD and BOD removal efficiencies were increased from 63% to 84% and from 66% to 87%, respectively. Moreover, at an HRT of 3 days, COD and BOD removal efficiencies were equal, reaching 93%. In the second phase of the research, the effect of adding nitrate with a concentration of 3000 mg/L at an influent organic loading rate of 10 g COD/L d was researched on the reactor performance and the amount of biogas produced. Denitrification took place almost solely in the first three compartments of the reactor, with efficiencies of 85%, 95% and 98%, respectively. The nitrite produced was only detected in the first and second compartments at concentrations of 138 and 24 mg/L, respectively. In addition, no accumulation of nitrite was detected in the reactor. Furthermore, the denitrification caused an increase in the total volume of produced biogas from 102 to 178 L/d. (C) 2009 Elsevier B.V. All rights reserved.