Biological treatment of high-strength carbon and nitrogen synthetic wastewater by two combined anaerobic-aerobic biofiltration systems packed with clay (system 1) or plastic media (system 2) has been studied for 143 days Four phases of operating conditions were carried Out With an influent COD and TKN concentrations varied from 2595 to 5969 mg/L and 5 10 to 1866 mg/L respectively. Both systems exhibited excellent COD removal performance and the average removal efficiencies were Superior to 90% for a loading of 2 to 4 kg COD/m(3)/d. The BOD5 average removal during the four operating phases varied from 91 to 98%. Total nitrogen removal occurred in the aerobic stage showing 60% and 77% of elimination obtained for aerobic filters of systems 1 and 2. respectively Nitrogen removal was related to many factors such as nitrification/denitrification. bacterial assimilation and NH3-stripping The aerobic filter packed with plastic media produced less nitrates than the other filter This may due to an enhancement of the denitrification or to the inhibition of nitrobacter by free NH3, Effluent nitrite concentrations confirmed the possibility of NH3-inhibition. Phosphorus removal was also at acceptable level for both systems with yields varied from 50% (system 1) to 70% (system 2) (C) 2009 Elsevier B V. All rights reserved.