BACKGROUND: Coking wastewater is a major pollutant, produced in large quantities in many countries worldwide. This study investigates the performance of a combined system for treating coking wastewater. The system is based on an upflow blanket filter (UBF) with a biological aerated filter (BAF). Efficiency is assessed according to organic pollutants and nitrogen removal. RESULTS: It was found that hydraulic retention time (HRT) had a greater influence on the removal efficiency of NH3-N than chemical oxygen demand (COD). The BAF facilitated simultaneous carbonaceous removal and nitrification, depending on the reactor height. The system removed 81.5% of COD and 96.4% of NH3-N when the total HRT was 46.7 h (15.4 h for UBF and 31.3 h for BAF). Gas chromatography/mass spectrometry analysis indicated that the main components of the coking wastewater were phenols and nitrogenous heterocyclic compounds. Certain refractory compounds decomposed in the anaerobic section, resulting in the production of intermediates. Although most organics present in the influent were absent from the final effluent, a few residual contaminants could not be fully eliminated by the system. CONCLUSION: The experimental results show that the present system is feasible for the treatment of coking wastewater. (C) 2007 Society of Chemical Industry.