BACKGROUND: Biological treatment efficiency of coking wastewater is rather poor, especially for chemical oxygen demand (COD) and ammonia-nitrogen (NH4+-N) removal due to its complex composition and high toxicity. RESULTS: A pilot-scale anaerobic/anoxic/oxic/oxic (A(2)/O-2) biofilm system has been developed to treat coking wastewater, focusing attention on the COD and NH4+-N removal efficiencies. Operational results over 239 days showed that hydraulic retention time (HRT) of the system had a great impact on simultaneous removals of COD and NH4+-N. At HRT of 116 h, total removal efficiencies of COD and NH4+-N were 92.3% and 97.8%, respectively, reaching the First Grade discharge standard for coking wastewater in China. Adequate HRT, anoxic removal of refractory organics and two-step aerobic bioreactors were considered to be effective measures to obtain satisfactory coking effluent quality using the A(2)/O-2 biofilm system. The correlation between removal characteristics of pollutants and spatial distributions of biomass along the height of upflow bioreactors was also revealed. CONCLUSION: The study suggests that it is feasible to apply the A(2)/O-2 biofilm process for coking wastewater treatment, achieving desirable effluent quality and steady process performance. (c) 2012 Society of Chemical Industry