Four-stage biofilm anaerobic–anoxic–oxic–oxic system for strengthening the biological treatment of coking wastewater: COD removal behaviors and biokinetic modeling

High-strength coking wastewater with a high chemical oxygen demand (COD) was efficiently treated by a novel pilot-scale four-stage biofilm anaerobic–anoxic–oxic–oxic (FB-A2/O2) system. The results demonstrated that the system played an important role in obtaining an overall COD removal efficiency at a hydraulic detention time (HRT) of 116 h and over 60% of COD was removed in A2 and O1 reactors. Three different mathematical models, including the Grau second-order model, modified Stover–Kincannon model, and the Monod-biological contact oxidation model (Monod-BCO model) were applicable to describe the COD removal efficiency of the FB-A2/O2 system. Based on a kinetics study and model evaluation, the Monod-BCO model was demonstrated to be more applicable to COD removal from the system with an average determination coefficient of 0.9661. According to the Monod-BCO model, the COD maximum utilization rates for a unit area of carrier for the A1, A2, O1, and O2 units were 0.1685 g (m2 d)−1, 0.8384 g (m2 d)−1, 0.4654 g (m2 d)−1, and 0.2689 g (m2 d)−1, respectively. The refractory organics in each reactor were further evaluated by the Monod-BCO model. The results proved that the system had great potential for treating high-COD coking wastewater. Step-feeding after the A1 reactor was strongly suggested to further optimize the feeding pattern of the system, depending on the model evaluation.