A new configuration of the moving-bed biofilm reactor process with pre-denitrification and nitrification was investigated in a pilot plant, which is fed with urban raw wastewater, the primary settler is located between the anoxic and the aerobic reactors, and primary sludge is recycled to the anoxic reactor as a hybrid pre-denitrification. The carriers used in the experiments are made of high-density polyethylene, with a diameter of 10 mm and a specific surface area of 400 m2/m3. The new process was compared with conventional pre-denitrification–nitrification using in-series reactors fed with settled wastewater. The new configuration achieved an increase of 45% for the denitrification rate and of 30% for the nitrification rate when compared with conventional configuration. These results were analysed in light of the calibration study of the mixed-culture biofilm (MCB) model and simulations in AQUASIM 2.1 platform. Regarding denitrification, the high values obtained in the new configuration were attributed to a higher removal of the slowly biodegradable substrate (XS) in the anoxic reactor due to the use of raw wastewater and sludge recycle. Accordingly, the amounts of heterotrophic biomass (XH) and XS obtained in simulations were higher in both the biofilm and the bulk liquid. Regarding nitrification, the higher values were attributed to a lower removal of XS in the aerobic reactors and accordingly, a lower accumulation of heterotrophic biomass in the biofilm was found in the simulations.

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