| Summary: | Moving bed biofilm reactor (MBBR) is considered as a promising technology for wastewater treatment owing to the high biomass retention and low cost. In this study, the performance of using MBBR for partial denitrification (PD) was investigated. Denitrifying biofilm was successfully formed after 40 days with the biomass and nitrite reduction rate of 40.83 mg VSS/g carriers and 51.52 mg N/(gVSS·h), respectively. Morphology analysis by scanning electron microscope (SEM) showed that the biofilm surface was dominant by <i>cocci</i>, filamentous bacteria, and extracellular polymeric substances (EPS). Investigation about the influencing factors of PD found that the optimal COD/NO<sub>3</sub><sup>−</sup>-N and pH for efficient nitrite production (nitrate to nitrite ratio: 96.49%) was 3 and 9, respectively. Moreover, <i>Saccharimonadales</i> was proved to be dominant functional microbes in the constructed PD systems with different influent conditions because its relative abundance exhibited good correlation with the nitrite accumulation. By analyzing the biofilm characteristics under different conditions, PD was observed to mainly occur in the range of 300–700 μm inside the biofilm, where most of the dissolved oxygen was consumed. This study confirmed the feasibility and superior performance of PD-MBBR system.
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