Operational optimization of a three-stage nitrification moving bed biofilm reactor (NMBBR) by obtaining enriched nitrifying bacteria: Nitrifying performance, microbial community, and kinetic parameters

Abstract

A two-sludge system consisting of A2/O (Anaerobic Anoxic Oxic) and NMBBR (Nitrification Moving Bed Biofilm Reactor) was developed. Stable and efficient denitrifying phosphorus removal can be realized by high-efficiency utilization of carbon sources in A2/O reactor with the electron acceptors of NOx−-N in a three-stage NMBBR (consisting of N1, N2, N3). The three-stage NMBBR was successfully started within 18 days without additional inoculation sludge. Then a long-term operation (22–120 d) for the optimization of nitrifying performance, microbial community, and kinetic parameters was investigated. The biofilm characteristics (MLSS and biofilm thickness) and real-time control parameters (DO and pH) initially revealed the differences of three stages, while FISH results confirmed the optimizing nitrifying bacteria populations including AOB, Nitrobacteria and Nitrospira (N1: 5.94 ± 0.12%; N2: 8.26 ± 0.42%; N3: 10.06 ± 0.27% on day 50), basically consisting with the qPCR results (N1: 4.05%; N2: 8.04%; N3: 14.14%). The specific ammonium oxidation rate (SAOR: 3.24–10.02 mg/(gMLSS·h)) and temperature coefficient (θ: 1.008–1.011) based on temperature variation (15–35 °C) exhibited a strong resistant ability to low temperature operation. Moreover, half-saturation constants (KN,AOB, KN,NOB, KO,AOB and KO,NOB) fitted by Monod equation proved that DO diffusion played a significant role than substrate utilization (NH4+-N and NO2−-N), but the diffusion resistance was negligible for flocs size smaller than 70 μm. Additionally, the dominant NOB (mainly Nitrospira) due to a higher KN,NOB and KO,NOB was more sensitive to mass transfer and diffusion resistance, which was helpful to understand the microbial competition for short-cut nitrification between AOB and NOB. Based on the above mechanism analysis, the MBBR optimization for the design and operation was put forward.