Porous polyurethane biocarriers could enhance system nitrification resilience under high organic loading by retaining key functional bacteria

Resilience to increasing organic loading rates (OLRs) is the key to maintaining stable performance in treating industrial wastewater. First, this study compared the stability, particularly the nitrification performance, of two lab-scale moving bed biofilm reactors (MBBRs) filled with porous polyurethane biocarriers with two conventional activated sludge reactors (ASRs) in the treatment of synthetic coking wastewater under OLRs increasing from 0.3 kg to 1.5 kg COD m^–3 day^–1. In comparison with the ASRs, which could only achieve complete nitrification (99.31 % ± 0.43 %) at an OLR of 0.7 kg COD m^–3 day^–1, the MBBRs could achieve efficient NH₄⁺-N removal (99.45 % ± 0.21 %) at an OLR as high as 1.3 kg COD m^–3 day^–1. Even at an OLR of 1.5 kg COD m^–3 day^–1 where nitrification was inhibited, the porous polyurethane biocarriers in the MBBRs still maintained a highly diversified bacterial community (Shannon index, 4.34 ± 0.31) by retaining the slow-growing nitrifying bacteria and phenol-degrading bacteria, including Methyloversatilis and Acinetobacter, whose phenol degradation functions were confirmed by metagenome-assembled genome extraction and analysis, while the ASRs lost diversity (Shannon index, 1.41 ± 0.45) due to the sequential occurrence of filamentous and viscous sludge bulking. The advantage of the MBBR was further verified in a full-scale coking wastewater treatment system, where a reactor series filled with 4.35 % porous polyurethane biocarriers exhibited better NH₄⁺-N removal of 99.57 % ± 0.34 % compared to 96.85 % ± 2.56 % for a conventional one under an OLR of 0.54 ± 0.12 kg COD m^–3 day^–1. The results could contribute to the development of more effective and resilient treatment systems for industrial wastewater.