Enhanced nitrogen removal in an inverted A2/O-MBR system coupled with chitosan/polyvinyl alcohol biochar aerogel fillers for low C/N aquaculture wastewater treatment

The low carbon-to‑nitrogen (C/N) ratio characteristic of actual aquaculture wastewater often leads to insufficient carbon sources for denitrification in conventional biochemical processes, resulting in poor nitrogen removal and potential membrane-fouling concerns. To address these bottlenecks, an inverted A 2 /O-MBR system coupled with self-developed chitosan/polyvinyl alcohol biochar aerogel (CPBA) fillers was investigated. Using low-C/N freshwater aquaculture wastewater as feed, the study evaluated pollutant removal efficiency, end-point membrane-foulant characteristics, and changes in microbial community composition. The results showed that at the optimal hydraulic retention time (HRT) of 16 h, the standalone system achieved TN removal below 75%. Addition of 20% CPBA fillers substantially increased TN removal to 85%–92% and simultaneously lowered effluent COD to 15–20 mg/L. Besides, EEM analysis showed lower fluorescence intensities in soluble microbial product- and protein-like regions after CPBA addition. Furthermore, high-throughput sequencing suggests that CPBA filler addition reduced overall microbial diversity but enriched norank_f__Microscillaceae and Sphingopyxis , genera with potential involvement in organic matter transformation and nitrogen cycling. Consequently, the inverted A 2 /O-MBR system with CPBA fillers offers an effective strategy for advanced treatment of low C/N aquaculture wastewater, providing a promising approach for similar carbon-deficient water treatment.