Microbial driving mechanisms of sludge reduction in modular wastewater treatment systems under surplus aeration...

The treatment and disposal of residual sludge pose a critical bottleneck to the sustainable development of wastewater treatment plants (WWTPs). Modular wastewater treatment systems have garnered significant interest due to their high efficiency and operational flexibility, making them well-suited for small-scale community applications. This study aims to investigate the microbial driving mechanisms underlying sludge reduction in such field-based systems under surplus aeration regulation. By comparing treatment performance, microbial community structure, and metabolic functions between the Surplus Aeration (SA) group and the Conventional (Conv.) group-coupling 16S rRNA high-throughput sequencing and metagenomic analysis -the microbiological basis of sludge reduction was systematically elucidated. Results demonstrated that the SA group achieved a 63.7% reduction in residual sludge while maintaining compliant effluent quality (GB 18918-2002), with COD and NH₄⁺-N removal rates both reaching more than 85%. 16S rRNA profiles indicated higher alpha diversity in the SA group and clear community separation from the Conv. group (PERMANOVA, p < 0.001). The SA group was enriched in taxa with documented extracellular polymeric substance (EPS) degradation potential, including Saccharimonadales, Saprospiraceae, and Caldilineaceae, whereas the Conv. group showed relatively higher abundance of taxa often associated with proliferation and EPS production (e.g., OLB17, Acinetobacter). Metagenomic functional annotation suggested higher representation of genes and pathways related to carbohydrate processing and energy metabolism in the SA group. As these omics results primarily reflect functional potential rather than confirmed in situ activity, we present a conceptual mechanism in which surplus aeration improves DO distribution and substrate utilization in the field system, thereby favoring EPS breakdown and energy-use efficiency-consistent with the observed reduction in sludge yield.