Turning influent fluctuations into opportunities: Enhanced nutrient removal in aerobic granular sludge via microbial...

Influent fluctuations are a common phenomenon. However, the underlying mechanisms by which influent fluctuations affect aerobic granular sludge (AGS) performance remain unclear. This study investigated the effects of different influent fluctuations (control group, carbon, nitrogen, and simultaneous carbon-nitrogen fluctuations) on AGS performance. The underlying mechanisms were explored based on the perspectives of removal performance, carbon allocation, and microbiota characterization. Results indicated that although influent fluctuations caused certain variations in effluent nitrogen, the fluctuation groups achieved total nitrogen removal of 77-79%, higher than that of the control group (67%). Meanwhile, chemical oxygen demand (COD) and phosphorus removal efficiencies in the fluctuation groups remained stable at approximately 96% and 90%, respectively. Further analysis revealed that carbon fluctuations favored exogenous denitrifying bacteria (DNBs) and glycogen-accumulating organisms (GAOs), and increased the abundance of denitrification-related genes. Nitrogen fluctuations enriched GAOs and polyphosphate-accumulating organisms, thereby promoting intracellular carbon storage. Carbon-nitrogen fluctuations promoted the enrichment of DNBs and strengthened carbon allocation toward exogenous denitrification. These findings demonstrate that influent fluctuations can enhance nitrogen removal through microbial regulation of carbon allocation, offering fundamental guidance for the engineering application of AGS systems under fluctuating influent conditions.