Enhanced simultaneous nitrification and denitrification via adding N-acyl-homoserine lactones (AHLs) in integrated floating fixed-film activated sludge process

Abstract Nitrogen removal from wastewater under low carbon/nitrogen (C/N) ratio and low dissolved oxygen (DO) level via simultaneous nitrification and denitrification (SND) can reduce energy consumption and exogenous carbon source, however, the stressful conditions would impair nitrogen removal performance. In this study, integrated floating fixed-film activated sludge (IFFAS) filled with novel surface-modified carriers was adopted and exogenous N-acyl-homoserine lactones (AHLs) were introduced to enhance SND. Results showed that efficient SND performance was achieved at low C/N ratio of 4 and DO concentration of 0.8 mg/L. AHLs could raise polysaccharide contents of extracellular polymeric substances (EPS) remarkably, which strengthened bacterial adhesion and provided structural and functional benefits to resource capture, digestive capacity and intercellular interactions. Bacterial community analysis indicated AHLs could enhance the abundance of mixtrophic denitrifying genera that participated in quorum sensing on carriers, such as anoxic denitrifiers (Flavobacterium, relative abundance 12.7 %), aerobic denitrifiers (Zooglaea, relative abundance 16.9 %) and autotrophic denitrifiers (Silanimonas, relative abundance 5.2 %). The efficient SND was mainly attributed to the structural/compositional-improved biofilm and coexistence of enriched mixtrophic nitrogen-removal microorganisms caused by AHLs. This study inspired a feasible strategy towards synchronous enhancement of biofilm structure and SND performance at low C/N ratio and low DO level through exogenous AHLs.