Intermolecular adhesion forces explain the formation of denitrifying granular sludge driven by acidic pH under ambient temperature

Abstract

Denitrifying granular sludge (DGS), with a unique microbial structure and high biomass retention, is tolerant to harsh environments (e.g., toxic, acidic and high temperature); therefore, its cultivation is of increasing interest. In this study, we explored the feasibility of DGS cultivation under acidic pH using a sequencing batch reactor and elucidated the mechanism. By lowering the influent pH (from 7.0 to 5.0), the content of extracellular polymeric substances (EPS) increased from 29.28 to 114.80 mg∙g-1 VSS, promoting successful DGS cultivation (mean particle size 241 µm) with high activity (VSS/SS = 0.96) and a total nitrogen removal efficiency above 99 %. Microbial community analysis revealed that Thauera (38.52 %) might be the main producer of EPS. During granulation, the extracellular protein (PN) and exopolysaccharide (PS) content increased by 2.78 and 12.49 times, respectively, with PN always being dominant. Further studies using atomic force microscopy combined with PN colloidal probes showed that both the PN-PN and PN-PS adhesion forces increased with decreasing pH (9.0, 7.0 and 5.0), indicating that the increasing contents of both PN and PS had a positive effect on sludge cell aggregation. At the same time, the PN-PN intercalation forces were greater than the PN-PS intercalation forces, especially at pH 5.0, where the former was 2.34 times greater than the latter, thereby demonstrating the dominant role of PN in DGS formation. This information provides more direct evidence for the role of PN and PS in granule formation, and this study provides an alternative strategy for the direct treatment of acidic nitrate-containing wastewater.