Formation and Granulation Mechanism of Granular Sludge Dominated by Denitrifying Glycogen-Accumulating Organisms

Abstract

In this study, granular sludge (GS) dominated by denitrifying glycogen-accumulating organisms (DGAOs) formed after 136 days of cultivation in a sequencing batch reactor under the low phosphate influent condition. The average phosphorus removal and denitrification efficiency in the stable operation stage were 84.9% and 54.3%, respectively. The percentages of DGAOs and denitrifying phosphate-accumulating organisms in GS were 17.1% and 0.5%, respectively. The main species of DGAOs in GS were Candidatus Competibacter and Candidatus Contendobacter. The excessive secretion of exopolysaccharide induced by the enrichment of DGAOs improved the viscosity of extracellular polymeric substances, thus promoting the initial sludge granulation and subsequent rapid enlargement of GS. The X-ray diffraction analysis revealed that the main composition of the inorganic crystal inside GS was CaCO3. The metabolism of DGAOs caused a high pH environment, leading to the preferential formation of CaCO3. This study reveals the important role of DGAOs in sludge granulation.

Keywords: Granular sludge, Denitrifying glycogen-accumulating organism, Extracellular polymeric substances, Calcium carbonate, pH