Revealing the characteristics and formation mechanisms of partial denitrification granular sludge for efficient nitrite accumulation driven by glycerol

Granular sludge is considered as a promising technology for wastewater treatment owing to the high biomass retention and excellent settleability. In this study, the formation and underlying mechanisms of granular sludge for stable and efficient nitrite accumulation in glycerol-driven partial denitrification (PD) reactors were investigated. The PD granular sludge (PDGS) was successfully formed with developing and mature stages. The mature PDGS exhibited superior settleability (mean particle size: 468.1 μm, and SVI5:16.4 mL/g SS), and high microbial activity (VSS/SS: 0.80–0.92). The nitrite was stably and efficiently accumulated with an average nitrate-to nitrite transformation ratio of 87.3%. Morphological analysis showed that the mature granules were dominated by cocci and exhibited compact structure with few inorganic components. The proteins (i.e. tyrosine, tryptophan and aromatic protein-like substances) in tightly-bound extracellular polymeric substances (EPS) were expected to play critical roles in the granulation process in view of their good correlation with particle size variation. The EPS contents (mainly polysaccharides and proteins) and living cells were distributed throughout the granule, suggesting the overall high stability and bioactivity of formed PDGS. Moreover, the glycerol-driven PD reactor caused the evident evolution of microbial structure, and the significant enrichment of Saccharibacteria was expected to be responsible for the granulation process, which was increased to 66.6% in the matured sludge. This study confirmed the feasibility and superior performance of PDGS fed with glycerol.