Unraveling the effects of the particle size on biomass properties, microbial community, and functional genes of denitrifying granular sludge

Abstract

A comparative study was conducted on the biomass properties, microbial community composition, and functional genes of four particle-size groups of denitrifying granular sludge (DGS) (S1: 0.1 ≤ d < 0.5 mm; S2: 0.5 ≤ d < 1.0 mm; S3: 1.0 ≤ d < 1.5 mm; S4: d ≥ 1.5 mm). The results showed that the settling velocity and wet density increased with an increase in particle size. The sludge volume index and water content were negatively correlated with the particle size. The production of extracellular polymeric substances (EPS) and the ratio of protein to polysaccharides increased initially and then decreased with an increase in the particle size. S3 achieved the highest EPS, which combined with Ca2+ ions, resulting in the highest integrity coefficient. A similar convex trend was observed for the specific denitrifying activity. Illumina sequencing and quantitative polymerase chain reaction indicated that microbial community diversity and richness increased with the particle size. S3 had the largest number of denitrifying genes. Statistical analysis revealed that Aquimonas and Pannonibacter, potential hosts of the narG/nirS/nirK/norB/nosZ genes and narG/nirS/nirK/nosZ genes, respectively, were crucial for nitrate removal in the larger-sized DGS (d ≥ 1.0 mm), whereas Lentimicrobium, a possible host of the napA gene, was primarily responsible for denitrification in the smaller-sized groups (d < 1.0 mm). These results indicated that DGS with a particle size of 1.0–1.5 mm had superior granular structural stability and denitrification capacity. This study provides a new perspective for cultivating DGS and enhancing its denitrification capacity and stability.