Enhanced aerobic sludge granulation by applying carbon fibers as nucleating skeletons

A novel granulation technique by adding carbon fibers (CFs) as the skeletons of aerobic granular sludge (AGS) was developed in this study. Three sequential batch reactors (SBRs) named R1, R2 and R3 were operated over 5 months under low hydraulic shear force (HSF). Comparatively, the R2 and R3 had short CFs and long CFs added to the sludge mixture, respectively. The results indicated that adding CFs accelerated the maturation of granules and improved the stability of AGS, while the AGS in R1 disintegrated twice due to the overgrowth of filamentous bacteria, and appeared in a mixture of flocs and AGS eventually. Total amount of protein and polysaccharide in extracellular polymeric substances (EPS) showed little difference among three reactors, whereas their contents in tightly bound EPS of AGS in R2 and R3 were much higher than that in loose bound EPS during granulation. Confocal laser scanning microscopy (CLSM) images showed that the beta-D-glucopyranose polysaccharides of AGS in R2 and R3 were more concentrated in the outer layer, but uniformly distributed in the AGS in R1, indicating that CFs might maintain the structural stability of AGS by providing skeletons together with beta-D-glucopyranose polysaccharides. High throughput sequencing analysis showed that the relative abundance of functional bacteria, especially Candidatus_Accumulibacter, was extremely increased due to the addition of CFs. This study indicated that adding CFs is an effective method for aerobic sludge granulation under low HSF.