Deciphering the genesis of anammox granular sludge floating from the perspective of microbial community

Sludge floating generally occurred in different anammox reactors, while the microbial difference between settling and floating sludge still unclear. In this study, the comprehensive investigation of microbial features between floating sludge and size-fractionated settling sludge was conducted in a stably operated anammox reactor. Firstly, scanning electron microscope image showed that mounts of long filament and a few gas tunnels were found in floating sludge, while cocci and bacilli were stacked in settling sludge. Microbial diversity was gradually decreased as the sludge aggregation, the microbial community structure and composition of floating sludge (FG) were more similar to large size-fractionated sludge (SG-L), and away from the small size-fractionated sludge (SG-S). Anammox bacteria Candidatus Jettenia tended to enrich in small size-fractionated sludge (accounting for 46.3%), while the large particle size feature and starved condition of floating sludge induced the decrease of Candidatus Jettenia (accounting for 23.5%). Meanwhile, denitrifying bacteria Dok59 rapidly enriched in large size-fractionated sludge and floating sludge, accounting for 8.9% and 9.8%, respectively. Network analysis showed that complex microbial interrelationships bring a robust microbial community in the SG-L sample, while the FG sample in the anammox reactor obtained an unstable microbial community. Moreover, the microbial community of FG has narrow niche breadths based on the low betweenness and node transitivity. These results obtained in this study extended our understanding of sludge floating in the anammox reactor and emphasized the crucial role of microorganisms.