Effect of fluctuating hydraulic retention time (HRT) on denitrification in the UASB reactors

Abstract This study cultivated denitrifying granular sludge in three up-flow anaerobic sludge blanket (UASB) reactors using different fluctuation hydraulic retention time (HRT) strategies (reactor 1 (RC): constant HRT (C-HRT); reactor 2 (RDF): downward fluctuation HRT (DF-HRT); and reactor 3 (RUF): upward fluctuation HRT (UF-HRT)). The results verified that these fluctuation HRT strategies could enhance microbial diversity, while robust against fluctuations in nutrient load of the denitrifying granular sludge. Microbial aggregates appeared in RC, RUF and RDF on days 22, 12 and 7, respectively. The sludge in RUF and RDF achieved complete granulation on days 22 and 14, respectively. Compared to the results of RC and RUF, the acyl homoserine lactones (AHLs) concentration rapidly increased, and changed the components of extracellular polymeric substances (EPS) resulted in the rapid formation of denitrifying granular sludge in RDF. These results demonstrate that microbial community, AHLs, EPS, and the denitrifying sludge granulation process were associated with each other. Informed from quorum sensing system, a mechanism for the granulation of denitrifying sludge using the DF-HRT strategy was proposed. The DF-HRT strategy is an economical and fast method to cultivate denitrifying granular sludge. We hope that the results of our research would provide some theoretical support for wastewater producing unstable plants.