Recirculation ratio regulates denitrifying sulfide removal and elemental sulfur recovery by altering sludge characteristics and microbial community composition in an EGSB reactor.

Expanded granular sludge blanket (EGSB) is regarded as a promising reactor to carry out denitrifying sulfide removal (DSR) and elemental sulfur (S) recovery. Although the recirculation ratio is an essential parameter for EGSB reactors, how it impacts the DSR process remains poorly understood. Here, three lab-scale DSR-EGSB reactors were established with the different recirculation ratios (3:1, 6:1 and 9:1) to evaluate the corresponding variations in pollutant removal, S recovery, anaerobic granular sludge (AGS) characteristics and microbial community composition. It was found that an intermediate recirculation ratio (6:1) could facilitate long-term reactor stability. Adequate recirculation ratio could enhance S recovery, but an excessive recirculation ratio (9:1) was likely to cause AGS fragmentation and biomass loss. The S desorbed more from sludge at higher recirculation ratios, probably due to the enhanced hydraulic disturbance caused by the increased recirculation ratios. At the low recirculation ratio (3:1), S accumulation as inorganic suspended solids in AGS led to a decrease in VSS/TSS ratio and mass transfer efficiency. Although typical denitrifying and sulfide-oxidizing bacteria (e.g., Azoarcus, Thauera and Arcobacter) were predominant in all conditions, facultative and heterotrophic functional bacteria (e.g., Azoarcus and Thauera) were more adaptable to higher recirculation ratios than autotrophs (e.g., Arcobacter, Thiobacillus and Vulcanibacillus), which was conducive to the formation of bacterial aggregates to response to the increased recirculation ratio. The study revealed recirculation ratio regulation significantly impacted the DSR-EGSB reactor performance by altering AGS characteristics and microbial community composition, which provides a novel strategy to improve DSR performance and S recovery.