The impact of reduced sulfur compounds on aerobic granular sludge formation and biological phosphorus removal

Biological sulfur conversion combined with phosphorus removal represents an innovative biotechnology for activated sludge (AS). However, its effect on the formation of aerobic granular sludge (AGS) remains unclear. This study aimed at determining the impact of sulfide and thiosulfate on the performance of enhanced biological phosphorus removal (EBPR) and on AGS formation. Using two lab-scale sequencing batch reactors (SBR), we compared EBPR performance, the degree of granulation, and the enriched microbial community with sulfide (SBR-S, 20 mg S/L) or thiosulfate (SBR-T, 140 mg S/L) as primary sulfur source. Results indicate that both reactors achieved comparable EBPR performance, with a phosphorus removal rate of 98% in SBR-T and 99% in SBR-S. SBR-T resulted in compact granules with fast settling properties, while SBR-S resulted in dense floccular sludge, referred to as "densified sludge". Both systems enriched a comparable microbial community with shared dominant genera, including Ca. Competibacter, Thiothrix, Ca. Accumulibacter and Dechloromonas. The higher abundance of Thiothrix unzii throughout the operating time in SBR-T, reaching 19.4%, is believed to have enhanced the granulation process. This study provides important insight into the role of reduced sulfur compounds in sludge densification and EBPR performance, relevant to sulfur-rich wastewaters such as those from industrial sources.