Understanding the effects of algae growth on algae-bacterial granular sludge formation: From sludge characteristics, extracellular polymeric substances, and microbial community

In this study, algae growth was evaluated for its effects on the formation of algae–bacterial granular sludge by comparing aerobic sludge granulation with and without algae. The results revealed that algae initially self-aggregated and subsequently provided attachment carriers for microbial aggregation, resulting in improved biomass growth (4.82 g/L) and sludge settleability (32.32 mL/g). Compared with aerobic granular sludge, algae–bacterial granular sludge was formed approximately 14 days earlier and its size was 15% larger (600 μm). The removal efficiency of COD, NH4+-N, TN, and TP in the algae–bacterial granular sludge system was enhanced, reaching 97.55%, 98.19%, 84.88%, and 85.19%, respectively. In addition, algae present in algae–bacterial granular sludge facilitated the production of extracellular polymeric substances and triggered alterations in their chemical composition. Analysis of the protein secondary structures of extracellular polymeric substances revealed that the contents of the β-turn helix and aggregated strands in algae–bacterial granular sludge were 5.56% and 7.01% more than those in aerobic granular sludge during granulation. Additionally, algae–bacterial granular sludge had a higher abundance of Gammaproteobacteria (46.09%). Altogether, this study provides a theoretical basis for understanding the formation of ABGS.