* Corresponding author.

As a key factor affecting the granulation of symbiotic sludge, the evaluation of the influence of algae sources on the ABGS system is expected to further optimize the cultivation of ABGS. In this study, formation of algalbacterial granular sludge (ABGS) cultivated with endogenous algae and exogenous algae was comparatively investigated. During ABGS formation, the presence of algae facilitated the settleability improvement and biomass growth. ABGS cultivated with endogenous algae achieved a faster granulation (39 days) than that with exogenous algae (46 days). Inoculating exogenous algae could significantly improve the nutrient removal efficiency at the start-up stage. Tightly-bound extracellular polymeric substances (TB-EPS) were crucial for ABGS formation and maintenance, especially protein (PN) components. More EPS enriched in the mature ABGS enhanced by endogenous algae. The spatial distribution of protein and alpha-D-glucopyranose was obviously affected by algal sources. During ABGS formation, there was a directional selection in algal species, and the operation condition of the system was the key factor affecting the structure of algae community. The effect of algae sources on the bacterial community structure of ABGS took time to manifest. Correlation analysis showed that endogenous algae contributed more to the metabolism and property improvement of sludge than exogenous algae.

Automated summary

The study compared the cultivation of ABGS with endogenous and exogenous algae, finding that algae sources have a significant impact on the formation and maintenance of granular sludge, with endogenous algae accelerating granulation and improving nutrient removal efficiency.