Rapid establishment of algal-bacterial granular sludge system by applying mycelial pellets in a lab-scale photo-reactor under low aeration conditions: Performance and mechanism analysis.

Light-driven algal-bacterial granular sludge (ABGS) is an innovative low-carbon technology with significant merits in treating municipal wastewater, but how to shorten the photogranulation process, especially under low aeration conditions, is largely unknown. Herein, two strategies were proposed to accelerate the start-up of the ABGS system in photo-sequencing batch reactors (PSBRs) with a low superficial gas velocity of 0.5 cm/s. Compared to directly dosing mycelial pellets (MPs), applying MPs to flocculate algae and using the formed algal-mycelial pellets (AMPs) as carriers enhanced the establishment of the algal-bacterial symbiosis. The ABGS system developed rapidly within 20 days, with a large particle diameter (mean diameter of 321 μm) and excellent settleability (SVI of 55.4 mL/g). More importantly, this system could be stably operated for at least 100 days, mainly attributed to the reinforced secretion of protein with unique secondary structure and elevated hydrophobic functional groups. As for the reactor performance, the average removal efficiencies of the ABGS system were 97.8% for organic matter, 80.0% for total nitrogen, and 84.4% for phosphorus. The enrichment of functional bacteria and algae, and the up-regulation of functional genes and enzymes involved in electron production and transport processes likely drove the transformation of the pollutants, underlining the inherent mechanism for the excellent nutrient removal performance. This study provides a promising approach to solve the problem of a long ABGS start-up period and unstable granular structure under low aeration conditions, which is significant for achieving effective wastewater treatment without energy intensive aeration.