Algal–Bacterial Granular Sludge Development from a Pond Microbiome: Rapid Granulation, Treatment Performance, and Nutrient Removal Pathways

Algal–bacterial granules (ABG) are attractive for mitigating greenhouse gas emissions, reducing aeration energy demand, and adopting circular economy in wastewater treatment plants. This study investigated ABG development from a pond-microbiome, nutrient removal capabilities, and microbial community succession in a photosequencing batch reactor (photo-SBR). The formation of compact, green-colored, and fast-settling granules was rapid and evident within 10 photo-SBR cycles. rRNA amplicon sequencing showed moderation of the microbial diversity during granulation and the presence of diverse communities including proteobacteria, cyanobacteria, and green algae in mature granules. Granulation was corroborated with the establishment of ammonium and phosphate removal pathways. ABG and photo-SBR conditions enabled the effective and simultaneous removal of chemical oxygen demand (COD; 89%), biochemical oxygen demand (BOD; 96%), NH4+-N (96%), and PO43–-P (90%). The effective removal led to lower COD (<50 mg/L), BOD (<5 mg/L), NH4+-N (<0.5 mg/L), and PO43–-P (<0.5 mg/L) in the effluent. Moreover, no nitrite or nitrate accumulation confirmed complete nitrogen removal. Increasing influent ammonium and phosphate or decreasing the hydraulic retention time from 34 to 14 h did not affect nutrient removal, demonstrating its robustness and resilience. Our study provided evidence for the rapid start-up of ABG–SBR for establishing nutrient removal pathways using a pond microbiome as the inoculum, contributing to algal–bacterial granular technology development for sustainable wastewater treatment.