Behavior of algal-bacterial granular sludge in a novel closed photo-sequencing batch reactor under no external O₂ supply

Algal-bacterial aerobic granular sludge (AB-AGS) as a symbiosis system possesses high potential for being operated without external O₂ supply. In this study, a novel lab-scale closed photo-sequencing batch reactor (PSBR) was developed for AB-AGS operation under successively open (Phase Ⅰ) and closed (Phase Ⅱ) conditions. Results show that AB-AGS maintained almost 100% of organics removal, exhibiting higher removals of phosphate (63 ± 20%), K⁺ (19 ± 12%) and Mg²⁺ (26 ± 12%), and higher chlorophylls content during Phase II. Meanwhile, only O₂ besides N₂ was detectable in the headspace of PSBR. The change of granular structure and faster algae growth during Phase Ⅱ may contribute to the increase of microbial activity and phosphorus bioavailability, in which lower extracellular polymeric substances content may account for low biomass retention. Results from this closed PSBR imply that AB-AGS has the potential to reduce some greenhouse gases like CO₂ and CH₄ emission.