Phosphorus removal mechanisms in an aerobic granular sludge sequencing batch airlift reactor (AGS-SBAR) system with a low abundance of phosphate-accumulating organisms

Aerobic granular sludge sequencing batch airlift reactor (AGS-SBAR) systems show considerable potential for efficient phosphorus removal. Conventionally, this performance is largely interpreted within the framework of classical enhanced biological phosphorus removal, in which enrichment of phosphate-accumulating organisms (PAOs) is considered essential. However, AGS can still achieve high phosphorus removal under low PAO abundance. This study established an AGS-SBAR system to reveal the mechanism of the aforementioned phenomenon. After 70 days, phosphorus removal efficiency exceeded 89%, with white granules, yellow granules and few flocs coexisting stably in the system. But typical PAOs, such as Acinetobacter remained at low abundances in yellow (0.23%) and white granules (0.5%). Despite the very low abundance, PAOs retained detectable metabolic activity, with carbon source utilization rate reaching 21.05%. indicating a non-negligible but non-dominant intracellular bioaccumulation (IB) contribution. In contrast, the inorganic phosphorus (IP) accumulated by microbial-induced phosphorus precipitation (MIPP) accounts for the majority both in yellow (97.15%) and white granules (98.3%), far exceeding that of IB and adsorption. Moreover, White granules contributed 60.7% of MIPP and 59.8% of total phosphorus removal. This study reveals the underlying mechanisms and division of different components that enable AGS-SBAR with low PAO abundance to achieve acceptable phosphorus removal performance.