Nitrogen assimilation may facilitate enhanced nitrogen removal and reservation in algal-bacterial granular sludge

Algal-bacterial granular sludge (ABGS) is promising for sustainable nitrogen management, yet its mechanistic advantages over bacterial aerobic granular sludge (BAGS) remain unclear. Two 16-L BAGS- and ABGS-sequencing batch reactors were operated in parallel to treat synthetic domestic wastewater and evaluated via nitrification inhibition by allylthiourea in addition to batch tests and microbial community analysis. Both biosystems achieved >99 % ammonia removal, in which nitrification/denitrification accounted for ∼99 % in BAGS while ∼53 % in ABGS with a substantial assimilatory contribution. When nitrification was inhibited, BAGS retained <2 % of its original ammonia removal capacity, whereas ABGS maintained ∼33 %, consistent with enriched phototrophs. Under normal operations, the specific ammonia removal rates of ABGS and BAGS were comparable; with allylthiourea inhibition ABGS demonstrated a >20-fold higher rate, supporting its dominance of assimilative pathways. Therefore, ABGS can couple nitrification/denitrification with assimilative nitrogen retention, offering a synergistic route for nitrogen removal and bionitrogen recovery from low-strength wastewater.