Multi-compartment denitrifying bioreactor for aquaculture tailwater treatment: Synergistic effects of temperature, HRT and nitrate loading on nitrogen removal and microbial community.

To address low nitrogen removal efficiency in aquaculture tailwater with a low carbon-to-nitrogen ratio, this study constructed a multi-compartment aerobic denitrification reactor (MPBR) integrating nitrification, slow-release carbon, anoxic denitrification, and simultaneous nitrification-denitrification zones. The synergistic effects of temperature (15 °C, 25 °C), hydraulic retention time (HRT; 4 h, 8 h, 10 h), and influent NO₃⁻-N load (20 mg/L, 50 mg/L) on nitrogen removal were investigated, along with microbial community dynamics via high-throughput sequencing. The partitioned structure ensured robust nitrification, with ammonia removal exceeding 90% under all conditions. Denitrification was highly sensitive to multi-parameter interactions: low temperature (15 °C) reduced NO₃⁻-N removal from 89.4% (25 °C) to 38.1%; high load (50 mg/L) increased absolute removal when HRT was sufficient (≥8 h) but caused inhibition when HRT was insufficient; HRT exhibited a threshold (∼8 h), with limited benefit at low temperatures. The simultaneous nitrification-denitrification zone maintained effluent NO₂⁻-N below 0.2 mg/L across all conditions. Pseudomonadota dominated the microbial community (75% average abundance), correlating positively with denitrification efficiency. At the genus level, Pseudomonas thrived at room temperature, whereas Simplicispira enriched significantly in the slow-release carbon zone under low temperature (up to 47.5%), exhibiting complementary succession and sustaining denitrification potential. Spatial niche differentiation across functional zones and functional compensation under stress supported system stability. This study elucidates the multi-factor regulation of aerobic denitrification and microbial compensation mechanisms, providing a theoretical basis for applying partitioned reactors in aquaculture tailwater treatment.