Concentration-dependent threonine induces a functional shift from anammox to denitrification in an anammox reactor

As amino acid-based pollutants increase, traditional biological nitrogen removal technologies face new challenges. This study, through continuous-flow experiments, systematically revealed the concentration-dependent dual regulatory effect of threonine in anammox systems. At low concentrations (50-150 mg/L), threonine acted as a supplementary carbon source, promoting co-metabolism between anammox and heterotrophic bacteria. This not only improved the removal efficiency of NH₄⁺-N and NO₂^--N (>95%), but also promoted NO₃^--N consumption, thereby achieving a closed nitrogen conversion loop. In addition, the NH₄⁺ and CO₂ produced during threonine degradation could indirectly promote anammox. Meanwhile, under stable operation at low-concentration threonine (50-150 mg/L), over 90% of threonine was degraded, and Candidatus Brocadia was enriched by 10.3-fold. In contrast, high threonine levels (300-600 mg/L) caused overgrowth of denitrifying bacteria (e.g., Thauera), shifting the dominant nitrogen removal function towards heterotrophic denitrification. During this stage, NH₄⁺-N and threonine accumulated, and substrate inhibition and free ammonia accumulation were identified as the key factors driving anammox failure. This study proposed a new mechanism that threonine-driven co-metabolism fosters microbial interaction and functional succession, providing theoretical guidance for treatment of amino acid-containing wastewater.