Development of a stable biochar-bacteria hybrid system with a non-N2O-accumulating denitrifier for efficient high-strength nitrate wastewater treatment.

Biological treatment of high-strength (above 500 mg/L NO3--N) nitrate wastewater is often limited by process instability, nitrite accumulation and nitrous oxide (N2O) emissions. This study developed a synergistic biochar-bacteria hybrid system by coupling a non-N2O-accumulating denitrifier (Citrobacter freundii XY-1) with biochar (BC550) derived from spent mushroom substrate. Pyrolyzed at 550 ℃, BC550 exhibited high electron transfer capacity and served as a multifunctional carrier, facilitating biofilm formation and enabling high-rate nitrate removal. In a continuous-flow biofilter treating 1200 mg/L NO3--N, the system maintained a nitrate removal efficiency exceeding 97.5% for over 100 days at a hydraulic retention time of 15 h and C/N ratio of 10, with effluent nitrite consistently below 3 mg/L. Microbial community analysis confirmed the stable dominance of the inoculated XY-1 strain (39.7%), demonstrating successful bioaugmentation and system resilience. This work presents a stable and environmentally friendly hybrid system for high-strength nitrate removal, achieved through the rational coupling of functional biochar with a specific beneficial microorganism to ensure high treatment efficiency and mitigate N2O emission risk.