GAC pretreatment enhanced the downstream ammonia treatment through improving microbial adaptation

Landfill leachate with high ammonia and organic loading poses a major challenge to biological nitrogen removal due to nitrifier inhibition and process instability. This study evaluated the effect of granular activated carbon (GAC) pretreatment during start-up on nitrification performance and microbial adaptation in sequencing batch reactors treating high-ammonia leachate (600–1000 mg NH₄⁺-N/L). Over 113 days, the pretreated reactor maintained ammonia removal above 99%, while the control deteriorated, with effluent NH₄⁺-N exceeding 200 mg/L. The ammonia oxidation rate was 4.6-fold higher in the treated system. Quantitative PCR showed higher amoA gene abundance, indicating stronger ammonia oxidation potential. Microbial analysis revealed enrichment of autotrophic ammonia-oxidising bacteria (AOB), particularly Nitrosomonas and Nitrosospira . These findings indicate that GAC pretreatment stabilises nitrification by supporting autotrophic nitrifiers under elevated ammonia loading. This approach provides a practical strategy for improving biological treatment of high-strength ammonia wastewater.