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The accumulation of soluble microbial production (SMP) in an anammox attached film expanded bed (AAFEB) and its effect on the reactor performance were investigated in this study. During the long-term experiment, an extended HRT resulted in the accumulation of SMP and the change of treatment performance. When the SMP increased from 10.5 ± 1.5 mg L⁻¹ to 31.7 ± 6.4 mg L⁻¹ with the increase of influent TN concentration from 313 mg L⁻¹ to 2500 mg L⁻¹, the TN removal efficiency was stable. However, when the influent TN concentration was 3500 mg L⁻¹, the SMP concentration increased higher than 100 mg L⁻¹, the reactor soon became inhibited. Bath tests indicated that both the specific anammox activity (SAA) and the substrate tolerance ability decreased during the stable operation phases, whereas the specific denitrification activity (SDA) was significantly enhanced. In addition, N₂O emissions in the …