Kinetic mechanism of methanol-fed partial denitrification anammox in tertiary moving bed biofilm reactors fed with real secondary effluent

• Achieved effluent TIN ≤ 3 mg/L in normal and ≤ 4 mg/L in peak loading operations. • Methanol savings of 31.6 % to 46.3 % were realized at influent DO of 0–3 mg/L. • Two distinct PdNA mechanisms identified based on wastewater strength. • Low rbCOD leads to nitrite sink; high rbCOD shifts mechanism to rate differential. • Methylotroph kinetics give methanol unique PdN mechanisms vs. other carbon sources. This research advanced our understanding of methanol-enabled partial denitrification anammox (PdNA) and elucidated the impact of operational conditions on the PdNA mechanism and performance shifts. Specifically, a pilot-scale tertiary moving bed biofilm reactor (MBBR) treatment train was operated onsite for 417 days in a local municipal wastewater treatment plant (WWTP) to understand the effectiveness and mechanisms of methanol-fed PdNA. This MBBR train was able to achieve effluent total inorganic nitrogen (TIN) ≤ 3 mg/L under normal loading operation and ≤ 4 mg/L under peak loading operation. The process enabled remarkable methanol savings ranging from 31.6 % to 46.3 %, at influent dissolved oxygen levels ≤ 3 mg/L, with projected oxygen savings of 1.1 to 1.6 tons per day for the WWTP. The research unravelled two coexisting mechanisms, i.e., in low strength wastewater treatment such as tertiary polishing where bulk COD ≤ 8 mg/L and bulk NO 3 − -N ≤ 4 mg/L, NO 2 − -N sink by anammox bacteria was found to be a dominant mechanism enabling partial denitrification (PdN); while in high strength wastewater with the opposite concentration ranges, the PdN mechanism shifted to the reliance on rate differential between denitratation and denitritation. Additionally, it offered a valuable framework for designing and optimizing full-scale methanol-fed PdNA processes, promoting low carbon nitrogen removal.