Enhancing nitrogen removal from low-strength municipal wastewater by partial denitrification coupled with anammox in plug flow pure biofilm system

Biofilm has been widely utilized to retain anaerobic ammonium-oxidizing bacteria (AnAOB). However, most research on partial denitrification/anammox (PD/A) focuses on hybrid systems, with limited insights into systems dominated by complete biofilms. This study established pure biofilm system within plug-flow reactor to treat low strength municipal wastewater. Strategies such as two-point feeding significantly increased the activity of AnAOB within anoxic biofilm, reaching 2.0 mg N/g Mixed Liquor Suspended Solids/h. The Delta NO3--N/Delta NH4+-N indicated a more balanced relationship between denitrification and anammox. The reactor operated stably for 240 days, achieving a substantial nitrogen removal efficiency of 75.5 +/- 3.6 % with influent carbon/nitrogen ratio of 3.2. Candidatus_Brocadia, whose abundance surged to 2.6 %, contributed 42.5 % to nitrogen removal, predominantly colonized the interior. Partial denitrification bacteria (Dechloromonas, Thauera) preferred the outer layer, exhibiting sustaining nitrite supply for AnAOB growth. This study highlights the potential of pure biofilm system for mainstream anammox application.