Mainstream　partial　denitrification　anammox　was　achieved　through　inoculation　of　side-stream　mature　partial　nitritation　anammox　biofilm　without　domestication.　The　contribution　of　anammox　to　nitrogen　removal　was　29.4　%.　Moreover,　prolonging　anoxic　hydraulic　retention　time　and　introducing　side-stream　nitrite　under　different　carbon/nitrogen　ratios　enriched　anammox　bacteria.　The　abundance　of　anammox　bacteria　increased　by　∼　10　times　((2.19　±　0.17)　×　1012　copies　gene　/　g　dry　sludge)　with　a　total　relative　abundance　of　18.51　%.　During　258　days　of　operation,　the　contribution　of　anammox　to　nitrogen　removal　gradually　increased　to　68.8　%.　The　total　nitrogen　in　the　effluent　decreased　to　8.84　mg/L　with　a　total　nitrogen　removal　efficiency　of　76.4　%　under　a　carbon/nitrogen　ratio　of　3.　This　paper　proposes　a　novel　way　to　rapidly　achieve　mainstream　partial　denitrification　anammox　via　inoculation　with　side-stream　mature　partial　nitritation　anammox　biofilm.　This　method　achieves　advanced　nitrogen　removal　from　municipal　wastewater,　even　under　low　carbon/nitrogen　ratios.　©　2024　Elsevier　Ltd