Energy-efficient　nitrogen　removal　from　carbon-limited　municipal　wastewater　poses　challenges.　This　study　developed　a　novel　partial　denitrification　and　anammox　(PD/A)　process　using　a　step-feed　strategy　in　a　single-stage　integrated　fixed-biofilm　activated　sludge　(IFAS)　system　operated　under　sequencing　batch　mode.　Despite　relatively　low　temperatures　(15.9-21.6　degrees　C),　biological　nitrogen　removal　efficiency　reached　80.5　%　without　the　addition　of　external　carbon　sources.　With　the　total　inorganic　nitrogen　(TIN)　of　48.9　f　1.2　mg　N/L,　and　C/N　of　4.0　in　influent,　the　TIN　in　effluent　achieved　9.6　f　1.5　mg　N/L.　Anammox　bacteria　(AnAOB)　activity　in　the　biofilm　was　27.96　mg　N/(g　VSS　&　sdot;d),　which　was　threefold　greater　than　in　floc　sludge.　Candidatus　Brocadia　(AnAOB)　presented　a　high　relative　abundance　in　biofilm　(0.53　%),　whereas　denitrifying　bacteria　including　Thauera　and　Candidatus　Competibacter　were　mainly　retained　in　floc　sludge.　Overall,　the　coupling　of　step-feed　strategy　and　IFAS　offers　novel　insights　into　the　efficient　nitrogen　removal　from　low　C/N　domestic　sewage　for　retrofitting　of　municipal　wastewater　treatment　plants,　not　only　saving　the　energy　consumption　for　aeration,　but　also　fully　utilizing　carbon　sources　and　ammonium　in　the　influent　for　the　PD/A　process.