Achieving　maintream　anammox　is　critical　for　energy-neutral　sewage　treatment.　This　study　presents　a　new　way　to　achieve　mainstream　anammox,　which　couples　anammox　with　denitratation　(nitrate　reduction　to　nitrite)　instead　of　nitritation　(ammonium　oxidation　to　nitrite).　An　anoxic/oxic　(A/O)　biofilm　system　treating　systhetic　domestic　wastewater　was　used　to　demonstrate　this　concept　for　over　400　days.　This　A/O　biofilm　system　achieved　a　total　nitrogen　(TN)　removal　efficiency　of　80　+/-　4%　from　the　influent　with　a　low　C/N　ratio　of　2.6　and　a　TN　concentration　of　60.5　mg/L.　Nitrogen　removal　via　anammox　was　found　to　account　for　70%　of　dinitrogen　production　in　the　anoxic　reactor.　Batch　tests　confirmed　that　the　anoxic　biofilm　could　oxidize　ammonium　using　nitrite　as　electron　acceptor,　and　that　it　had　a　higher　nitrate　reduction　rate　than　the　nitrite　reduction　rate,　thus　producing　nitrite　for　the　anammox　reaction.　Metagenomic　analysis　showed　that　Candidatus　Jettenia　caeni　and　Candidatus　Kuenenia　stuttgartiensis　were　the　top　two　dominant　species　in　anoxic　biofilm.　Genes　involved　in　the　metabolism　of　the　anammox　process　were　detected　in　anoxic　biofilm.　The　abundance　of　nitrate　reductase　(73360　hits)　was　much　higher　than　nitrite　reductase　(13114　hits)　in　anoxic　biofilm.　This　system　can　be　easily　integrated　with　the　high-rate　activated　sludge　technology,　which　produces　an　effluent　with　a　low　C/N　ratio.　While　this　new　design　consumes　21%　more　oxygen　in　comparison　to　the　currently　studied　nitritation/anammox　process,　the　nitrite-producing　process　appears　to　be　more　stable.