Partial　denitrification　was　one　of　most　effective　ways　to　provide　nitrite　for　annamox;　whereas　very　limited　research　has　been　done　to　achieve　nitrite　accumulation　in　biofilm　system.　In　this　study,　partial　denitrification　was　studied　in　a　lab-scale　denitrifying　biofilter　(DNBF).　The　results　showed　biofilm　structure　variations　caused　the　differences　between　nitrate　specific　reduction　rate　(NaSRR)　and　nitrite　specific　reduction　rate　(NiSRR),　which　led　to　nitrite　accumulation　in　different　degree　at　different　biofilm　formation　phases.　Hydrodynamic　conditions　also　significantly　influenced　biofilm　structure,　nitrate　and　nitrite　reduction　activities.　At　the　filtration　velocity　of　3.86　m　h(1),　not　only　biofilm　structure,　NaSRR　and　NiSRR　kept　relatively　stable,　but　also　60%　of　nitrite　accumulation　and　no　nitrate　in　the　effluent　were　achieved.　Furthermore,　Thauera　genus　bacteria,　benefited　for　nitrite　accumulation,　became　the　dominant　communities　in　high　nitrite　accumulation　conditions.　The　partial　denitrification　combine　with　anammox　in　biofilter　have　the　great　potential　applied　in　WWTPs.　(C)　2017　Elsevier　Ltd.　All　rights　reserved.