The　overall　influence　of　dissolved　oxygen　(DO)　concentrations　and　biofilm　thicknesses　on　single　stage　anammox　process　performance　has　been　evaluated　in　this　study.　Results　indicated　the　biofilm　displayed　a　rapid　initial　increase　and　followed　by　a　relatively　slower　formation　rate　during　the　operational　period.　The　optimal　DO　concentration　could　be　determined　from　a　variety　of　biofilm　thicknesses　and　as　well　the　best　biofilm　thickness　was　required　among　different　DO　levels.　In　our　lab-scale　single　stage　anammox　reactor　with　a　constant　hydraulic　retention　time　of　1.0h　and　influent　ammonium　of　400　mg　L-1,　an　optimal　nitrogen　removal　capacity　was　acquired　(TN　removal　loading　of　2.18　kg-N　m(-3)　d(-1))　at　the　DO　level　of　0.6　mg　L-1　and　biofilm　thickness　of　700　mu　m.　Species　identification　showed　that　Nitrosomonas　related　aerobic　ammonium-oxidizing　bacteria　(AerAOB)　and　Candidatus　Brocadia　fulgida-like　anaerobic　ammonium-oxidizing　bacteria　(AnAOB)　were　the　predominant　functional　bacteria　mixed　together　with　each　other　and　exhibited　no　distinct　niche.　However,　AerAOB　exhibited　higher　biodiversity　at　the　thinner　biofilm　while　AnAOB　showed　a　stable　but　lower　biodiversity.　Moreover,　the　population　of　AnAOB　was　smaller　along　with　more　scattered　cells　at　the　thinner　biofilm　while　they　trended　to　form　specific　irregular　cauliflower-like　zooglea　as　biofilm　thickness　increased.　(C)　2016　Elsevier　B.V.　All　rights　reserved.