In　order　to　investigate　the　effects　of　decreasing　settling　time　and　biofilm　on　the　rapid　start-up　of　aerobic　granular　sludge　reactor　for　nitrification,　the　real　domestic　wastewater　was　treated　in　three　sequencing　batch　reactors　(SBR,　named　as　R1,　R2　and　R3)　and　one　sequencing　batch　airlift　reactor　(SBAR,　named　as　R4).　After　decreasing　the　settling　time　from　20　min　directly　to　2,　4　and　2　min　respectively　in　R1,　R2　and　R4,　for　the　reason　that　the　selective　pressure　of　settling　velocity　was　so　strong　that　the　activated　sludge　was　all　washed　out　of　the　reactors,　the　three　SBRs　lost　their　function　of　treating　wastewater.　Then　the　biofilm　grew　on　the　walls　of　the　three　reactors,　and　lots　of　soft　granular-like　sludge　appeared　in　the　reactors,　35-40　days　later.　The　biofilm　made　the　effluent　ammonia　of　R1,　R2　and　R4　lower　than　1　mg/L.　The　soft　granular-like　sludge　was　not　the　aerobic　granular　sludge,　but　there　were　lots　of　rotifers　both　around　the　soft　granular-like　sludge　and　the　aerobic　granular　sludge.　The　phenomena　showed　that　aerobic　granular　sludge　was　one　type　of　biofilm.　In　R3　the　settling　time　was　shortened　gradually　from　20　min　to　8,　6,　5　and　4　min.　When　the　settling　velocity　was　10　cm/min,　the　activated　sludge　began　to　granulate,　and　when　the　settling　velocity　reached　12　cm/min,　the　aerobic　granular　sludge　became　dominant　in　the　reactor,　but　the　sludge　floe　coexisted　with　aerobic　granular　sludge.　It　took　33　days　to　cultivate　aerobic　granules　in　R3.　The　average　diameter　of　the　aerobic　granular　sludge　was　0.5　mm,　the　ratio　of　aerobic　granular　sludge　to　sludge　floe　by　weight　was　2:1,　and　the　ammonia　oxidized　rate　of　aerobic　granular　sludge　was　5　times　as　the　sludge　floe＇s.