A　pilot-scale　activated　sludge　bioreactor　was　filled　with　immobile　carrier　to　treat　high　ammonium　wastewater.　Autotrophic　nitrogen　elimination　occurred　rapidly　by　inoculating　nitrifying　activated　sludge　and　anammox　biofilm.　As　the　ammonium　loading　rate　increased,　nitrogen　removal　rate　of　1.2　kg　N　m(-3)　d(-1)　was　obtained　with　the　removal　efficiency　of　80%.　Activated　sludge　diameter　distribution　profiles　presented　two　peak　values,　indicating　simultaneous　existence　of　flocculent　and　granular　sludge.　Red　granular　sludge　was　observed　in　the　reactor.　Furthermore,　the　results　of　morphological　and　molecular　analysis　showed　that　the　characteristics　of　granular　sludge　were　similar　to　that　of　biofilm,　while　much　different　from　the　flocculent　sludge.　It　was　assumed　granular　sludge　was　formed　through　the　continuous　growth　and　detachment　of　anammox　biofilm.　The　mechanism　of　granular　sludge　formation　was　discussed　and　the　procedure　model　was　proposed.　According　to　the　experimental　results,　the　integrated　fixed-biofilm　activated　sludge　reactor　provided　an　alternative　to　nitrogen　removal　based　on　anammox.　(C)　2015　Elsevier　Ltd.　All　rights　reserved.