Two　reactors　with　different　configurations　were　operated　at　low　substrate　concentration.　One　was　the　UASB　reactor　using　sponge　as　carrier　(bio-film　reactor)　and　the　other　was　filled　with　granular　sludge.　The　results　show　that　the　granular　reactor　has　a　better　nitrogen　removal　efficiency　than　the　bio-film　reactor　when　the　NH4+-N　and　NO2--N　mass　concentrations　of　the　influent　are　(17.03±2.16)　mg/L　and　(19.17±2.33)　mg/L,　respectively.　By　maintaining　the　average　removal　efficiency　of　NH4+-N　and　NO2--N　in　the　granular　reactor　over　90%,　and　shortening　the　hydraulic　retention　time　(HRT)　stepwise,　the　nitrogen　removal　rate　of　the　granular　reactor　achieves　3.55　kg·N/(m3·d),　compared　to　0.94　kgN/(m3·d)　of　the　bio-film　reactor.　Furthermore,　the　mass　concentration　of　NO2--N　to　NH4+-N　in　the　influent　has　certain　influence　on　the　stoichiometry　of　the　reactor.