The　objective　of　this　study　was　to　investigate　the　nitritation　performance　in　a　biofilm　reactor　for　treating　domestic　wastewater.　The　reactor　was　operated　in　continuous　feed　mode　from　phases　1　to　3.　The　dissolved　oxygen　(DO)　was　controlled　at　3.5-7　mg/L　throughout　the　experiment.　The　biofilm　reactor　showed　excellent　nitritation　performance　after　the　inoculation　of　nitrifying　sludge,　with　the　hydraulic　retention　time　being　reduced　from　24　to　7　hr.　Above　90%　nitrite　accumulation　ratio　(NAR)　was　maintained　in　phase　1.　Afterwards,　nitratation　occurred　with　the　low　NH4+-N　concentration　in　the　reactor.　The　improvement　of　NH4+-N　concentration　to　20-35　mg/L　had　a　limited　effect　on　the　recovery　of　nitritation.　However,　nitritation　recovered　rapidly　when　sequencing　batch　feed　mode　was　adopted　in　phase　4,　with　the　effluent　NH4+-N　concentration　above　7　mg/L.　The　improvement　of　ammonia　oxidizing　bacteria　(AOB)　activity　and　the　combined　inhibition　effect　of　free　ammonia　(FA)　and　free　nitrous　acid　(FNA)　on　the　nitrite　oxidizing　bacteria　(NOB)　were　two　key　factors　for　the　rapid　recovery　of　nitritation.　Sludge　activity　was　obtained　in　batch　tests.　The　results　of　batch　tests　had　a　good　relationship　with　the　long　term　operation　performance　of　the　biofilm　reactor.　(C)　2015　The　Research　Center　for　Eco-Environmental　Sciences,　Chinese　Academy　of　Sciences.　Published　by　Elsevier　B.V.