In　order　to　treat　the　digested　sludge　dewatering　liquor　bypass　and　reduce　influent　ALR　of　wastewater　treatment　plant,　the　startup　and　maintenance　of　nitritation　was　investigated　in　an　A/O　reactor.　NOB　activity　was　inhibited　and　nitritation　was　stated　up　successfully　by　increasing　the　influent　ALR　(in　N,　the　same　below)　step　by　step　from　0.23　kg/(m3·d)　to　0.78　kg/(m3·d),　in　spite　of　normal　temperature,　long　SRT　and　high　DO　concentration.　Due　to　the　deficiency　of　alkalinity　in　influent,　nitritation　could　be　maintained　steadily　by　the　associated　inhibition　of　free　ammonia　(FA)　and　free　nitrous　acid　(FNA).　The　nitrite　accumulation　ratio　always　kept　at　above　90%　during　the　period　of　maintaining　nitritation　for　30　days,　and　correspondingly　the　average　concentration　of　DO　was　higher　than　2　mg/L.　The　nitrite　accumulation　ratio　did　not　decline　even　if　the　average　DO　concentration　increased　to　4.8　mg/L,　which　demonstrated　that　high　DO　could　not　destroy　the　nitritation　built　by　the　associated　inhibition　of　FA　and　FNA.　The　concentration　of　FNA　at　the　last　chambers　was　increased　by　reducing　the　influent　ALR　in　order　to　enhance　the　inhibition　of　FNA.　The　nitritation　was　still　maintained　for　two　months　although　the　inhibition　of　FA　was　weakened.　But　the　nitritation　was　destroyed　completely　in　3　days　after　the　inhibition　of　FNA　was　eliminated,　which　showed　the　inhibition　of　FNA　to　NOB　was　very　critical　to　maintain　steadily　nitritation.