This　study　assessed　the　technical　feasibility　of　removing　nitrogen　from　municipal　wastewater　by　partial　nitrification　(nitritation)　in　a　continuous　plug-flow　step　feed　process.　Nitrite　in　the　effluent　accumulated　to　over　81.5　+/-　9.2%　but　disappeared　with　the　transition　of　process　operation　from　anoxic/oxic　mode　to　the　anaerobic/anoxic/oxic　mode.　Batch　tests　showed　obvious　ammonia　oxidizing　bacteria　(AOB)　stimulation　(advanced　ammonia　oxidation　rate)　and　nitrite　(NOB)　oxidizing　bacteria　inhibition　(reduced　nitrite　oxidation　rate)　under　transient　anoxic　conditions.　Two　main　factors　contributed　to　nitritation　in　this　continuous　plug-flow　process:　One　was　the　alternating　anoxic　and　oxic　operational　condition;　the　step　feed　strategy　guaranteed　timely　denitrification　in　anoxic　zones,　allowing　a　reduction　in　energy　supply　(nitrite)　to　NOB.　Fluorescence　in　Situ　Hybridization　and　quantitative　real-time　polymerase　chain　reaction　analysis　indicated　that　NOB　population　gradually　decreased　to　1.0　+/-　0.1%　of　the　total　bacterial　population　(dominant　Nitrospira　spp.,　1.55　x　10(9)　copies/L)　while　AOB　increased　approximately　two-fold　(7.4　+/-　0.9%,　1.25　x　10(10)　copies/L)　during　the　above　anoxic　to　anaerobic　transition.　Most　importantly,　without　addition　of　external　carbon　sources,　the　above　wastewater　treatment　process　reached　86.0　+/-　4.2%　of　total　nitrogen　(TN)　removal　with　only　7.23　+/-　2.31　mg/L　of　TN　in　the　effluent,　which　met　the　discharge　requirements.　(C)　2014　Elsevier　Ltd.　All　rights　reserved.