Nitrogen　removal　via　nitrite　pathway　was　investigated　in　a　lab-scale　modified　University　of　Cape　Town(MUCT)　process　treating　real　domestic　wastewater　with　low　C/N　ratio.　The　experimental　results　of　121　d　showed　that　nitritation　was　achieved　at　hydraulic　retention　time(HRT)　of　8　h,　dissolved　oxygen(DO)　of　0.3-0.5　mg·L-1,　sludge　recycle　ratio　of　80%,　anoxic　recycle　ratio　of　120%　and　internal　recycle　ratio　of　300%　after　startup　period　of　87　d.　Nitritation　was　stabilized　for　35　d　with　an　average　nitrite　accumulation　rate(NAR)of　62%,　the　highest　NAR　up　to　80%,　average　ammonia　removal　efficiency　of　65%　and　the　highest　of　87%.　Mechanisms　of　nitritation　indicated　that　the　low　DO　concentrations　and　short　HRT　were　really　important　to　nitritation,　whereas　pH,　free　ammonia(FA)　and　free　nitrous　acid(FNA)　had　no　effect　on　nitritation.　And　temperature　and　sludge　retention　time(SRT)　possibly　had　a　slightly　positive　effect　on　nitritation.　The　fluorescence　in　situ　hybridization(FISH)　assays　demonstrated　that　the　percentage　of　ammonia　oxidizing　bacteria(AOB)　in　biomass　increased　when　the　performance　of　MUCT　reactor　was　converted　from　complete　nitrification　to　nitritation,　and　the　highest　AOB　percentage　was　up　to　9.3%.　Meanwhile,　the　percentage　of　nitrite　oxidizing　bacteria(NOB)　decreased,　and　NOB　were　phylogenetically　related　to　Nitrospira.　Nitrogen　removal　via　nitrite　pathway　saves　the　aeration　costs　and　external　carbon　sources　as　compared　to　nitrate　pathway.　For　the　treatment　of　domestic　wastewater,　nitritation　and　denitritation　are　highly　beneficial　because　the　organic　carbon　source　in　it　is　typically　limited.　©　All　Rights　Reserved.