Nitrogen　removal　via　nitrite　and　simultaneous　nitrification-denitrification　(SND)　are　two　processes　for　nitrogen　removal　from　wastewater.　The　combination　of　these　two　processes,　i.e.,　simultaneous　nitritation-denitritation　or　SND　via　nitrite,　is　highly　beneficial　for　the　domestic　wastewater　treatment　in　terms　of　lower　carbon　requirements,　reduced　oxygen　demand　and　less　biomass　production.　A　lab-scale　sequencing　batch　reactor　(SBR)　treating　domestic　wastewater　with　low　C/N　ratios　was　operated　to　investigate　simultaneous　nitritation-denitritation　without　addition　of　external　carbon　sources　under　limiting　aeration　and　normal　temperatures　(19　+/-　1　degrees　C).　The　results　showed　that　at　a　longer　sludge　retention　time　(SRT)　of　50-66　d　and　an　average　dissolved　oxygen　(DO)　concentration　of　0.65　mg/L,　nitritation　was　successfully　achieved　with　nitrite　accumulation　rate　over　95%.　Fluorescence　in-situ　hybridization　(FISH)　analysis　proved　that　ammonia　oxidizing　bacteria　(AOB)　became　dominant　nitrifying　bacteria.　Furthermore,　denitritation　occurred　during　the　above　aerobic　period.　The　average　total　nitrogen　(TN)　removal　through　simultaneous　nitritation-denitritation　was　maintained　at　52%　with　a　maximum　of　63.1%.　Low　DO　concentration　under　limited　aeration　is　the　key　factor　to　achieve　simultaneous　nitritation-denitritation.　Under　long-term　operation　with　low　DO　concentrations,　the　altering　of　nitrifying　communities,　establishment　of　anoxic　micro-environment　for　denitrifiers　growth　and　the　characteristics　of　COD　and　NF4+-N　biodegradation　promoted　the　occurrence　of　simultaneous　nitritation-denitritation.