Simultaneous　partial　nitritation　and　denitritation　(SPND)　coupled　with　anammox　was　established　in　this　study　to　treat　domestic　wastewater.　Two　lab-scale　bioreactors,　namely　SPND-SBR　and　ANA-UASB,　were　used　in　the　two-stage　system.　In　SPND-SBR,　stable　nitrogen　removal　efficiency　of　51.1%　was　achieved　with　a　high　ammonia　oxidation　rate　of　0.117　kg　N/(m(3).d).　Besides,　successful　out-selection　of　nitrite-oxidizing　bacteria　(NOB)　under　low-DO　of　0.1　mg/L　during　the　steady　period,　resulting　in　an　average　effluent　NO2--N/NH4+-N　ratio　of　1.04.　In　ANA-UASB,　the　abundance　of　Candidatus　Brocadia　and　Candidatus　Kuenenia　increased　from　8.21%　and　4.01%　to　21.33%　and　6.41%　with　low　influent　substrate　contents　of　only　38　mg　N/L.　The　effluent　total　inorganic　nitrogen　(TIN)　was　only　8.4　+/-　1.1　mg　N/L　and　the　nitrogen　removal　efficiency　reached　88.24%.　Overall,　the　study　demonstrated　that　the　novel　low-DO　two-stage　process　for　nitrogen　removal　is　a　promising　technique　for　wastewater　of　low　C/N　ratio.