A　combined　process　including　a　partial　nitritation　SBR　(PN-SBR)　followed　by　a　simultaneous　sludge　fermentation,　denitrification　and　anammox　reactor　(SFDA)　was　established　to　treat　low　C/N　domestic　wastewater　in　this　study.　An　average　nitrite　accumulation　rate　of　97.8%　and　total　nitrogen　of　9.4　mg/L　in　the　effluent　was　achieved　during　140　days＇　operation.　The　underlying　mechanisms　were　investigated　by　using　Illumina　MiSeq　sequencing　to　analyze　the　microbial　community　structures　in　the　PN-SBR　and　SFDA.　Results　showed　that　the　predominant　bacterial　phylum　was　Proteobacteria　in　the　external　waste　activated　sludge　(WAS,　added　to　the　SFDA)　and　SFDA　while　Bacteroidetes　in　the　PN-SBR.　Further　study　indicated　that　in　the　PN-SBR,　the　dominant　nitrobacteria,　Nitrosomonas　genus,　facilitated　nitritation　and　little　nitrate　was　generated　in　the　PN-SBR　effluent.　In　the　SFDA,　the　co-existence　of　functional　microorganisms　Thauera,　Candidatus　Anammoximicrobium　and　Pseudomonas　were　found　to　contribute　to　simultaneous　sludge　fermentation,　denitrification　and　anammox.　(C)　2016　Elsevier　Ltd.　All　rights　reserved.