Free　nitrous　acid　(FNA)　has　only　been　studied　as　the　pretreatment　of　waste　activated　sludge　(WAS).　Integrated　fermentation　and　nitrogen　removal　using　FNA　as　a　primary　means　of　treatment　are　seldom　investigated.　WAS　fermentation　was　characterized　under　various　FNA　concentration.　The　production　of　COD,　protein,　and　carbohydrate　increased　with　FNA　concentration　(in　the　range　of　0.197-1.97　mg/L)　before　the　denitrification　process.　Volatile　fatty　acids　(VFA)　were　only　produced　after　complete　denitrification.　Potential　FNA　impact　on　fermentation　step　found　FNA　facilitated　both　solubilization　and　hydrolysis　but　inhibited　acidification,　acetogenesis,　and　methanogenesis　processes.　The　types　of　fermentation　were　determined　using　threedimensional　excitation-emission　matrix　(EEM)　fluorescence　spectroscopy.　Protein-like　substances　and　Tyrosine/Tryptophan　were　the　most　dominant　dissolved　organic　matters　(DOMs).　The　cell　decay　rate　increased　from　0.044　to　0.102/d　based　on　the　nonlinear　fitting　for　the　FNA　concentration　of　0.197-1.97　mg/L.　The　microbial　biomass　mortality　ree　-　i　bstances　(T-EPS)　exceeded　0.04　mg/L　In　addition,　the　microbial　diversity　and　microbial　structure　were　substan　a　operation,　while　the　bacterial　abundance　associated　with　hydrolysis　and　acidification　increased　significantly.