The　sequencing　batch　biofilm　reactor　(SBBR)　is　widely　applied　in　the　wastewater　treatment　due　to　its　strong　adaptability　to　the　unstable　influent　substrate　concentrations.　The　growth　environment　for　microorganisms　is　different　in　the　outer　and　inner　space　of　the　biofilm,　which　leads　to　different　microbial　community　structure　in　different　zones　of　the　system.　Salinity　is　one　of　the　key　factors　that　affect　biological　nitrogen　removal　(BNR)　performance　for　domestic　wastewater　treatment.　Higher　salinity　could　also　promote　the　nitrite　accumulation.　In　particular,　nitrite　accumulation　was　considered　to　be　a　major　parameter　for　affecting　the　emission　of　N2O　in　both　nitrification　and　denitrification　stages,　and　therefore　mitigate　the　environmental　benefits　of　nitrogen　removal　process.　In　present　study,　the　feasibility　of　simultaneous　nitrification　and　denitrification　process　achievement　in　a　SBBR　was　evaluated　treating　domestic　wastewater　with　NaCl　addition　(0,　5,　10,　15　and　20　g/L)　salinity　addition.　For　more　detailed　insights,　the　changes　of　polyhydroxyalkanoate　(PHA)　and　glycogen　(Gly)　were　also　analyzed　to　evaluate　the　salinity　effect　on　nitrite　accumulation　and　N2O　emission.　The　results　showed　that　with　the　increase　of　NaCl　concentration,　the　nitrogen　removal　efficiency　decreased,　while　the　N2O　emission　ratio　increased.　The　NH4+　removal　efficiency　was　more　than　95%　as　the　NaCl　concentration　was　no　more　than　10　g/L.　When　the　NaCl　concentration　increased　to　20　g/L,　the　average　NH4+　decreased　to　70.6%.　As　the　NaCl　increased　from　0　to　20　g/L,　the　increment　of　PHA　and　Gly　decreased　from　43.6　mg/g　and　34.5　mg/g　to　28.2　mg/g　and　22.7　mg/g,　respectively,　while　the　NO2-　accumulation　and　the　N2O　emission　ratio　increased　from　1.12　mg/L　and　4.08　%　to　18.87　mg/L　and　13.60%.　The　more　NaCl　was　added,　the　higher　the　ratio　of　NO2-　to　NOx-　accomplished.　The　accumulated　NO2-　contributed　to　the　occurrence　of　nitrifier　denitrification　(ND)　by　AOB.　Most　nitrous　oxide　emission　was　via　ND　process　with　NH4+　as　electron　donor　and　NO2-　as　electron　acceptor.　The　higher　amount　of　N2O,　formed　in　the　transition　zone,　could　be　consumed　in　deeper　regions　of　the　biofilm　when　the　COD　was　sufficient.　In　the　absence　of　external　carbon　source,　both　PHA　and　glycogen　Gly　were　used　as　internal　carbon　source　for　the　endogenous　denitrification.　The　higher　NaCl　concentration　inhibited　the　PHA　and　Gly　production,　which　decreased　the　internal　electron　donors　for　denitrification.　The　competition　for　electron　between　Nir　and　Nos　during　the　endogenous　denitrification　process　in　the　deeper　region,　as　well　as　the　nitrifier　denitrification　of　AOB　in　the　transition　region,　contributed　to　the　high　N2O　emission,　especially　in　the　high　NaCl　concentration　of　15　and　20　g/L.　Furthermore,　higher　NaCl　concentration　reduced　the　density　of　the　biofilm,　which　made　it　possible　for　more　DO　diffusing　into　the　biofilm.　It　can　not　be　ignored　that　DO　possessed　high　inhibition　on　Nos,　which　attributed　to　the　high　N2O　emission　under　high　NaCl　concentration.　The　＇feast＇　time　increased　at　a　high　salinity,　revealing　the　inhibition　of　microbial　activity.　High　salinity　hindered　the　denitrification　rate,　and　the　inhibition　degree　was　dependent　on　the　influent　COD/N　and　terminal　electron　acceptors.　Compared　with　the　nitrate　reduction　rate　(DNAR)　and　the　nitrite　reduction　rate　(DNIR),　the　nitrous　oxide　reduction　rate　(DN2OR)　was　much　more　reduced　by　high　salinity.　In　saline　wastewater　BNR　process,　the　higher　NO2-　accumulation,　the　competion　between　Nir　and　Nos,　as　well　as　the　higher　DO　concentration　in　the　inner　region　of　the　biofilm,　led　to　the　increase　in　N2O　yield.　©　2020,　Editorial　Department　of　the　Transactions　of　the　Chinese　Society　of　Agricultural　Engineering.　All　right　reserved.