In　this　study,　the　synergy　of　high　nitrite　(NO2--N)　accumulated　partial-denitrification　(PD)　and　anammox　in　a　continuously　fed　upflow　anaerobic　sludge　blanket　(UASB)　reactor　was　verified　for　simultaneous　nitrate　(NO3--N)　and　ammonia　(NH4+-N)　removal.　A　225-days　operation　demonstrated　that　the　relatively　low　total　nitrogen　(TN)　concentration　of　6.56　mg/L　in　effluent　could　be　achieved　with　influent　NH4+-N　and　NO3--N　both　of　30　mg/L,　resulting　in　a　high　TN　removal　of　89.1%　at　17.5　degrees　C.　Batch　tests　revealed　that　the　NO3--N-to-NO2--N　transformation　ratio　(NTR)　of　PD　stabilized　at　90%　during　the　whole　operation,　which　played　a　crucial　role　in　the　desirable　performance.　However,　the　PD　and　anammox　activity　was　negatively　impacted　by　the　limited　mass　transfer　with　serious　sludge　flotation.　Significantly,　hydrodynamic　mixing　optimization　by　adjusting　liquid　recirculation　ratio　effectively　enhanced　the　nitrogen　removal.　Moreover,　protein　composition　and　tightly-bound　structure　of　EPS　played　an　important　role　in　the　sludge　stability.