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.