Performance Optimization and Nitrogen Removal Mechanism of Up-Flow Partial Denitrification/Anammox Process

This study aimed to remediate the problems of sludge floating and uneven mass transfer in up-flow partial denitrification/anammox (PDA) reactor and dissect the nitrogen removal mechanism. Two up-flow reactors were operated, whereby in R1 combined biological carriers were added, while in R2 mechanical stirring was applied, the reactors were inoculated with PD sludge and anammox sludge. Results showed the TN removal rates at the end of the operation were 88.98% (R1) and 92.16% (R2). The addition of both strategies suppressed the occurrence of sludge upwelling and deterioration of settling performance, even when the mean size reached 2 mm. 16SrRNA sequencing revealed a relative abundance of anammox bacteria, denitrifying bacteria, and ammonia-oxidizing bacteria in R1 (in the biofilm and in the granular zone) and R2 were 7.51%, 7.03%, 9.06% and 33.1%, 28.82%, 67.11% and 10.7%, 12.2%, 5.29%. And the nitrogen removal was contributed by anammox and denitrification (NO2--N2)in combination, with contributions of 82.17%, 17.83%, and 85.07%, 14.93%, respectively. The determined sludge properties and bacterial activity complemented each other and influenced TN removal