Autotrophic biological nitrogen removal in a non-aerated algae-partial nitritation /anammox system: Long-term...

This study demonstrated the long-term process stability of algal-partial nitritation/anammox (A-PN/A) in an operational operation condition and gained insight into the mechanism during the photoperiod. Results showed that an efficient nitrogen removal characteristic was obtained under the operational conditions: algae (Oocystis borgei) to PN/A sludge mass ratio of 1:5, light intensity of 2000 lux, and photoperiod of 12:12. Moreover, in a long-term operation, the total inorganic nitrogen removal efficiency could be stabilized at 86 %. Based on Flow cytometry analysis and high-throughput sequencing, the proportion of Chlorophyta exhibited a distinct upward trend, which could provide oxygen for ammonia-oxidizing bacteria and protect anammox bacteria from photooxidative damage. In a typical light-dark cycle assay, unexpectedly, little nitrite accumulated in a typical photoperiod, indicating the partial nitritation and anammox process co-occurred in the whole experiment. There was a higher nitrogen removal rate and higher transcript levels of amoA and hzsA in light period than dark period. While the dark period played a key role in the suppression of nitrite-oxidizing bacteria genus Nitrospira and nxrB inhibition to maintain stable A-PN/A, which was proved by whole-light batch experiments.