Regular articlePhysicochemical and microbial properties of settled and floating anammox granules in upflow reactor

- The anammox granules' flotation was related to NRR from its initiation in upflow reactor.
- The key chemical properties of anammox granules were identified both for settled and floated ones.
- The microbial differences were illustrated between settled and floated anammox granules.

A lab-scale UASB reactor was used to assess the high-rate performance of anammox process after long-term operation by shortening HRT. The nitrogen removal rate (NRR) was developed to 15.51 kg N m−3 d−1 with a relative high nitrogen removal efficiency (NRE) of 90.5% under the short HRT of 1.0 h. However, continuous granules' flotation consequently initiated and was enhanced under the high nitrogen gas production condition, accompanying by the decrease in NRE to 69.8%. The physical, chemical and microbial characteristics of the settled and floated anammox granules were compared. Results indicated that the alternation of chemical properties (e.g. EPS over-secretion) of granules helped to prevent gas escaping out, thus decreasing sludge density. Therefore, granules' flotation becomes inevitable in high-rate anammox reactors. Microbial analysis showed that “Brocadiacea” dominated in settled and floated granules with abundance of 42% and 36%, respectively. Whereas, the abundance of “Chloroflexi” in floated sludge (20.69%) was much higher than that in settled sludge (12.66%).