Insight into the response of anammox granule rheological intensity and size evolution to decreasing temperature and influent substrate concentration.

Anammox granules are advantageous because of their relatively higher nitrogen removal rate (NRR) and biomass retention capacity in ammonia-containing wastewater treatment. However, little attention has been paid to granule rheological intensity and size evolution, especially under low temperature and substrate concentration conditions. In this study, the size evolution and variations in rheological properties associated with biochemical characteristics of anammox granules were investigated at decreasing temperatures (35 → 13 °C) and influent substrate concentrations (300 → 50 mg NH-N L). Both the specific anammox activities (SAA) and yield stress (τ) (or storage modulus (G')), which reflected granules' intensity, decreased with decreasing temperature and influent substrate concentration. An exponential correlation was found between SAA and τ (or G'). Granule size strongly decreased at low temperature (13 °C) and influent substrate concentration (50 mg NH-N L), despite slight variations in τ (or G'). A threshold τ (or G') that is closely related to the hydrodynamic shear force in the reactor may exist for the anammox granules. Once the τ (or G') of the anammox granules was lower than this threshold value (τ = 10.13-15.63 kPa), granules that could not endure hydrodynamic shear forces would disintegrate and their size would decrease substantially. Candidatus Kuenenia was the dominant genus in the expanded granular sludge bed reactor, reaching a minimum abundance of 14.6% at 16 °C because of the low-temperature shock, but increasing in abundance to 57.0% at 13 °C, indicating it has a competitive advantage at low temperatures. This contributed to achieving a high reactor nitrogen loading rate (>1.0 kg N m d) even at 13 °C or with 50 mg NH-N L influent. Overall, the results of this study will facilitate management of anammox bioreactors that run at various temperatures and influent substrate concentrations by clarifying the correlation between rheological intensity and anammox granule sludge activity and identifying the τ (or G') threshold.