Effect Mechanism of Shear Stress on Hollow Anammox Granules in Low Strength Wastewater Treatment

Hollow anammox granules have been concerned in the field of anammox granular sludge technology. So far, little information is available about the effect of shear stress on hollow anammox granules. The evolution of hollow granular sludge at three shear stresses (τ = 0.014, 0.041, and 0.075 Pa) was investigated in three laboratory scale continuous stirred tank reactors. It was found that the lower shear stress (0.014 Pa) was beneficial to the retention of granules and thus ensured a higher NRR. A higher shear stress led to a stronger granular sludge abrasion and thus smaller granule size. The results showed that the shear stress affected the production of EPS by stimulating the secretion of c-di-GMP synthetase and degrading enzymes. A higher shear stress resulted in a higher content of EPS (LB/TB-EPS). Then, the shear stress had a physical impact on the EPS, which in turn affected the physical and chemical properties of the particles. The increasing shear stress could make LB-EPS loose and thus facilitated mass transfer, and then enhanced the permeability of anammox granules. On the contrary, the lower shear stress not only weakened the penetration of the substrate, but also hindered the outflow of gas to the outside of the granules. Therefore, the occurrence of cavity changed the internal structure of the anammox granules and affected the distribution of bacteria. In conclusion, this research clarified the impact mechanism of shear stress on the characteristics of hollow anammox granules and provided theoretical guidance for the application in engineering