Effects of a higher hydraulic shear force on denitrification granulation in upflow anoxic sludge blanket reactors

• The sludge stability in two USB reactors was first characterized by RSD-N3 and Kss.
• Kss (0.000045) was smaller than other reports under a higher hydraulic shear force.
• The sludge stability mechanism with hydraulic shear force was discussed in detail.

The objectives of this study were to culture a more stable denitrification granular sludge and to investigate the effects of hydraulic shear force on the stability of the granular sludge. The stability consists of shear stability and removal stability, which are characterized by the shear sensitivity (Kss) and by the relative standard deviation of the specific nitrogen removal rate for three consecutive days (RSD-N3), respectively. Two upflow granular sludge blanket (USB) reactors under different hydraulic shear conditions were used to culture granular sludge. The Kss of the mature granular sludge in USBH (G = 24.7 s−1) and USBL (G = 14.5 s−1) were 0.000024 and 0.0051, respectively. The USBH only required 56 days to obtain mature granular sludge, whereas the USBL required 70 days. These results indicated that higher hydraulic shear tended to shorten the granulation time and enhance the shear stability of the granular sludge. The RSD-N3 of the USBL during the maturation period was only 3.68%, which is approximately 32.84% of the value for the USBH, indicating better removal stability for the nitrogen with the USBL. SEM indicated bacillus bacteria were the largest component of the granular microbial community, and metagenomics using high-throughput sequencing identified Methyloversatilis and Azospira as the dominant microorganisms. These findings are important for the development of technologies in this field and have extensive applications in the denitrification of granular sludge.