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Evolution of bioaggregate strength during aerobic granular sludge formation

Paper ID Volume ID Publish Year Pages File Format Full-Text
3687 182 2011 10 PDF Available
Title
Evolution of bioaggregate strength during aerobic granular sludge formation
Abstract

This work investigated the modification of aggregate properties during the formation of granular sludge in a sequencing batch airlift reactor (SBAR). The cohesion of biological aggregates was quantified by subjecting sludge samples to two different controlled shear stresses in a stirred reactor. For reference sludge (without granules), flocs broke and reformed easily, indicating that floc size was controlled by the turbulence micro-scale (Kolmogorov scale, here from 17 μm to 62 μm). In contrast, granules showed high strength which enabled them to resist turbulence and their size was no longer imposed by the Kolmogorov micro-scale. Different steps were observed during the granulation process: a first increase of aggregate cohesion associated with a decrease in sludge volume index (SVI), a growth of aggregates with detachment of fragile particles from the surface and, finally, an increase in the sizes of small and large granules to reach a pseudo-stable size distribution. Results suggest that small particles could have formed the seeds for new granules, as they were maintained in the bioreactor. Here, granular sludge was formed in an SBAR with a conventional settling time (30 min), i.e. without particle washout, and with a low superficial air velocity (SAV = 0.6 cm s−1): it is thus demonstrated that high SAV and low settling time are not necessary to produce granules, but probably only accelerate the accumulation of granules. It is shown that the increase of cohesion is the initial phenomenon explaining the granule formation concomitantly with bacterial aggregates densification. It seems important, in the future, to investigate the reasons for this cohesion increase, which is possibly explained either by bacterial bounding interactions or the excretion of extracellular polymeric substances (EPS).

► High SAV and low settling time are not necessary to produce granules. ► Size of conventional sludge flocs is linked to the turbulence Kolmogorov microscale. ► First step of granules formation is an increase of cohesion and a decrease of SVI. ► When no hydraulic selection is performed, bimodal particle size distribution is observed. ► High energy dissipation on mature granules leads to surface detachment (50 μm).

Keywords
Aerobic granular sludge; Aggregate strength; Shear; Cohesion
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Publisher
Database: Elsevier - ScienceDirect
Journal: Biochemical Engineering Journal - Volumes 58–59, 15 December 2011, Pages 69–78
Authors
, , , , , ,
Subjects
Physical Sciences and Engineering Chemical Engineering Bioengineering
Get Full-Text Now
Don't Miss Today's Special Offer
Price was $35.95
You save - $31
Price after discount Only $4.95
100% Money Back Guarantee
Full-text PDF Download
Online Support
Any Questions? feel free to contact us