Hydrodynamic Shear Force

In a bubble column SBR, hydrodynamic shear force is mainly created by aeration that can be described by the upflow air velocity. A study showed that higher shear force favored the formation of more compact and denser aerobic granules, while the stimulated production of extracellular polysaccharides and the microbial activity at high shear force was also observed (Fig. 5.3) (Tay et al., 2001b; Liu and Tay, 2002). It is well known that extracellular polysaccharides can mediate both cohesion and adhesion of cells and play a crucial role in maintaining structural integrity

Superficial air upflow velocity (cm/s)

Fig. 5.3. Effects of superficial air upflow velocity on the PS/PN ratio and SOUR of aerobic granules (Tay et al., 2001b). (•) PS/PN; (o) SOUR.

Superficial air upflow velocity (cm/s)

Fig. 5.3. Effects of superficial air upflow velocity on the PS/PN ratio and SOUR of aerobic granules (Tay et al., 2001b). (•) PS/PN; (o) SOUR.

in a community of immobilized cells (Liu et al., 2004b). Consequently, the enhanced production of extracellular polysaccharides at high shear force can make granule structure more compact and stronger.

Similar to the formation of a biofilm, aerobic granules can form at different levels of hydrodynamic shear forces. Therefore, hydrodynamic shear force is not a primary inducer of aerobic granulation in SBR (Liu and Tay, 2002). However, the structure of mature aerobic granules is hydro-dynamic shear force-related. High shear in terms of superficial upflow air velocity could lead to more compact, denser, rounder, stronger, and smaller aerobic granules, as showed in Figs 5.4 and 5.5 (Tay et al., 2004).

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0.85

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Superficial upflow air velocity (cm/s)

Fig. 5.4. Effect of superficial upflow air velocity on granule size and aspect ratio (Tay et al., 2004).

SV TS

Fig. 5.4. Effect of superficial upflow air velocity on granule size and aspect ratio (Tay et al., 2004).

Superficial upflow air velocity (cm/s)

Superficial upflow air velocity (cm/s)

Fig. 5.5. Effect of superficial upflow air velocity on SVI and biomass density (Tay et al., 2004).

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