Shape and Size of the Granules

Typical shape of the granules are sphere or ellipsoid (Fig. 6.1a). The roundness is evaluated by the ratio of the shortest and longest axis of ellipsoid. Probably, elongation of the granule depends on air upflow velocity in SBR. The granules can be significantly elongated at a high upflow velocity of 1.3cm/s, and after several months of cultivation (Fig. 6.1b).

However, there may be the granules with irregular shape (Fig. 6.1c) or small granules merged together and producing grape-shaped aggregate (Fig. 6.1d). The size of aerobically grown microbial granules varied in a wide range, from 0.2 to 10 mm, and depended on the balance of biomass growth, production of cell binding exopolysaccharides, and cell detachment from the granule. The relationships between these processes are shown in Table 6.1.

Fig. 6.1. Shape of the aerobically grown microbial granules (a) spherical and ellipsoid granules; (b) granules of irregular shape; (c) super-elongated granules produced at high upflow air velocity (photo from Dr. Liu Yongqiang); (d) granules produced by filamentous microorganisms (fungi, actinomycetes, filamentous bacteria). (See Color Plate Section before the Index.)

Fig. 6.1. Shape of the aerobically grown microbial granules (a) spherical and ellipsoid granules; (b) granules of irregular shape; (c) super-elongated granules produced at high upflow air velocity (photo from Dr. Liu Yongqiang); (d) granules produced by filamentous microorganisms (fungi, actinomycetes, filamentous bacteria). (See Color Plate Section before the Index.)

Table 6.1. Effect of cultivation conditions on granule size

S"

TO Ci

TO S

Cell growth

Production of Cell detachment polysaccharides, which from the granules are bound cells in granule due to aeration or stirring

Granule size*

Type of surface and its hydrophobicity

Fast growth due to rich medium and optimal conditions Fast growth due to rich medium and optimal conditions Slow growth due to limitation by nitrogen and phosphorus (unbalanced catabolism and anabolism) Slow growth due to limitation by nitrogen and phosphorus (limitation of anabolism; unbalanced catabolism and anabolism) Slow growth due to inhibition by toxic substances (inhibition of catabolism) Slow growth due to inhibition by toxic substances (inhibition of anabolism; unbalanced catabolism and anabolism)

No No Yes

Weak

Strong

Weak

Strong

Weak or strong

Weak or strong

Big Filamentous hydrophilic or hydrophobic surface Medium Smooth hydrophobic surface

Medium Smooth hydrophilic surface

Small Smooth hydrophilic surface

Small Smooth hydrophobic surface

Small Smooth hydrophilic surface

*The granules at stable period of cultivation in SBR with size less than 1 mm, between 1 and 5 mm, and bigger than 5 mm were considered conventionally as small, medium, and big ones, respectively.

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