The surface of the granules may be rough (filamentous) or smooth (Fig. 6.2) depending on the balance of biomass growth rate, production rate of cell binding exopolysaccharides, and cell detachment rate from the granule (Table 6.1). In some cases, cells of protozoa are attached to the surface of the granules (Fig. 6.2c,d). There are hydrophilic sites on the surface of the granules due to the presence of OH, COO-, HPO42-, NH2, and other polarized groups of polysaccharides and proteins. Together with this, there may be a hydrophobic site caused by the presence of aliphatic chains and aromatic rings of lipids and proteins. The rule of the thumb is that the surface of the granule is dominantly hydrophilic if there is no production of exopolysaccharides in the granule (Table 6.1).
In cases with absence of excessive production of polysaccharides and strong aeration, the granules are covered by skin-like envelope, which is
composed of dead cells. This skin-like envelope reduces cells detachment from the granule and protects it from mechanical destruction. If the pressure changes or mechanical impulses become too strong, the destruction of granule is due to crack in skin-like envelope of granule, and gel from the granule is released to the environment.
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