Granules as Cellular Aggregates

A multicellular aggregate is formed and separated from its surrounding environment due to:

(1) Aggregation by hydrophobic force, electrostatic interactions, or salt bridges;

(2) Loose polysaccharide or inorganic matrix (iron hydroxide as example) combining the cells together by mechanical embedding, chemical bonds, hydrogen bonds, electrostatic forces, or hydrophobic interactions;

(3) Formation of mycelia, which is a net of branched cell filaments;

(4) Polysaccharide matrix with a filamentous frame;

(5) Structured matrix with layers parallel to the boundary or subaggregates, which are perpendicular to the boundary;

(6) Coverage by a common sheath of organic (polysaccharides, proteins) or inorganic origin (iron hydroxide, silica, calcium carbonate);

(7) Coverage by a common sheath ("skin" of microbial aggregate) consisting of dead cells.

Aerobically grown microbial granules are aggregates which are specified as follows:

(1) with regular shape (spherical, egg-shaped, or elongated oval in cross section);

(2) with size from 0.5 to several mm;

(3) with high-settling velocity from 0.2 to 2cm/s;

(4) with high density and sludge-to-volume index (SVI) from 20 to 80 g/l.

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