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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