Based on microstructure of UASB granules observed under scanning electron microscope, Wiegant (1998) proposed a spaghetti model for anaerobic granulation. This model hypothesizes that development of UASB granules is initiated by attachment of filamentous Methanosaeta on precursors, followed by a formation of a three-dimensional network through a branched-growth process. Other bacteria, such as Methanosarcina, could be easily entrapped in this network (Sanchez et al., 1994; Wu et al., 1996). The structured aggregates further develop through cellular multiplication of the entrapped bacteria, and become denser and spherical by the action of hydrodynamic shear force attributed to upflow liquid and biogas. It must be emphasized that in the spaghetti model, formation of the structured aggregate is a crucial stage of the overall granulation process.
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