Physical forces

◦ opposite charge attraction;

◦ thermodynamic forces, e.g. free energy of surface, surface tension;

◦ hydrophobicity;

◦ cross-link or bridge of individual bacteria by filamentous organisms.

In this step, cell surface hydrophobicity would play a crucial role in the initiation of aerobic granules (Tay et al., 2000; Liu et al., 2003, 2004a). According to the thermodynamics theory, increasing the cell surface hydrophobicity would cause a corresponding decrease in the excess Gibbs energy of the surface, which in turn promotes cell-to-cell interaction and further serves as a driving force for bacteria to self-aggregate out of liquid phase (hydrophilic phase). In addition, in this step, filamentous organisms would assist in building up a three-dimensional structure or backbone, which provides a stable environment for the growth of attached bacteria.

• Chemical forces:

◦ hydrogen liaison;

o formation of ionic pairs;

◦ formation of ionic triplet;

◦ interparticulate bridge and so on.

• Biochemical forces:

◦ cellular surface dehydration;

◦ cellular membrane fusion.

Tay et al. (2000) postulated that cellular surface dehydration and membrane fusion would play a part in initiating self-immobilization of bacteria, while some environmental conditions would induce cellular surface dehydration and further membrane fusion (Xu et al., 1993).

Step 3: Microbial forces to make aggregated bacteria mature

• production of extracellular polymers, such as exopolysaccharides, etc.;

• growth of cellular cluster;

• metabolic change and genetic competence induced by environment, which facilitate and further strengthen the cell-cell interaction, and finally result in the high density of adhering cells.

Step 4: Stable three-dimensional structure of microbial granules shaped by hydrodynamic shear forces. The microbial granules would be shaped by hydrodynamic shear force to form a certain structured community. The outer shape and size of granules would result from the interactive strength/pattern between granules and hydrodynamic shear force, micro-bial species and substrate loading rate, and so on. For microbial cells to aggregate, a number of conditions have to be fulfilled. Shear force has been demonstrated to play an important role, and also influences the structure and metabolism of aerobic granules (Tay et al., 2001b; Liu and Tay, 2002). More recently, increasing evidence shows that selection pressure would be the most important factor influencing the formation of aerobic granules (McSwain et al., 2004; Qin et al., 2004a,b; Hu et al., 2005; Liu et al., 2005a; Wang et al., 2005a).

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