3. Enhancement of granulation by growth nuclei

One of the contributing factors to the development of granules from suspended sludge is the presence of nuclei or bio-carriers for microbial attachment. The attachment of cells to these particles has been proposed as the initiation step for granulation. Since the second step was the formation of a dense and thick biofilm on the cluster of the inert carriers, this step could be considered as biofilm formation. In other words, once

the initial aggregates are formed, subsequent granula-tion could be regarded as a mere phenomenon of an increase of biofilm thickness. Hence, the sludge granulation process in UASB reactors with added inert particles might be interpreted as a biofilm-forming phenomenon.

Several investigators have studied the effect of inert particles in granulation. Hulshoff demonstrated the importance of inert support particles in the granula-tion process. When the inert particles (40–100 mm in size) were removed from the inoculated sewage sludge, granulation was not observed within the period of time required for granulation of dispersed sludge with no removal of inert particles in the seed sludge.

Others shows that inert materials can accelerate sludge granulation.