Although much attention in granulation theories goes to the conditions affecting bacterial adhesion, still the selective wash-out of dispersed sludge, resulting in an increased growth of retained (heavier) sludge agglomer-ates is more crucial for the granulation process. In this respect, the presence of inert particles serving at surfaces on which bacteria can adhere is clearly advantageous.

Nevertheless, the particles should be well settle-able, if not it may cause unwanted sludge wash-out.