Yu et al. proposed the following guidelines for the carefully choice of the inert materials to be used in order to enhance sludge granulation:

 

*     high specific surface area

*     specific gravity similar to anaerobic sludge

*     good hydrophobicity

*     spherical shape

 

Lettinga et al. stated that clay and other inorganic particles seemed to be harmful to the formation of granular sludge. Also Hickey et al. did not find any difference in thermophilic granulation with or without the addition of sand (50–10 mm in size) into inoculated digested sludge, although the granules formed included sand particles. This can be attributed to the greater specific gravity of some inert particles, like sand particles, in relation to the biomass. More biomass may accumulate in the upper portion of the reactor while the sand particles tend to accumulate in the reactor bottom. Therefore, the chance of contact between the particles and biomass, which is beneficial for microbial attachment, may be    significantly reduced, resulting in no significant enhancement of granulation.