In this investigation the impact of floc associated cations on enhanced anaerobic digestion, and the enhancement of anaerobic digestion using mechanical and ultrasonic disintegration in an internal recycle were studied. The data show that the degree of digestion enhancement in terms of volatile solids destruction varies, 2.6 to 68.9% and 12% to 80.4% for mechanical and ultrasonic disintegration, respectively. Mechanical shear disintegration operates primarily through particle size reduction and sulfide-enhanced disintegration. However, mechanical shear was strongly influenced by the iron and aluminum content of the floc. Ultrasonic disintegration was found to target material primarily associated with divalent cations. Trivalent metals iron and aluminum were found to have a strong negative impact on ultrasonic disintegration, likely due to wave attenuation and particle recoagulation. This study shows that the metal composition in floc and thus the biochemical composition of sludge has an impact on the degree of digestion enhancement. Therefore, the applicability of specific disintegration technologies can be estimated from measurements of floc metal content.