The influence on batch gravity sedimentation behavior of the cation valence and concentration of inorganic salts added to activated sludge pretreated by ultrasonication was examined to clarify the synergistic flocculation effect of ultrasonication and salt addition. Among the salts tested, the addition of those with divalent cations such as Ca2+ and Mg2+ were found to be most effective in improving the sedimentation properties of ultrasonicated sludge. For instance, the addition of a tiny amount of Ca2+ formed enormously large flocs with high apparent densities, thereby upgrading the sedimentation performance. The addition of 10 mM Ca2+ increased the constant rate sedimentation velocity during the initial stage of sedimentation by 40.4 times and decreased the sediment volume index after 300 min by 23 points compared to the values for untreated sludge. Mg2+ addition showed a similar trend, suggesting the non-specific binding of divalent cations to intracellular metabolic polymer substances released from cells disrupted by ultrasonication, rather than the interaction and gel formation specific to Ca2+ known as the egg-box structure of alginate that is seen in the presence of Ca2+. The constant rate sedimentation velocity for ultrasonicated sludge increased with increasing concentration of monovalent cations such as Na+ and K+. However, the addition of trivalent cations such as Fe3+ and Al3+ to ultrasonicated sludge was ineffective in improving sedimentation performance, even though their addition to untreated sludge is most effective among the monovalent, divalent, and trivalent cations tested in this research. This ineffectiveness is probably due to steric hindrance between polymeric substances induced by trivalent cations. The results obtained from the cation addition to untreated sludge were qualitatively in line with the Schulze. Hardy rule, supported by the DLVO theory elucidating the compression of the electrical double layer occurring in association with cation addition.