ructural properties of activated sludge flocs were found to be sensitive to small changes in ionic strength. This study investigates the effect of ionic strength on settling of activated sludge. Samples were taken from activated sludge process of Ghazvin Sasan soft drink wastewater treatment plant, then treated with different ionic strengths of KCl and CaCl₂ solution, after that the turbidity of supernatant was measured. The results indicated that low ionic strength resulted in a steeper slope of the accumulated turbidity data for both KCl and CaCl₂ as a result of a release of floc fragments and free cells into the supernatant, which did not precipitate during the incubation period. By increasing the ionic strength, the sedimentation improved because of less fragmentation of the sludge flocs. Also at low ionic strengths, the dissociation coefficient was high as a result of a substantial fragmentation of the sludge flocs. The dissociation of the flocs decreased as the ionic strength increased, resulting in a minimum of floc degradation in ionic strength between 0.006 and 0.06M.