The solid liquid separation characteristics of activated sludge are significantly improved by a freezing and thawing treatment. This improvement is much affected by the freezing rate when using the same freezing column. In this study, the effect of the freezing column diameter (inner diameter: 16-147 mm) on the freezing and thawing treatment is investigated. This parameter Is important for scale up purposes. For inner diameters greater than 40 mm and at a constant freezing rate of 2 mm/h, changes In the freezing column inner diameter had a minimal impact on both the solid liquid separation characteristics and the decreasing ratio of the biological activity. Consequently, for the freezing columns with inner diameters of 40 mm and above, scale up of the freezing and thawing equipment can be undertaken with confidence.