This report contains the results of a study to develop a mechanical device for dewatering alum sludge by freeze-thaw. This proposed device is a combination of two conventional unit operations: a vacuum filter and a blast freezer. Bench-scale studies were conducted to evaluate this concept and develop preliminary design criteria. The results of filter leaf tests indicate that a suitable sludge layer could be collected on a cloth medium at a vacuum level of only 100 mm of Hg and a 5.0-minute filtration time. The volume of sludge was reduced by 67%. The freezing tests indicated that low freezing rate and a high initial solids content had a tendency to produce large alum sludge particles. However, fast freezing rates could be achieved without reducing the effective grain size below that of a fine sand. Curing time had no effect on grain size. The electrical cost of freezing sludge with this device was estimated to be $0.004/m3, which is not expensive in relation to the total cost of water treatment which is approximately $0.25 to $0.50/m3.