This paper presents a new mechanical freezing concept for freezing alum or other hydroxide sludges as a conditioning step for dewatering. The basic concept is to freeze a thin layer of sludge on a continuously moving fabric belt. Sludge is attached to the belt by a vacuum drum belt filter which also removes one-half of the water and thus reduces the amount of sludge to be frozen. Filter leaf tests were conducted to determine the operational parameters and approximate production rates of this concept. These tests show that freezing alum sludge in thin layers will separate out the water as ice crystals and transform the solids into the same type of granular material produced in natural freezing beds. The average production rate of frozen sludge was 6.5 kg/hr.m2 at −20°C. The belt area needed for a 10,000-m3/day plant was estimated to be 48 m2. This concept has been patented by the U.S. Patent Office.