After several years of research, the U.S. Army Cold Regions Research and Engineering Laboratory (CRREL) has developed a new unit operation for sludge dewatering called a freezing bed. The freezing bed uses natural freeze-thaw to dewater sludge. Essentially, it is the cold regions counterpart to the drying bed. Pilot and full-scale studies conducted in the United States have demonstrated that freezing beds can dewater both water and wastewater treatment sludges to greater than 20% total solids content. The best results were obtained with alum sludge, which was dewatered to 82% solids. The design of freezing beds will depend on the depth of sludge that can be frozen and thawed at the proposed site. Mathematical models capable of calculating the design depth are presented.