Heat causes breakdown of the water containing colloid gel structures. This phenomenon, known as heat syneresis, has been shown to be effective as an aid to dewatering municipal sewage sludges. Samples of primary sludge, and 1:1 mixtures of primary plus activated sludges, and primary sludge plus trickling filter humus were heated to various temperatures for a variety of holding times. The heat treated sludge was then cooled and its dewaterability measured by both filter leaf and specific filtration resistance tests. The primary sludge exhibited a marked improvement in filterability when held at a temperature of 170 to 185 °C for 20 minutes or more. The 1:1 mixture of primary and activated sludges showed this excellent dewaterability when held at temperatures as low as 145 °C for 20 minutes. Very little increase in filterability was noted when the holding time was increased to 40 minutes. The mixture of primary sludge and trickling filter humus showed no such improvement when heat treated at any of the typical good conditions for the other mixtures. Design and cost evaluations of two plant scale systems (one with high pressure sludge pumping; one without) for heat treating domestic sludge show the process to be economically feasible for a design population of 100,000 persons. The annual cost of the heat treatment system was estimated to be one and one-half times that of conventional anaerobic digestion followed by lagoons or sand beds and approximately 50 to 75 per cent of the annual cost of present sludge treatment by vacuum filtration. The easier handling of the heat treated sludges and the economics of the synereis operation could have an important influence on the design, cost, and operation of waste treatment plants in the United States. / Ph. D.