PURPOSE: To obtain the titled dehydrator which can perform the efficient dehydration of sludge, while contriving to shorten a time required for the freeze-thawing of sludge by rapidly thawing frozen sludge, by adopting the constitution that the thawing of frozen sludge in a freeze-thawing tank is performed with heat formed by an applied electric current.

CONSTITUTION: When conc. sludge in a tank 1 for freeze-thawing sludge is freezed, an electric current is applied from an AC power source 14 to each heat-transmitting plate 2. An electric current is then applied between the heat-transmitting plates 2b and 2a, 2c, 2d and 2c, 2c, etc. through the sludge containing large amounts of electrolytes such as Cl- and SO42- in its components. Hence, the thawing of the freezed sludge is performed with heat formed by the applied electric currents. By this freeze-thawing sludge dehydrator, the thawing speed of the freezed sludge can be desirably adjusted by controlling the applied electric currents. In addition, the surfaces of the heat-transmitting plates 2 can be held at a low temp. by holding cold brine standing in the heat-transmitting plates 2 during the application of an electric current, so that the corrosion of the heat-transmitting plates 2 caused by the applied electric current can be inhibited. In the drawing, numerals 7, 9, 10 and 11 represent a cooler, an automatic change-over valve, a tank for reserving conc. sludge and a pump for applying an electric current, respectively.