PURPOSE: To obtain the titled apparatus for freeze-thawing sludge improved in the corrosion prevention of the heat exchanger of a freeze-thawing tank existent in contact with sludge and the flocculativity of sludge, by electrically connecting the heat-transmitting plates of the heat exchanger to Al electrodes provided at positions facing to said heat-transmitting plates.

CONSTITUTION: When sludge is received in a freeze-thawing tank 1, the sludge existent between Al electrodes 4 and heat-transmitting plates 2a act as an electrolyte to form a corrosion cell with the Al electrodes 4 and the heat-transmitting plates (iron plates) 2a as the opposite electrodes. Hereon, Al and iron act as a cathode and an anode, respectively, since the electrolytic potentials of iron and Al in sea water, for instance, are at 0.45W0.65V and about -0.8V, respectively. Consequently, Al is affected by corrosion as a sacrifice anode, while iron is prevented from the corrosion. In addition, electrolytic reaction is brought out between the Al electrodes 4 and the heat-transmitting plates 2a through the sludge, a corrosion current is let flow, and Al3+ exudes at the cathode and forms Al(OH)3 with OH- formed at the anode. Said Al(OH)3 accelerates the flocculating action of anionic colloid in the sludge. Thus, the dehydratability of the sludge can be improved.