PURPOSE: To reduce the cost of a dehydration treatment by adding poly-Al chloride to sludge contg. adhesive agents, etc., to form fine flocs and adding slaked lime thereto, then adding an alkali metal salt of a boric acid to the flocs to form the coarser flocs.

CONSTITUTION: The poly-Al chloride is added to the sludge contg. the materials having adhesiveness such as adhesive agents to form the fine flocs and thereafter, the slaked lime is added thereto to adjust the concn. of hydrogen ions at the time of dehydrating said sludge. The poly-Al chloride and slaked lime are otherwise added simultaneously to the sludge to form the fine flocs and the concn. of the hydrogen ions is adjusted. The alkali metal salt (e.g., sodium borate) of the boric acid or the hydrous salt thereof (e.g., borax) is added thereto and the coarse flocs formed in such a manner are dehydrated. As a result, the running cost is reduced by averting the use of a dehydration assistant and the amt. of the dehydrated cake is decreased to a greater extent, by which the treatment cost is reduced. The adhesiveness of the dehydrated sludge is eliminated and the use of a general-purpose dehydrator is permitted.