PURPOSE: To gelatinize sludge, reduce the weight thereof, simultaneously solidify the sludge and make it easy to handle by blending an alkaline salt, e.g. a phosphate or a carbonate of a specified water-soluble polymer with the sludge, subsequently reacting a sparingly soluble polyvalent metal salt of a strong acid therewith and carrying out dehydration.

CONSTITUTION: With sludge existing in lakes and mashes, rivers, harbors, etc., (A) a phosphate such as a dibasic phosphate, a tribasic phosphate or a tri metaphosphate or a carbonate and/or (B) an alkaline salt of a water-soluble polymer substance having carboxylic groups, e.g. sodium alginate, sodium polyacrylic acid, sodium cellulose glycolate, potassium alginate, potassium polyacrylate or potassium cellulose glycolate is blended. One or more sparingly soluble polyvalent metal salts of a strong acid are then reacted therewith and gelatinization is subsequently carried out. The resultant gelatinized material is subjected to syneresis for removal of water and reduction of the weight and solidification are then carried out.