PURPOSE: To obtain a dehydration effect particularly outstanding in improving the filterability of a sludge basin and decreasing the moisture content of cake by using a flocculating agent consisting of chitosan of 67W75mol% in the rate of deacetylation of chitin as an effective component.

CONSTITUTION: Chitin is obtd. by grinding finely the shells of crabs and shrips, removing calcium carbonate with a dilute hydrochloric acid and removing protein with a dilute alkali. Chitosan is obtd. by deacetylating the chitin at a high temp. with an alkali of a high concn. The rate of deacetylation is adjustable by the concn. of alkali, the concn. of reaction and the time for reaction. Sodium hydroxide or potassium hydroxide is usually used for the alkali and the concn. is made 20W50wt%. The temp. is regulated to 100W180°C and the time for reaction in a range of about 30minW5hr. The chitosan obtd. in such a way is better as the viscosity is higher and since the chitosan does not dissolve in water, the chitosan is added in the form of a dilute acidic soln. to waste water and sludge.