PURPOSE: To obtain a starch flocculant more effective in making a diluted and concentrated sludge suspension unstable by heating and hydrolyzing a starch in the coexistence of lime and alum.

CONSTITUTION: An aqueous dispersion containing wheat starch, potato starch, corn starch, etc., of about 4-12 wt.% solid content, lime and alum is heated at about 80°C so that the dispersion may reach the maximum viscosity. A metal salt, e.g. tribasic sodium phosphate, of about 50-100%, preferably about 65%, of a stoichiometric amount based on the lime is added to the dispersion, and the dispersion with added metal salt is heated to about 95°C. Further, the formed aggregated material or aggregated lump is allowed to be maintained at the temperature for about one hour.