Patent ID: 7906029

Claim:
A sludge thickening method of a differential rotation rate rotary thickener in which a screw is disposed in a rotatable outer cylinder screen; a flocculant is fed to a raw slurry at a chemical feed rate a to form a raw slurry sludge; the raw slurry sludge is fed into a feed end of the outer cylinder screen; filtered liquid is separated from the outer cylinder screen with rotating the screw at a variable speed; and thereafter a thickened sludge of a thickened sludge concentration is discharged from a discharge end of the outer cylinder screen, the sludge thickening method comprising: setting in advance an upper limit of the chemical feed rate a max of the flocculant, a lower limit a min of the chemical feed rate of the flocculant, an upper limit X max of the thickened sludge concentration, a lower limit X min of the thickened sludge concentration, an upper limit C max of a rotational speed of the outer cylinder screen, a lower limit C min of the rotational speed of the outer cylinder screen, an upper limit S max of a rotational speed of the screw, and a lower limit S min of the rotational speed of the screw; when a thickened sludge concentration X of the thickened sludge, which is discharged from the differential rotation rate rotary thickener, has been increased to not less than the upper limit X max of the thickened sludge concentration, decreasing a chemical feed rate α of the flocculant stepwise until the thickened sludge concentration X becomes not greater than the upper limit X max of the thickened sludge concentration or until the chemical feed rate α of the flocculant becomes equal to the lower limit a min of the chemical feed rate of the flocculant; and, when the flocculant has become equal to the lower limit α min of the chemical feed rate, increasing a rotational speed S of the screw stepwise; and when the thickened sludge concentration X of the thickened sludge, which is discharged from the differential rotation rate rotary thickener, has been decreased to not greater than the lower limit X min of the thickened sludge concentration, decreasing the rotational speed S of the screw stepwise until the thickened sludge concentration X becomes not less than the lower limit X min of the thickened sludge concentration or until the rotational speed S of the screw becomes equal to the lower limit S min of the rotational speed; and, when the rotational speed S of the screw has become equal to the lower limit S min of the rotational speed, increasing the chemical feed rate α of the flocculant stepwise.