Method for thickening or dehydrating sludge

The invention relates to a method for thickening or dehydrating sludge, which includes at least: a) a step of adding flocculants to the sludge to be treated; b) a step of flocculation by agitation of the sludge with the flocculants thus added in order to form a mixture of flocks and an aqueous solution; c) a step of mechanical separation of the flocks and the aqueous solution formed during the preceding step; d) a step of recovering the aqueous solution and the flocks that make up a treated sludge; characterised in that: the added flocculants are made up of at least one cationic starch (A) and at least one cationic polyacrylamide (P); the cationic starch or starches (A) including a fixed weight percentage of nitrogen of at least 2 %; and the weight ratio (R) mA/(mA+mP), expressed as solids, is comprised between 0.6 and 0.99.