A method for dewatering of biological sludge is disclosed. The method includes addition of a flocculant to a biological sludge, which includes an aqueous phase and a suspended solid organic material, flocculating and dewatering the sludge. The flocculant includes a polymer composition, which includes a cationic crosslinked first polymer, which is selected from crosslinked polyamines, and a cationic second polymer, which is a polymer obtained by polymerization of (meth)acrylamide and cationic monomers, the second cationic polymer being polymerized in presence of the cationic first polymer.