Sludges having a high solids content, e.g., from industrial wastewater or sewage treatment, are dewatered using a high-mol. poly(di-Me diallyl ammonium chloride) and a water-sol., high-mol., highly crosslinked copolymer of di-Me diallyl ammonium chloride and 0.1-3.0 mol % of a crosslinking polymer chosen from >=1 of Me triallyl ammonium chloride, tetraallyl ammonium chloride, triallyl amine hydrochloride, N,N,N',N'-tetraallyl ethylene diamine hydrochloride, and methylene bisacrylamide. The latter copolymer is used in amts. of 0.01-0.5 %, based on dry solids, at 276-310 K. [on SciFinder (R)]