An examination of the rheological properties of activated sludges from three sewage treatment works and a digested sludge from a dual mesophilic-thermophilic anaerobic digester (TAD-MAD) showed that all the sludges exhibited yield stresses the magnitude of which varied with the suspended solids' concentration and the type of treatment which they had received. In particular, the TAD-MAD sludge had a yield stress which was an order of magnitude lower than the activated sludges at comparable solids' concentrations. Treatment of the sludges with ultrasound reduced the values of the yield stress and this effect was irreversible. The surface charge carried by the sludge particles and the bound water of the sludge were both found to be related to the yield stress. The bound water content of the activated sludge samples was related to the filtration characteristics, as measured by the capillary suction time method. However, the TAD-MAD sludges were found to have poor filtration characteristics. Microscopic examinations showed that the TAD-MAD sludges contained a predominance of small particles (<100 μm) which would cause blinding of the filters.