Rheological Characterization of Sludges during Belt Filtration Dewatering Using an Immobilization Cell

An immobilization cell was successfully coupled to a controlled stress rheometer to quantify rheological properties of a sludge during its dewatering. An anaerobically digested sludge and a synthetic sludge were analyzed and conditioned at various doses with a cationic flocculant. Direct strain-controlled oscillatory analyses could not be performed due to rapid dewatering, but controlled shear rate analysis quantified the increases in sludge viscosity as the solid's concentration increased. Immobilization times determined by these experiments-viscosity versus dewatering time-agree with capillary suction times, since both indicate the time required for water removal (r(2) from 0.81 to 0.99). However, capillary suction time tests were more strongly influenced by filtrate viscosity at high polymer doses. The immobilization cell allowed quantified amounts of shear to be imposed during dewatering, with greater shearing found to provide more rapid immobilization. This finding is consistent with the design of belt filtration dewatering devices, but demonstrates that current models do not account for a critical aspect of this process.