Improved Characterization of Mixing for Sludge Conditioning

The dewatering characteristics of polymer conditioned sludges are examined for sludge-conditioning regimes effected by agitation in a baffled, cyclindrical mixing vessel using different impellers operated at constant values of $\overline{G}$ and constant mixing time. Dewatering characteristics are evaluated by different laboratory tests, characterizing sludge filterability, settleability, and drainability. Only one type of slude and polymer was applied for the conditioning experiments. The experimental results suggest that $\overline{G}$ is deficient as the only descriptor of the impact of turbulent mixing intensity on sludge-conditioning performance. Based on the results, it is questioned whether laboratory-scale sludge conditioning performed by impeller agitation in a baffled, cylindrical mixing vessel is comparable to the pipe-flow conditioning taking place at many full-scale sludge-dewatering installations. It is suggested that a more appropriate characterization of mixing regimes for sludge conditioning should take into account how the applied power for mixing is converted into steady flow and turbulent shear, respectively, and which of these conditions is more important at a particular mixing time and polymer dosage. The three parameters: (1) Mixing intensity; (2) mixing time; and (3) polymer dosage need to all be properly accounted for in order to evaluate sludge-conditioning performance.