Differences in rheological and fractal properties of conditioned and raw sewage sludge

Rheological tests for raw and conditioned activated sludge (AS) or anaerobic digested sludge (ADS) show that power-law relationships can be used to describe the evolution of several rheological parameters, i.e., limiting viscosity (η∞), yield stress (τy), cohesion energy of the sludge network (Ec), and storage modulus (G'), with total suspended solid (TSS) content in raw and conditioned sludge. A gel-like structure that behaves similar to weak-link flocs/aggregates was observed in AS and ADS. As derived from the double-logarithmic plots of G'-TSS content, the mass fractal dimensions of the raw and conditioned AS or ADS flocs/aggregates were 2.70 and 2.53 or 2.85 and 2.79, respectively. The rheological tests also indicate that both polymer conditioning and increased TSS content led to improved elastic behavior, cohesion energy, and yield stress of the sludge network, as well as expanded the corresponding linear viscoelastic range. The porosity of AS or ADS flocs/aggregates will be improved by polymer conditioning.