Deswelling and flocculation of gel networks: application to sludge dewatering

Organic sludge dewatering, chemically induced by a complexation mechanism with oppositely charged reagents, was investigated through chemical modelization using low charged density anionic gels. Flocculation, which allows an easy removal of interparticular water by filtration, proceeds through specific surface interactions. Initially negatively charged, an aggregation of the porous beads can be achieved by complexation with oppositely charged polycations, either around full complexation (global balance of original negative sites on the gel by positive charges added) with low molecular weight species (LaCl3 or cationic polymers of low molecular weight) or at low complexation extent when specific surface interactions are favoured with high molecular weight species. On the other hand, the collapsed state of the particles can be reached only at full complexation of the anionic charges whatever is the size of the polycation. This complexation study, based upon electrostatic interactions, was transposed to organic colloids contained in wastewater sludge after characterization of their charge content. The results obtained with a set of municipal sludges, issued from different steps of the wastewater treatment, evidence that flocculation proceeds through a similar mechanism to the one depicted with the anionic network model.