Deswelling and flocculation of gel networks: Application to sludge dewatering

Organic sludge dewatering, chemically induced by a complexation mechanism w ith oppositely charged reagents, was investigated through chemical modeliza tion using low charged density anionic gels. Flocculation, which allows an easy removal of interparticular water by filtration, proceeds through speci fic surface interactions. Initially negatively charged, an aggregation of t he porous beads can be achieved by complexation with oppositely charged pol ycations, either around full complexation (global balance of original negat ive sites on the gel by positive charges added) with low molecular weight s pecies (LaCl3 or cationic polymers of low molecular weight) or at low compl exation extent when specific surface interactions are favoured with high mo lecular weight species. On the other hand, the collapsed state of the parti cles can be reached only at full complexation of the anionic charges whatev er is the size of the polycation. This complexation study, based upon elect rostatic interactions, was transposed to organic colloids contained in wast ewater sludge after characterization of their charge content. The results o btained with a set of municipal sludges, issued from different steps of the wastewater treatment, evidence that flocculation proceeds through a simila r mechanism to the one depicted with the anionic network model. (C) 1998 El sevier Science Ltd. All rights reserved.