Méthodologie de choix de polymères biosourcés pour la coagulation-floculation de l’eau et des boues

Wastewater treatment plants rely on large amounts of non-biodegradable chemicals to agglomerate and increase the density of matter to facilitate its separation from water. These chemicals can then be discharged into river waters and/or in sludge that ultimately returns to the ground. The challenge in the coming years is to reduce the environmental footprint of wastewater treatment plants by the complete or partial replacement of these reagents with bio-sourced polymers. Bio-sourced polymers have shown promising results for this type of application, but only a few of them have gone beyond the laboratory scale to be deployed at the scale of the wastewater treatment plant. This article illustrates the approach undertaken within the framework of the BIOFLOCS project funded by the Institut Carnot Eau et Environnement to select bio-sourced products to treat one or more types of matrices (wastewater, sludge). This multi-scale approach involves an inventory of locally sourced products available on an industrial scale, a detailed characterization of the biopolymers and flocs/agglomerates formed with the selected matrices, an investigation of the formation of flocs/agglomerates and their separation from water and tests on a pilot scale to be more representative of the constraints they will undergo in industrial equipment. Biopolymers based on tannin, starch and chitosan were particularly studied. The study confirms industrial feedback of lower performance in water clarification and solid dehydration for biopolymers compared to polyacrylamide, even at doses up to 10 times higher due to their lower molecular weight and the structure of the flocs formed. However, our results demonstrate that partial replacement of petro-sourced polymers by coupling with them with biopolymers (replacing metal salts) can significantly reduce their dose.