Abstract
Phase separation processes of digestate from anaerobic digestion of sewage sludge still need to be optimized to ensure a noble valorization of the liquid and solid phases obtained. Adding coagulationflocculation reagents, notably cationic polyacrylamides, is a relevant tool to this optimization. Currently there is a rising interest for natural biopolymers made from renewable resources and being environmentally-friendly. Chitosan is one of the world’s most abundant biopolymers, and has been the subject of several studies in coagulation-flocculation of wastewater and digestate. However, its application is limited to acidic conditions since it is not soluble at pH higher than 6. In order to improve its solubility on the whole pH range, it is possible to functionalize it with glycidyltriméthylammonium chloride to graft a cationic charge (quaternary ammonium). During this study, three chitosans functionalized at different quaternization degrees (DQ) were synthesized and tested on a digestate obtained from wastewater treatment plant and compared to an usual commercial flocculant. The effects of the chitosan DQ were assessed on physico-chemical properties of the digestate and its rheological behaviour, as well as its settleability and filterability. Tests have shown that the addition of quaternized chitosan has low impact on pH and electrical conductivity of digestate, and decreases its viscosity contrarily to the commercial polymer. Regarding digestate phase separation, flocculants improve settleability and lowers specific resistance to filtration. Whatever the parameters studied, it seems that the modified chitosan efficiency improves with the increase of DQ and applied dosage.