Résumé
Solid and liquid phase separation of sewage sludge digestate may be improved by addition of coagulation-flocculation reagents. Currently there is a rising interest for natural biopolymers made from renewable resources such as chitosan. If chitosan use is limited to acidic conditions allowing its solubility, its functionalization, by grafting for example a cationic charge (quaternary ammonium), may extend its application. In this study, chitosans, with different origin and molar weight, are functionalized at different quaternization degrees (glycidyltrimethylammonium chloride use) and tested as flocculents of two sewage sludge digestates. The effects of functionalized-chitosan addition on physico-chemical properties of digestates, as well as on their filterability and settleability, is assessed for different quaternization degrees, origins, molar weights and doses, and compared with the ones obtained with two usual commercial flocculents. The particle size distribution and rheological behavior of the digestate are focused as main physico-chemical parameters. Functionalized-chitosan addition results (i) in a monomodal distribution, different from the one obtained with commercial flocculents and (ii) in a decrease of the digestate viscosity, contrary to the commercial flocculents addition. Regarding solid and liquid phase separation, functionalized-chitosan addition results in an efficient phase separation close to the ones obtained with usual commercial flocculents. Separation efficiency is mainly correlated with the dose rather than with the quaternization degree, and an optimum is reached for a given molar weight and origin.