A process of separating suspended solids from a fermentation liquor by subjecting the liquor to a solids-liquid separation stage, wherein the fermentation liquor is produced in a fermentation process for the production of a fermentation product, and which liquor comprises lignin, wherein the solids-liquid separation stage is assisted by a treatment system, characterized in that the treatment system comprises an anionic polymer, with the proviso that the treatment system and does not include a cationic polymer having an intrinsic viscosity (IV) of at least 4 dl/g.