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, which liquor comprises water, lignin and BOD, wherein the solids liquid separation stage is assisted by a treatment system, characterised in that the treatment system comprises either, (i) a cationic polymer having an intrinsic viscosity (IV) of at least 4 dl/g at a dose of above 2 kg/tonne based on dry weight of suspension, or (ii) a cationic polymer having an intrinsic viscosity (IV) of at least 4 dl/g and, (iii) an anionic polymer, and/or (iv) a cationic polymer of intrinsic viscosity of below 4 dl/g and a cationic charge density of at least 3 meq/g and/or (v) inorganic coagulants and/or (vi) charged microparticulate material.