liquid slurry such as black liquor preferably the acidifying is performed by adding so 2 g organic acids hcl hno 3 carbon dioxide or sulphuric acid in the form of fresh sulfuric acid or a so called u201c spent acid u201d from a chlorine dioxide generator or mixtures thereof to said liquid slurry preferably black liquor most preferred by adding carbon dioxide or sulphuric acid especially preferred is the use of carbon dioxide when acidifying in step c above it is intended throughout the present description that the expression u201c dewatering u201d embraces any means for dewatering preferably the dewatering is performed by using centrifugation a filter press apparatus a band filter a rotary filter such as a drum filter or a sedimentation tank or similar equipment most preferred a filter press apparatus is used according to a preferred embodiment of the first aspect of the invention the alcohol in step c is methanol ethanol or a mixture thereof preferably methanol according to a preferred embodiment of the first aspect of the invention the membrane separation of step b is achieved by using ultrafiltration preferably by using a membrane with a cut off of from 1000 to 50000 da according to a preferred embodiment of the first aspect of the invention the polymeric pentose containing liquid slurry is a black liquor provided after alkaline delignification preferably a black liquor resulting from the.