NOVELTY - Treating a lignocellulosic biomass feed stream comprising solids, C5's, C6's, lignin, and water, comprises: (A) pretreating the lignocellulosic biomass feed stream; (B) separating at least a portion of the lignocellulosic biomass liquid from the pre-treated lignocellulosic biomass feed stream; (C) separating at least a portion of the suspended solids from the pre-treatment lignocellulosic biomass liquid using filters and/or centrifuge to create a clarified liquid stream; and (D) nanofiltering at least a portion of the clarified liquid stream to create a nanofiltered permeate stream.    USE - The process is useful for treating a lignocellulosic biomass feed stream comprising solids, C5's, C6's, lignin, and water.    DETAILED DESCRIPTION - Treating a lignocellulosic biomass feed stream comprising solids, C5's, C6's, lignin, and water, comprises: (A) pretreating the lignocellulosic biomass feed stream by contacting the lignocellulosic biomass with water in the temperature of 40-210 degrees C to create a pre-treated lignocelluosic biomass comprised of a pre-treatment lignocellulosic biomass liquid comprised of suspended solids, C5's, C6's, and acetic acid, where the ratio of the C6's to C5's is less than 0.8-1, and a pretreated lignocellulosic biomass solids; (B) separating at least a portion of the pre-treatment lignocellulosic biomass liquid from the pre-treated lignocellulosic biomass feed stream; (C) separating at least a portion of the suspended solids from the pre-treatment lignocellulosic biomass liquid using filters, and/or centrifuge, to create a clarified liquid stream; and (D) nanofiltering at least a portion of the clarified liquid stream to create a nanofiltered permeate stream comprising acetic acid and water and a nano filtered retentate stream comprised of C5's, C6's, acetic acid and water, where the ratio of acetic acid to the total amount of C5's and C6's in the clarified liquid stream is greater than the ratio of the acetic acid to the total amount of C5's and C6's in the nano-filtered retentate.