NOVELTY - The method comprises discharging a biomass slurry from a pretreatment reactor vessel (12) to a biomass slurry retention device (22), adding a recovered wash liquid (18) to dilute the biomass slurry in the retention device, discharging the diluted biomass slurry from the retention device to a drainer device, separating wash liquid with dissolved solids from the diluted biomass slurry in the biomass slurry drainer device and then discharging a concentrated biomass slurry from the biomass slurry drainer device, and recovering the wash liquid from the diluted biomass slurry.    USE - The method is useful for washing and removing dissolved solids from biomass such as: lignocellulosic material including wood chips and sawdust; fibrous plants; agricultural residues including stalks, stover and hulls; straws and grasses or forest; and sawmill residues, and for extracting five carbon sugars and six carbon sugars that are useful for producing bio-fuel and chemicals.    ADVANTAGE - The method is capable of efficiently washing and removing dissolved solids from biomass with improved diffusion of dissolved solids without adding heat or steam in a processing vessel.    DETAILED DESCRIPTION - The method comprises discharging a biomass slurry from a pretreatment reactor vessel (12) to a biomass slurry retention device (22), adding a recovered wash liquid (18) to dilute the biomass slurry in the retention device, discharging the diluted biomass slurry from the retention device to a drainer device, separating wash liquid with dissolved solids from the diluted biomass slurry in the biomass slurry drainer device and then discharging a concentrated biomass slurry from the biomass slurry drainer device, recovering the wash liquid from the diluted biomass slurry in the biomass slurry drainer device and transferring the recovered wash liquid to the retention device, discharging a portion of the recovered wash liquid to a hydrolysate recovery device (32), introducing a catalyst or a solvent to the biomass slurry, introducing the concentrated biomass slurry from the biomass slurry drainer device to a second biomass slurry retention device (34), adding the wash liquid recovered from a second biomass slurry drainer upstream of the second biomass slurry retention device to dilute the concentrated biomass slurry in the second biomass slurry retention device, discharging the diluted biomass slurry from the second biomass slurry retention device to the second biomass slurry drainer device, separating wash liquid with dissolved solids from the diluted biomass slurry in the second biomass slurry drainer device and then discharging a concentrated biomass slurry from the second biomass slurry drainer device, recovering the wash liquid from the diluted biomass slurry in the second drainer device and transferring the recovered wash liquid to the second retention device, discharging the concentrated biomass slurry from the first drainer device and/or the second drainer device to a high compression extruder, compressing the concentrated biomass slurry in the high compression extruder to concentrate the biomass slurry and then recovering the wash liquid from the high compression extruder, adding heat energy to or maintaining an elevated pressure on the biomass slurry, while the biomass slurry is placed in the retention device or in the drainer device, and transferring the concentrated biomass slurry or the further concentrated biomass slurry to a fermentation unit in which six carbon sugars in the biomass ferment. The recovered wash liquid is extracted from the biomass slurry drainer device upstream of the retention device. The pretreatment vessel is a pre-hydrolysis reactor vessel in which the biomass slurry undergoes hydrolysis. The recovered wash liquid includes hemi-cellulosic material dissolved from the biomass slurry. The retention device is a pressurized tank. The drainer device is an inclined drainer. The inclined drainer includes a screw and an auger, a lower inlet for the biomass slurry, an upper outlet for the biomass slurry, and/or a lower liquid outlet. A portion of the recovered wash liquid is introduced to a lower portion of the prehydrolysis reactor vessel. INDEPENDENT CLAIMS are included for:    (1) a system for washing processed biomass and removing dissolved solids from the biomass; and    (2) a system to process biomass.    DESCRIPTION OF DRAWING(S) - The figure shows a flow chart of a method of extracting hemi cellulosic components from biomass and reducing the dissolved solids content of biomass.    Pretreatment vessel (12)    Recovered wash liquid (18)    First retention device (22)    Hydrolysate device (32)    Second retention device. (34)