NOVELTY - Controlling the total solids contents of biomass discharged from a plug screw feeder into a pretreatment digester, comprises (a) measuring the flow rate of biomass, (b) measuring the total solids content of biomass, (c) measuring the flow rate of a effluent slurry, (d) calculating the total solids content of the biomass discharged from the plug screw feeder into the pretreatment digester, and (e) adjusting e.g. at least one of the number of open drain openings in the plug screw feeder throat section.    USE - The method is useful for controlling the total solids contents of biomass, producing a pretreated biomass feedstock (all claimed) and producing ethanol, which is useful as an additive or replacement for petroleum-based fuels such as gasoline.    ADVANTAGE - The method is capable of uniformly controlling the total solids contents of biomass and producing pretreated biomass with high yield, cellulose digestibility and solubilization of hemicellulose and reduced degradation of solubilized xylose to furfural/condensation of xylose with lignin to form pseudo-lignin, which does not interfere with enzyme accessibility to cellulose to downstream enzymatic hydrolysis.    DETAILED DESCRIPTION - Controlling the total solids contents of biomass discharged from a plug screw feeder into a pretreatment digester, where the plug screw feeder comprises a throat section having a number of drain openings, the digester is connected to a damper, which contacts the biomass material as it enters the digester, and the damper is driven forward by a piston, which exerts a force on the damper to create a back pressure against the biomass, comprises (a) measuring the flow rate of biomass introduced into the plug screw feeder, (b) measuring the total solids content of biomass introduced into the plug screw feeder, (c) measuring the flow rate of a effluent slurry discharged from the plug screw feeder, (d) calculating the total solids content of the biomass discharged from the plug screw feeder into the pretreatment digester based on the flow rate and total solids content of biomass introduced into the plug screw feeder and the flow rate of the effluent slurry discharged from the plug screw feeder, and (e) adjusting at least one of the number of open drain openings in the plug screw feeder throat section, the force exerted on the damper by the piston, the rotational speed of the plug screw feeder and the total solids content of biomass introduced into the plug screw feeder based on the calculated total solids content of the biomass discharged from the plug screw feeder. An INDEPENDENT CLAIM is included for a method-I for producing a pretreated biomass feedstock, comprising: contacting a biomass feedstock with an aqueous acid solution to form an acid-impregnated biomass stream; dewatering the acid-impregnated biomass stream to produce a effluent slurry and dewatered biomass; controlling a concentration of acid in the effluent slurry; introducing the dewatered biomass and steam into a pretreatment digester; controlling a total solids content of dewatered biomass introduced into the pretreatment digester; controlling the temperature of the pretreatment digester; and controlling an average residence time of biomass in the pretreatment digester.