NOVELTY - Pretreating a biomass comprises: (a) allowing an effective amount of a liquid to pass through the biomass, the biomass being held in a vessel, where at least one component of the biomass dissolves in the liquid and exits the vessel in the form of an effluent; (b) allowing the effluent to pass through a filter, where more than 50% by weight of the at least one component in the effluent is retained by the filter as a retentate while the rest of the effluent passes through the filter forming a filtrate; and (c) allowing the filtrate to flow back and pass through the biomass.    USE - The method or system is useful for pretreating biomass (claimed).    ADVANTAGE - The invention provides new and improved biomass conversion system using high-temperature flow-though pretreatment and a nanoporous membrane to provide more digestible biomass for subsequent conversion to biofuels.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are:    (1) a system for pretreating a material comprising: (a) a vessel for holding the material, the vessel having an inlet and an outlet, where the inlet allows for the infusion of a liquid into the vessel, and the outlet allows for the liquid to exit the vessel forming an effluent; (b) a filtering means operably connected to the outlet of the vessel, where the filtering means is configured to retain at least one component in the effluent while allowing a portion of the effluent to pass through the filtering means forming a filtrate; and (c) a conveying means for transporting the filtrate to the inlet of the vessel; and    (2) a method for pretreating a biomass, comprising:(a) loading the biomass into the system; (b) allowing an effective amount of a liquid to pass through the biomass, where at least one component of the biomass dissolves at least partially in the liquid and exits the vessel in the form of an effluent; (c) allowing the effluent to pass through the filtering means, where more than 50% by weight of the at least one component in the effluent is retained by the filtering means as a retentate while the rest of the effluent passes through the filtering means forming a filtrate; and (d) allowing the filtrate to flow back and pass through the biomass in the vessel.