The subject of the invention is a process for treating lignocellulosic biomass comprising the following steps: a) conditioning the lignocellulosic biomass, b) washing said particles, c) separating the aqueous solution from the washed biomass particles, d) impregnating said lignocellulosic substrate with an acid liquor, e) carrying out a solid/liquid separation of the impregnated lignocellulosic substrate, f) pretreating said substrate from step e) by cooking, g) carrying out an enzymatic hydrolysis of the pretreated lignocellulosic substrate, h) carrying out a fermentation of the hydrolyzate from step g), and introducing the used aqueous washing solution into a step of said biomass treatment process which is after the pretreatment step f), and/or into an enzyme production step and/or into a step of producing/propagating the microorganisms necessary for steps g) or h).