Method for treating a lignocellulosic biomass

A process for treating lignocellulosic biomass, comprising:

• b) a step of acidic or neutral pretreatment of the biomass previously placed under acidic or neutral pH conditions in a pretreatment reactor, to produce an acidic or neutral pretreated biomass,
• the performance of said step b) alternating with
• b′) a step of basic pretreatment of the biomass previously placed under acidic, neutral or basic conditions, in the pretreatment reactor, with optional addition of base, to produce a basic pretreated biomass,
• followed by
• c) a step of enzymatic hydrolysis in a hydrolysis reactor (16) of the acidic or neutral pretreated biomass obtained at the end of step b) and/or of the basic pretreated biomass obtained at the end of step b′), to obtain a hydrolyzed biomass,
• d) a step of solid/liquid separation of the hydrolyzed biomass obtained at the end of step c) in the form of sugar(s) or a step of solid/liquid separation of the hydrolyzed biomass in the form of sugar(s) obtained at the end of step c) then treated in one or more other steps subsequent to the enzymatic hydrolysis step c), in order to obtain a separated juice and an unconverted solid residue,
• e) a step of recycling at least part of said unconverted solid residue obtained in step d) to step b′) of basic pretreatment.