NOVELTY - A detrimental species-containing cellulosic biomass solid is introduced from an inlet into a treatment vessel, and an acidic solution having a pH value of 4 and an aqueous solution having a pH value of 5 or more are introduced, to produce washed cellulosic biomass solid. A removal stream comprising a gas stream or an organic solvent solution is then introduced, to produce a pretreated cellulosic biomass solid. The pretreated solid is discharged from an outlet, and at least a portion of the discharged solid is transferred to a digestion and/or reaction zone.    USE - Method for selective removal of detrimental metals and their anion species from cellulosic biomass solid (claimed).    ADVANTAGE - The method enables selective removal of detrimental metals and their anion species from cellulosic biomass solid without generation of waste products by simple process.    DETAILED DESCRIPTION - A detrimental species-containing cellulosic biomass solid is provided (step a). The detrimental species-containing cellulosic biomass solid is introduced from an inlet into a treatment vessel, where the cellulosic biomass solid has a true interstitial volume in the vessel (step b). An acidic solution having a pH value of 4 is then introduced in an amount to fill 5-40% bed volume of true interstitial volume, from the inlet into cellulosic biomass solid-containing vessel at 0-60 degrees C (step c). An aqueous solution having a pH value of 5 or more is introduced from the inlet into acidic cellulosic biomass solid-containing vessel, to produce washed cellulosic biomass solid having reduced detrimental species content compared to the detrimental species-containing cellulosic biomass solid and an acidic water effluent, where the content of acidic water effluent is 3 parts with respect to 0.5-1 part detrimental species-containing cellulosic biomass solid (dry basis) (step d). A removal stream comprising a gas stream or an organic solvent solution is then introduced from an inlet (b) into the vessel containing the washed cellulosic biomass solid, to produce a pretreated cellulosic biomass solid with reduced water content compared to the water-washed cellulosic biomass solid and aqueous effluent (step e). The pretreated cellulosic biomass solid is discharged from an outlet and at least a portion of the discharged pretreated biomass solid is transferred to a digestion and/or reaction zone (step f). At least a portion (a1) of the aqueous effluent is recycled to produce acidic solution (step g), and at least a portion (a2) of the aqueous effluent is recycled as an aqueous solution (step h).