A method for selective removal of chlorine and phosphorus from a cellulosic biomass solids comprising: a. providing a detrimental species-containing cellulosic biomass solids,, wherein the detrimental species comprises at least one of chlorine and phosphorus; b. introducing said detrimental species-containing cellulosic biomass solids from an inlet into a treatment vessel to provide a bed of detrimental species-containing cellulosic biomass solids in said vessel; wherein said bed of cellulosic biomass solids has an interstitial volume that is the amount of liquid added to just cover the top of said bed of cellulosic biomass solids when the biomass solids are saturated with liquid; c. introducing a base solution having a pH of greater than 9 from an inlet to said cellulosic biomass solids containing vessel at a temperature in the range of 0°C to 60°C in an amount to fill at least 5% to at most 40% of said interstitial volume; d. subsequently to step c, introducing an aqueous solution having a pH of at most 8 from an inlet to said basic cellulosic biomass solids-containing vessel, thereby producing washed cellulosic biomass solids having reduced detrimental species content compared to the detrimental species-containing cellulosic biomass solids and a basic water effluent, wherein said basic water effluent is in the range of 3 parts to 0.5 parts relative to 1 part of detrimental species-containing cellulosic biomass solids (dry basis); e. introducing a removal stream comprising a gas stream or an organic solvent solution from an inlet to the vessel containing the washed cellulosic biomass solids thereby producing a pretreated cellulosic biomass solids with reduced water content compared to the water washed cellulosic biomass solids and an aqueous effluent; f. discharging the pretreated cellulosic biomass solids from an outlet and transferring at least a portion of the discharged pretreated biomass solids to a digestion and/or reaction zone; g. recycling at least a first portion of the aqueous effluent to produce base solution; and h. recycling at least a second portion of the aqueous effluent as an aqueous solution. a. providing a detrimental species-containing cellulosic biomass solids,, wherein the detrimental species comprises at least one of chlorine and phosphorus; b. introducing said detrimental species-containing cellulosic biomass solids from an inlet into a treatment vessel to provide a bed of detrimental species-containing cellulosic biomass solids in said vessel; wherein said bed of cellulosic biomass solids has an interstitial volume that is the amount of liquid added to just cover the top of said bed of cellulosic biomass solids when the biomass solids are saturated with liquid; c. introducing a base solution having a pH of greater than 9 from an inlet to said cellulosic biomass solids containing vessel at a temperature in the range of 0°C to 60°C in an amount to fill at least 5% to at most 40% of said interstitial volume; d. subsequently to step c, introducing an aqueous solution having a pH of at most 8 from an inlet to said basic cellulosic biomass solids-containing vessel, thereby producing washed cellulosic biomass solids having reduced detrimental species content compared to the detrimental species-containing cellulosic biomass solids and a basic water effluent, wherein said basic water effluent is in the range of 3 parts to 0.5 parts relative to 1 part of detrimental species-containing cellulosic biomass solids (dry basis); e. introducing a removal stream comprising a gas stream or an organic solvent solution from an inlet to the vessel containing the washed cellulosic biomass solids thereby producing a pretreated cellulosic biomass solids with reduced water content compared to the water washed cellulosic biomass solids and an aqueous effluent; f. discharging the pretreated cellulosic biomass solids from an outlet and transferring at least a portion of the discharged pretreated biomass solids to a digestion and/or reaction zone; g. recycling at least a first portion of the aqueous effluent to produce base solution; and h. recycling at least a second portion of the aqueous effluent as an aqueous solution.