A continuous process for treating a ligno-cellulosic raw biomass stream, comprising water insoluble contaminants and a ligno-cellulosic biomass comprised of a ligno-cellulosic component, non-ligno-cellulosic water soluble compounds and non-ligno-cellulosic water insoluble compounds, said process comprising the steps of: a. introducing the ligno-cellulosic raw biomass stream into a separation pool, containing an extraction solution comprising water and dissolved water soluble species and having an extraction solution density, wherein at least a portion of the extraction solution derives from the extraction of a portion of ligno-cellulosic biomass previously treated and contains dissolved water soluble species derived from the non-ligno-cellulosic water soluble compounds of the previously treated ligno-cellulosic biomass; b. separating the water insoluble components according to their apparent mass densities to create at least a heavy stream comprising at least a portion of the water insoluble contaminants and a light stream comprising at least a portion of the ligno-cellulosic component, wherein the apparent density of the water insolubles in the heavy stream is greater than the extraction solution density and the apparent density of the water insolubles in the light stream is lower than the extraction solution density; and c. removing the light stream from the separation pool, wherein the light stream is removed from the separation pool by a mechanical removal system extending to an upper position of the separation pool with respect to the gravity, said mechanical removal system comprising a conveyor belt which extracts the light stream from an outlet zone of the separation pool and drains a dirty liquid stream while lifting the light stream to the upper position, wherein the dirty liquid stream comprises water and at least a portion of the non-ligno-cellulosic water soluble compounds. a. introducing the ligno-cellulosic raw biomass stream into a separation pool, containing an extraction solution comprising water and dissolved water soluble species and having an extraction solution density, wherein at least a portion of the extraction solution derives from the extraction of a portion of ligno-cellulosic biomass previously treated and contains dissolved water soluble species derived from the non-ligno-cellulosic water soluble compounds of the previously treated ligno-cellulosic biomass; b. separating the water insoluble components according to their apparent mass densities to create at least a heavy stream comprising at least a portion of the water insoluble contaminants and a light stream comprising at least a portion of the ligno-cellulosic component, wherein the apparent density of the water insolubles in the heavy stream is greater than the extraction solution density and the apparent density of the water insolubles in the light stream is lower than the extraction solution density; and c. removing the light stream from the separation pool, wherein the light stream is removed from the separation pool by a mechanical removal system extending to an upper position of the separation pool with respect to the gravity, said mechanical removal system comprising a conveyor belt which extracts the light stream from an outlet zone of the separation pool and drains a dirty liquid stream while lifting the light stream to the upper position, wherein the dirty liquid stream comprises water and at least a portion of the non-ligno-cellulosic water soluble compounds.