It is described herein a continuous process for soaking a ligno-cellulosic biomass stream in an extraction solution comprising water and dissolved water soluble species derived from a previously treated ligno-cellulosic biomass. In the process, water insoluble contaminants are separated according to their apparent mass densities. The ligno-cellulosic biomass stream may be further subjected to a second optional soaking step in a counter flow configuration. The process is useful to remove non-ligno-cellulosic water soluble compounds from the ligno-cellulosic biomass with a low consumption of water.