Abstract: Herein disclosed is a method comprising a) fermenting biomass to produce a fermentation broth; b) separating the fermentation broth into a liquid stream and a solid or slurry stream; c) passing the liquid stream through a reverse osmosis membrane to obtain a permeate and a retentate; and d) concentrating the retentate. Herein disclosed is a method comprising a) fermenting biomass in a first fermentor to produce a first fermentation broth; b) separating the first fermentation broth into a first liquid stream and a first solid or slurry stream; c) introducing the first solid or slurry stream into a second fermentor to produce a second fermentation broth, wherein the second fermentor comprises a lower fermentation products concentration than the first fermentor; d) separating the second fermentation broth into a second liquid stream and a second solid or slurry stream; and e) passing the second liquid stream through a reverse osmosis membrane.