Alright.

Desalination of water via hollow wound fiber membranes, reverse osmosis.

You need shittonnes of pressure. I am also unware of the solvent activity of butanol in water. This is a considerable problem, if the solvent activity is too low, nothing will flow. This would actually be a difficult problem to solve, because it is typically designing the polymer membranes which is difficult. You would need to select for a polymer membrane which was specific for butanol and not ionic molecules. (or specific for water and not butanol, then you collect the rejection). Efficiency will be high, but prefiltration will be needed, 1 for dirt, 1 for bugs, 1 for smaller particles.

Even larger problems will occur if butanol does not dissolve in solution, the micelles will not want to go through any of the above filters, so you would have to mess with the water chemistry at that stage.

You would probably be better off using gaseous stripping, collect water at some location, bubble significant volumes of air through it, cool that air, reheat the exit water with the exchange heat, collect the resulting butanol.

toxicity is not a problem so long as you can keep the concentration low. However the pressures on each side of gas / liquid interface are proportional, I have seen studies about alcohol distillation noting some 30% of the produced ethanol in batch was lost by gaseous stripping. The extracted ethanol was then refluxed into the original mix.

G