A process for operating a cooling tower (10) comprising: introducing a feed solution into a cooling tower, evaporating solvent from the feed solution to produce a concentrated solution, removing a portion of the concentrated solution as blow-down, passing a portion of the blow-down through a first membrane, such that some of the solute in the blow-down are retained as a concentrated blow-down solution on the retentate-side of the first membrane, contacting a portion of the concentrated blow-down solution (42) with one side of a second membrane (12) and contacting a solution having a lower solute concentration than the combined concentrated blow-down solution and the return from the cooling tower with the opposite side of the second membrane, such that solvent flows across the second membrane to dilute the concentrated blow-down solution by direct osmosis, and re-introducing the diluted blow-down solution to the cooling tower.