A method of enhancing the concentration of a first inorganic compound in a first aqueous solution of a first process of a heavy chemical plant, the method comprising (a) feeding the first solution having the first compound at a first concentration and a first water vapor pressure to an osmotic membrane distillation means comprising a hydrophobic, gas and water vapor permeable membrane separating (i) a first chamber for receiving the first solution, from (ii) a second chamber for receiving a receiver feed aqueous solution having a second water vapor pressure lower than the first water vapor pressure; (b) feeding the receiver aqueous feed solution to the second chamber as to effect transfer of water vapor through the membrane from the first chamber to the second chamber, and to produce (i) a resultant first solution having a second concentration of the first compound greater than the first concentration and (ii) a diluted receiver feed aqueous solution; and (c) collecting the resultant first solution. The apparatus and method are of particular value in electrolytic haloalkali production plants in offering reduced capital and operating re-concentration and dilution costs.