Saving of water and chemicals in the tanning industry by membrane processes

Some results on pressure-driven membrane operations able to improve chromium recovery from spent chromium tanning baths and to desalinate water discharged from filterpress after Cr(III) precipitation are reported. Nanofiltration was employed to concentrate chromium(III) using a spiral-wound module. Chromium concentration and COD in permeate were respectively about zero and 2-3 g/l as compared to initial feed values of 2.8 and 5.2 g/l; the high chloride concentration in the permeate suggested to reuse this solution in the pickling step saving fresh water and salts. The concentrate solution presented a higher chromium purification with respect to initial feed. It was tested in retanning, and after further concentration by chemical precipitation-dissolution method, in tanning operations. Chemical and physical analyses on leather treated with these solutions showed similar characteristics with respect to use of traditional chromium solution. Reverse osmosis was employed to reduce the high salt content in waste water from filter pressing of chromium hydroxide panels. Good rejection of reverse osmosis membrane to chloride and sulphate ions suggested the use of this operation for obtaining water, to be used in washings, or to sent to biological treatment of final waste water.