Membrane treatment by nanofiltration of exhausted tannin liquors of the leather industry

A rational use of water in the leather industry is described by applying the criteria of "clean technologies" by means of membrane processes. A nanofiltration (NF) process was used for recovering tannins and water from exhausted baths and reusing them as tanning agents and washings. Results of experiments performed on laboratory and semi-industrial pilot scale plants, operating and fluid-dynamic conditions and mass balance of the nanofiltration process are reported and discussed. Chemical and physical analytical values measured on skins treated with the recovered solutions were very similar to those measured on control skins tanned with standard solutions. According to the obtained results a process scheme is suggested that starting from exhausted tanning baths and by using a nanofiltration membrane purification/concentration, permits to recover water and to increase the tannin/non tannin (T/NT) ratio in the retentate solution. The final volume reduction factor (VRF) of tested exhausted baths was 5.25, the increase of T/NT ratio was from 1.1 to 2.3 and the average permeate flux was 12.5 l/m2h. Advantages are in terms of: reduction of environmental impact (up to 75% of COD reduction of the global effluent), simplification of cleaning-up processes of wastewaters, decrease of disposal costs, saving of chemicals and water.