Pickling wastewater reclamation by means of nanofiltration

One of the main environmental problems of tannery industries is due to the liquid effluents generated in their processes. It contains high amounts of organic matter, sulphates and chlorides. In fact, the wastewater stream that comes from the pickling process can reach nearly 75 mS/cm of conductivity. The present work is focused on the reclamation of pickling wastewater after applying a nanofiltration (NF) process. The retentate stream, with a high sulphates concentration, can be reused in pickling baths whereas the permeate stream, with a high chlorides concentration, can be pumped to the soaking drums. In this work a detailed study of the use of the NF permeate stream in the soaking process and its effect on the quality of the final leather is performed. At the same time, a reduction of the salt content in the final tannery effluent is accomplished. NF experiments with pickling wastewater were performed in a laboratory plant equipped with a pressure vessel that contains one spiral wound membrane element (Desal-5 DK from GE-OSMONICS) with an effective membrane area of 2.51 m2. The operating conditions of the experiments were a transmembrane pressure of 15 bar, 400 L h-1 of feed flow rate and a temperature of 25°C. It was studied the influence of the feed pH on the membrane performance (flux and salts rejection). The results showed that the maximum permeate flux was obtained at wastewater pH. The measured quality parameters of the final leathers soaked with NF permeates demonstrated that leathers did maintain the necessary quality for the final product.