Design and performance of the two-stage/two-pass RO membrane system for chromium removal from tannery wastewater. Part 3

In the chromium tanning process leather consumes 60-80% of the applied chromium salt and the rest is usually discharged into sewage without adequate treatment, causing serious environmental impact. Chemical analysis of the spent chromium effluents from two local medium-size tannery shops indicated that the chromium content ranged from 1300-2500 mg Cr6+/l, while NaCl concentration varied from 40,000-50,000 mg/l. The laboratory investigation and the bench-scale study of the physical and chemical pretreatment of the spent liquors were published earlier as Part 1. This was followed by the study of the reverse osmosis (RO) membrane separation of chromium from the pretreated solution in a pilot scale investigation, that proved its applicability for small and medium tanning workshops, published separately as Part 2. Based on the earlier pilot plant experimental results, the performance of the low pressure reverse osmosis membrane (LPROM) was investigated in this work. The flux production for such a membrane was mathematicaly modeled. The present work focuses on the design of a full-scale RO membrane treatment plant capable of handling the chromium-rich wastewater economically. It was found that the two-stage/ two-pass RO system is necessary to recover the spent chromium effectively in order to comply with the local national limits (<5 mg Cr6+/l) for discharging into the public sewarage system. The relevant assessment of technical aspects is discussed in detail and economically evaluated.