Study On Multi-Stage Membrane Treatment And Inorganic Salt Recovery Of Tannery Chromium Removal Wastewater

The tanning industry consumes a lot of fresh water resources in the process of leather production.At the same time,a large amount of tanning wastewater also has an adverse impact on the environment.To solve this problem,this topic selects a leather company’s tanning and chromium removal wastewater in Fujian Province,deeply treats the wastewater through the coupling process of chemical flocculation and membrane separation technology,and recovers the available resources,so as to provide new process technology for the resource utilization of tanning and chromium removal wastewater.Firstly,the chrome removal wastewater from tanning was pretreated by chemical flocculation ultrafiltration,and the effects of different flocculants,addition amount,and p H on the flocculation effect were studied.The results show that the flocculation effect is the best when 1.0‰poly aluminum chloride（PAC）and PAM are used together under the condition of p H=8,and the removal rates of chromaticity,turbidity,and COD are 35%,64.6%,and 31.5%respectively.On this basis,four different types of ultrafiltration membranes were compared,and the effects of operating pressure and recovery on membrane flux and ultrafiltration effect were investigated.The optimized conditions are as follows:the retained molecular weight is 150 k Da,the operating pressure is 0.2 MPa,the concentration dilution ratio of produced water is 1:5,and the removal rates of turbidity,chroma,and COD of wastewater are 90.7%,28.7%,and20.8%respectively.Secondly,the inorganic salts contained in wastewater were deeply studied.The ultrafiltration produced water was concentrated by nanofiltration.The effects of operating pressure and feed temperature on membrane flux and concentration-effect were investigated.Through a multistage concentration process,90%of fresh water resources in wastewater can be reused.The experimental results show that the salt content of primary fresh water is 200-500 mg/L under the conditions of the operating pressure of 1.8 MPa and feed temperature of 25-30℃.When the operating pressure of secondary concentration is 3 MPa and the feed temperature is 25-30℃,the salt content of secondary fresh water is 800-1000 mg/L,which meets the reuse standard of the factory.TDS and COD of secondary concentrated wastewater are 44000-46000mg/L and 10000-13000 mg/L respectively.Finally,the inorganic salts in nanofiltration concentrated wastewater were recovered by electrodialysis.The experimental results show that a 5%Na₂SO₄solution is selected as the polar solution,and the limit current of electrodialysis is 4.0A.when the voltage is 25 V,the circulating flow is 600 L/h,the feed flow is 4 L/h,and the salt content in the concentration chamber reaches 150 g/L,the desalination rate after primary electrodialysis desalination treatment is 52.2%.Subsequently,by raising the temperature of the wastewater in the desalination chamber to 31℃,the wastewater was subjected to two-stage electrodialysis to remove the sulfate in the wastewater for the second time,and the total desalination rate reached 61.9%.Then,the reverse osmosis method is used to further recover the fresh water in the wastewater,and then the flocculation membrane separation coupling process is formed.The reuse rate of fresh water can reach 95%.This study provides a new reference basis and technical support for the advanced treatment of tanning chromium removal wastewater.