Desalination From Oily And High-salt Wastewater By Vacuum Membrane Distillation And The Improvement Of Membrane Performance

In recent years,water pollution,as one of the world’s top ten environmental problems,has become a major concern of people,among which the treatment of oil-bearing and high-salt wastewater from a wide range of sources has become the top priority.Membrane distillation has a promising application prospect in the treatment of oily wastewater with high salinity.Because it has the advantages of simple operation,low energy consumption,and is not limited by the concentration of the solution.At present,it is difficult for the membrane materials on the market to meet the requirements of anti-pollution performance and anti-wetting performance during the membrane distillation process.Therefore,the preparation of suitable membrane materials has become the key problem to solve the problem of oil-containing and high-salt wastewater treatment.In this paper,the PVDF membrane was modified to enhance the hydrophobic performance,anti-fouling performance and anti-wetting performance of the membrane.The effects of the properties of different membranes and different operating conditions on membrane flux,inorganic salt rejection rate and oil rejection of the membranes in the vacuum membrane distillation（VMD）process were investigated.In this paper,PVDF membrane was prepared and characterized firstly,the vacuum membrane distillation desalination experiment of oil-bearing and high-salt wastewater was carried out.After continuous 10 h VMD experiment,the rejection rate of PVDF membrane for inorganic salts was more than 99%,and the rejection rate for oily organic compounds was88%.The maximum membrane flux recovered of PVDF membrane was 78%,and the rejection rate of oil-bearing organic compounds was 85%.Therefore,the hydrophobic modification of PVDF membrane is needed to further improve the hydrophobic performance and anti-pollution performance of PVDF membrane.Secondly,The hydrophobic properties and anti-pollution properties of PVDF membrane were further improved by hydrophobic modification.The hydrophobic materials graphene and carbon nanospheres were added into the PVDF membrane to prepare G/PVDF and C/PVDF membrane.The results show that the contact angles of G/PVDF membrane and C/PVDF membrane increase from 92.70°to 107.77°and 108.11°,respectively.The maximum membrane flux of G/PVDF membrane increases from 14.7 kg/m²·h to 15.2 kg/m²·h after 10hours continuous VMD experiment.The maximum membrane flux of C/PVDF membrane is15.6 kg/m²·h,and the rejection rate of inorganic salts is more than 99%,and the rejection rate of oil organic compounds is 89%and 89%,respectively.After membrane cleaning,the maximum membrane flux recovered of G/PVDF and C/PVDF membrane is more than 90%.The rejection rates for oily organic compounds are 88%and 88%.Finally,TiO₂/G/PVDF and TiO₂/C/PVDF films were prepared by adding nano-TiO₂with photocatalytic properties to improve the anti-fouling performance of the membrane.The results show that the contact angles of TiO₂/G/PVDF and TiO₂/C/PVDF membrane are120.07°and 128.72°under ultraviolet irradiation.The maximum membrane fluxes of TiO₂/G/PVDF and TiO₂/C/PVDF membranes can reach 25.1 kg/m²·h and 22.5 kg/m²·h respectively after 10 h VMD experiment,and the rejection rate of inorganic salts can reach more than 99%.After membrane cleaning,the membrane flux recovered is as high as 98%.The lowest rejection rate of oil organics was 91%and 89%,respectively.It shows good hydrophobic property,anti-pollution property and anti-wetting property for modified membrane.