Desalination From Organic Wastewater Containing High Concentration Salt By Membrane Distillation And The Improvement Of Membrane Performance

Along with the rapid development of the industry,the large amount of industrial wastewater generated is of great harm to the natural environment.In order to solve the problem of high salinity organic wastewater treatment in industrial wastewater,researchers have developed many treatment methods.Membrane distillation is a separation method which has attracted much attention in recent years.This method has the advantages of no limition to the concentration of solution,low vacuum pressure and low temperature of solution treatment.However,the membrane on the market is difficult to meet the requirements of hydrophobicity and self-cleaning performance of the membrane distillation process,so the development of membrane materials suitable for membrane distillation applications has become the focus of this field.In this paper,polyvinylidene fluoride?PVDF?membranes were modified by nanographene?GR?and nanographene/titanium dioxide?GR/TiO₂?.The effects of membrane properties and operating conditions on membrane flux and retention rate during vacuum membrane distillation were studied.Firstly,the effects of operating conditions such as feed temperature,feed flow,system vacuum and feed concentration on membrane flux,temperature polarization and rejection were studied.Experimental results indicated that the increase of system vacuum and liquid temperature resulted in the increase of the transmembrane mass transfer driving force,and the membrane flux also increased.However,the temperature polarization phenomenon was aggravated.The increase of the flow rate of the feed liquid can reduce the thickness of the boundary layer on the surface of the membrane,increase the membrane flux during the distillation process of the vacuum membrane,and reduce the temperature polarization phenomenon.The concentration of salt in the feed solution increased,which led to serious membrane fouling and the decrease of membrane flux.The retention rate of salt in the whole vacuum membrane distillation process reached more than 99.95%.Secondly,the membrane was subjected to hydrophobic modification when grapheme was added into the casting solution.The effect of graphene content on the properties and structure of PVDF membranes was studied by characterizing the membranes.Studies have shown that the contact angle of the membrane surface increased with the increasing of the concentration.When the graphene concentration was 0.8wt%,the contact angle of the membrane surface reached 111.6°.The average pore size of the membrane increased with the increase of the graphene concentration,but the LEP value decreased with the increase of the graphene concentration.In the vacuum membrane distillation experiment,the membrane flux increased with the increase of graphene concentration.To ensure the purity of product water,the LEP value should be higher than 2 times of the system vacuum.Finally,on the basis of graphene-PVDF membrane,nano-TiO₂ particles were introduced by blending method,and the photocatalytic performance of TiO₂ was used to improve the self-cleaning ability of the membrane surface.The results showed that the addition of TiO₂ reduced the pore structure of the sponge and increased the pore size of the membrane.Although the surface roughness was increased,the contact angle of the membrane surface was lowered to 86.6°due to the hydrophilicity of TiO₂.In the vacuum membrane distillation experiment,the TiO₂-graphene PVDF membrane showed good anti-pollution ability.And the self-cleaning ability of the membrane can be further enhanced by ultraviolet assistance.Under the condition of ultraviolet assistance at wavelength of254nm as well as the induction of air injection,the membrane flux was up to 22.58kg/?m²·h?and can maintain a high flux for a long time.