Experimental Investigation Of High Salinity Water Treatment By Membrane Distillation Crystallization

Membrane distillation(MD)is a thermal-membrane coupling separation technology.It has the advantage that the salt rejection ratio is not obviously restricted by the salt concentration of feed before the crystal fouling appears on the membrane interface.MD is considered an emerging approach that integrates high salinity water treatment and crystal separation.In the application of treating high salinity water based on MD,the nucleation and growth processes of crystal fouling behaviors on membrane interface are studied.These are the key to prevent interfacial crystal fouling and ensure the stable operation of MD system.The non-woven fibrous membranes were used as the interfacial barriers in this paper.The various salt concentrations of NaCl aqueous solution and working conditions were investigated.The performance analysis and failure analysis of MD system were carried out.The regeneration experiments were carried out on the membranes with interfacial crystal fouling by reducing the salt concentration of feed.The recovery mechanisms and performance of MD after regeneration were studied.The results showed that the crystal fouling of NaCl solution on the membrane interface will cover the membrane pores,reduces the effective area of membranes and the transmembrane flux of MD.Membrane failure will also occur when the salt concentrations and temperature of feed are high.The regeneration of membranes before membrane failure can restore the transmembrane flux of MD,and the restoration ratio is negatively correlated with the crystal growth degree.According to the morphology characterization of natural evaporation crystal fouling and crystal fouling during MD process of NaCl aqueous solution on the membrane interface,the crystal nucleation and growth mechanism of NaCl aqueous solution on the membrane interface were explored.The results showed that NaCl crystals firstly adhere to the fibers in the form of tiny grains,and then form large crystals on the membrane interface near the feed liquid,the growth processes are affected by the orientation of fibers.The fibers of the nonwoven fibrous membranes cross each other,and the crystals gather at the intersection of the fibers and form a block.The fiber orientation of quasi parallel fibrous membranes is regular,the crystals grow longer along the fibers,and wraps multiple fibers together in a bundle.The complex components high salinity water is also studied in this paper,different concentrations of simulated landfill leachate are configured according to the composition of RO concentrated landfill leachate.The MD experiments of simulated landfill leachate and the characterization experiment of crystal fouling on the membrane interface are carried out,and the experimental results are compared with those of NaCl aqueous solution.The results showed that the humic acid in the simulated landfill leachate will reduce the saturated vapor pressure of the solution.Under the same working conditions,the transmembrane flux of the simulated landfill leachate is less than that of NaCl aqueous solution.Some insoluble substances in humic acid will settle at the bottom of the feed cavity,some will adhere to the membrane interface,NaCl will heterogeneous nucleate on the surface of insoluble substances at the bottom,consuming the supersaturation of the system.When the salt concentration is low,crystal fouling occurs on the membrane interface,but membrane failure does not occur.It is proved that the MD can realize the concentration treatment of high salinity water.