Investigation of flux attenuation and crystallization behavior in submerged vacuum membrane distillation (SVMD) for SWRO brine concentration

Abstract

This work studied the concentration of seawater reverse osmosis (SWRO) brine by a submerged vacuum membrane distillation (SVMD) process. Factors for flux attenuation including CaSO4 crystallization,vapour pressure depression, the increase of heat transfer resistance and membrane fouling were distinguished by the combination of experiment and theoretical calculation. It was suggested that CaSO4 crystallization was the main reason for flux attenuation, which should be eliminated by appropriate feed pretreatment with additional of Na2CO3 and HCl. The combined effect of other three factors was another important reason for flux attenuation. Although the effect of CaSO4 crystallization was eliminated, there still existed a critical VCF after which membrane fouling and increase of heat transfer resistance could not be neglected. The result indicated that this critical VCF was about 2.5 under the experimental condition, which was far less than the VCF at which NaCl solution reached saturation. Therefore, finding an effective method to mitigate membrane fouling was still necessary. It was concluded that decreasing the concentration rate by changing the operation mode could effectively increase the critical VCF to 3.7. Meanwhile, the crystallization behavior of NaCl and the size and quality of crystal products would be affected.