**3. Blending method**

The introduction of inorganic nanomaterials (eg. TiO2, ZnO, GO, Al2O3, and SiO2) into polymer dope to make mixed matrix membrane (MMM) has been favored over the other methods including coating and grafting due to its simplicity. These nanoparticles are attractive in wastewater treatment, because of their porous textures, high surface area to volume ratio, high pore volume and their surface functional groups (-OH) which impart enhanced hydrophilicity and surface properties of the resultant membranes. They form new water pathways, increase solute rejection and control the degree of fouling.

MMM can be configured in both flat sheet and hollow fiber forms based on phase inversion process. Homogenous solution is prepared by dispersing the nanomaterials in a suitable organic solvent (N-methyl-2-pyrrolidone, dimethylformamide, dimethylsulfoxide, dimethylacetamide). A certain amount of polyethyleneglycol or polyvinylpyrrolidone as pore former is introduced to the solution. After sonication, the suspension is slowly added into the main membrane forming material (PSf, PAN, PWDF etc.). The dope solution is continuously stirred overnight, followed by casting process with the required thickness. An asymmetric membrane is then produced by placing the composite film into coagulation bath for nonsolvent induced phase inversion. The incorporation of the inorganic nanoparticles alter the bulk structure of the membrane as they hinder the interaction between polymer and solvent molecules [60]. Their hydrophilic character changes the solvent exchange rate during phase inversion process leading to a thin and dense selective layer with a finger like support layer. The hydrophilic nature of the nanoparticles causes them to move towards the surface during phase inversion process. This enhances the surface properties of the resultant membrane.

#### **3.1 Nanocomposite membranes prepared by blending method**

The research in improving the properties of nanofiltration membranes is recently increased tremendously. Various nanomaterials including GO, CNT, metal organic frameworks (MOF) (ZIF-8), TiO2, SiO2, zeolite, have been incorporated to form a nanocomposite membrane with high performance. In the following part, a review will be provided to highlight the performance of these composite membranes.
