## 3 Strategic Development of Ultrafiltration Membrane
### Surface Modifications  
Surface modification to increase the performance is preferable in order to get the double advantages of structural support and hydrophilicity [183]. Next, this modification aims to increase flux (membrane performance), parallel to improve selectivity, as illustrated in Figure 6A,B, respectively. Low membrane permeability could be explored for additional modification to improve permeates flux owing to fouling reduction at the same time with resolving the selectivity issues. One research aimed to functionalize charge properties of 100 kDa PES by cationic styrene polymerization inside the pore and following activation by sulfuric acid treatment to create open-structure PES [186]. Comparing to the pristine membrane, selectivity enhancement was nearly five-fold at pH 7.2 for \(\beta\)-lactoglobulin ascribable to lower membrane pore size (reduction in molecular sieving) and increase of electrostatic repulsion in negatively charged PES with \(\beta\)-lactoglobulin. Adjustment of pH would allow better selectivity as performed in a work that at pH 7.2, membrane was 50% more selective than at pH 3.2, of which this finding was supported by other researches [187; 188]. Regarding the hydrophilic enhancement, many strategies can be employed. _First_, this could be performed by metal-oxide nanoparticles such as TiO\({}_{2}\), ZrO\({}_{2}\) and ZnO to modify hydrophobic PES mixed matrix membrane by phase inversion technique [191]. When treating cheese whey effluent, 1.5 wt.% TiO\({}_{2}\), compared to 2.0 wt.% ZrO\({}_{2}\), 1.5 wt.% ZnO, and pristine membrane, also provided the highest average flux (25.09 L m\({}^{-2}\) h\({}^{-1}\)) and lowest flux reduction (39%) owing to the increase in pore radius to be 4.16 nm and surface free energy (109.33 mJ m\({}^{-2}\)). Following that, more than 94% of total whey protein could be rejected while individual protein rejection was 96% for BSA, and 90% for lysozyme and \(\alpha\)-lactalbumin [191]. _Second_, not only via embedding metal oxide nanoparticles, atmospheric pressure jet plasma can alternatively elaborate hydrophilic feature on PES by incorporating oxygen-derived functional groups [183]. From this method, the ratio of reversible fouling to total fouling was decreased by 51% from 94% in pristine membrane, and formation of cake layer was wholly eliminated in parallel with reduced minimum roughness and transformation of maximum surface free energy from \(-\)14.92 \(\pm\) 0.89 mJ m\({}^{-2}\) to +17.57 \(\pm\) 0.67 mJ m\({}^{-2}\). Furthermore, when used to treat whey proteins, fouling rejection capability was equal with the initial condition. However, permeate flux promptly declined because of lessened hydraulic permeability emerged from plasma-induced surface cross-linking. Nonetheless, permeate flux recovery after cleaning with 0.1 N NaOH was obtained in a great result while using this modified membrane [183]. Ultrafiltration Performance and Selectivity Enhancements: Controls of Fouling and Concentration Polarization from Operating Conditions Perspectives  
Fouling and concentration polarization influence the dynamics layer of membrane surface, which in turn control the mass transfer during UF process [192]. Generally, fouling built-up ranges over the duration of UF process, whereas concentration polarization is only below one minute and reversible in nature [29; 193]. In this paper, we gathered distinguished examples of how operating parameters play a role to control fouling and concentration polarization, i.e., protein concentration, pH, and ionic strength (salt concentration) by modulating protein-protein and protein-surface interactions, and TMP toward hydrodynamics. Owing to the unique findings resulted from different types of protein sources, the discussion of each operating parameters will be divided into marine, dairy and legume proteins. In Section 4.1, membrane characteristic like hydrophilicity, MWCO and UF mode were taken into discussion.