## 3 Results and Discussion
### Fouling and Rejection Test  
As the PSf/GO membrane was found to be the most efficient with respect to flux testing and characterization, its antifouling effect and rejection efficiency were evaluated. The fouling test was conducted in four steps to compare PSf/GO and PSf/rGO using a synthetic raw water containing kaolin  
Figure 4: Pure water fluxes of PSf and PVDF membranes with differing graphene contents: (**a**) GO-blended PSf membrane; (**b**) rGO-blended PSf membrane; (**c**) GO-blended PVDF membrane; and (**d**) rGO-blended PVDF membrane. (All experiments were performed at 20 \({}^{\circ}\)C and pH 6.5 \(\sim\) 6.8). Figure 5: Flux change trends in graphene-based membranes: (**a**) GO-blended PSf membrane and rGO-blended PSf membrane and (**b**) GO-blended PVDF membrane and rGO-blended PVDF membrane. (250 NTU as turbidity) and alginate (10.8 mg/L as total organic carbon) (Figure 6). Step 1 involved physical backwashing at the point when the flux was reduced by 50%. Step 2 used a starting flux of 166 L/m\({}^{2}\)/h, and 69.4% of the flux was recovered when the GO content was 0.10 wt%. Step 3 was also followed by physical cleaning, and a slight decrease in the flux recovery occurred. Finally, in Step 4, flux changes were observed after chemical cleaning when using NaOCl (2000 mg/L) and an NaOH solution (5000 mg/L). For the 0.10 wt% PSf/GO membrane, 72.7% of the flux was recovered in the final step, and the antifouling effect was higher than that of the PSf blank membrane. However, the rGO-blended membrane showed no antifouling effect at all compared to the PSf blank membrane. Therefore, these results showed that in addition to its other attributes, the 0.10 wt% PSf/GO membrane also provided the highest antifouling performance. The fouling phenomenon that caused the flux decline could be quantified by using the membrane resistance model. The synthetic raw water that was used as the model foulant in this study was employed in the resistance-in-series model to determine the fouling mechanism of the PSf/GO  
Figure 6: Membrane fouling and backwashing test conducted on (**a**) PSf/GO and (**b**) PSf/rGO membranes (Step 1: initial flux. Steps 2 and 3: after physical cleaning. Step 4: after chemical cleaning by using 2000 mg/L of NaOCl and 5000 mg/L of an NaOH solution. All experiments were performed at 20 \({}^{\circ}\)C and pH 6.7 \(\sim\) 7.0). membrane [21]. Flux decline can be determined by using [PERSON]'s law as a theoretical basis and is as follows:  
\[J=\frac{\Delta P}{\mu\times R_{\mathrm{t}}} \tag{2}\]  
where \(J\) is the permeate flux, \(\Delta P\) the change in transmembrane pressure (TMP), \(\mu\) the water viscosity, and \(R_{\mathrm{t}}\) the total fouling resistance. The total resistance, \(R_{\mathrm{t}}\), can be obtained by the sum of all the individual resistances:  
\[R_{\mathrm{t}}=R_{\mathrm{m}}+R_{\mathrm{c}}+R_{\mathrm{f}} \tag{3}\]  
where \(R_{\mathrm{m}}\) is the inherent membrane resistance, \(R_{\mathrm{c}}\) is the membrane resistance due to the cake layer formed on the membrane surface, and \(R_{\mathrm{f}}\) is the fouling resistance caused by pore plugging and the irreversible adsorption of foulants on the membrane pore wall or surface. As shown in Figure 7, overall, fouling resistance contributed the most to total resistance, and it is notable that although no significant changes in cake resistance occurred, the resistance of the membrane itself was lowest in case of a GO content of 0.10 wt%. In particular, fouling resistance was the lowest with the 0.10 wt% PSf/GO membrane. The results of the model calculation indicated that fouling was mainly caused by the adsorption of foulants in the membrane pores or onto the membrane surface. Therefore, \(R_{\mathrm{f}}\) contributed the most to total resistance. In addition, resistance, \(R_{\mathrm{f}}\), is an irreversible resistance that cannot be recovered by physical cleaning, and it can only be recovered by using a desorption strategy, such as chemical cleaning [22].