## 5 Characterization of Membranes
### Morphology Analysis
Additionally, the difference of the final fluxes between PVDF membranes blended with PVDF-g-PHEA and PVDF pristine membranes were 6.56 and 11.57 LMH at 20 ppm HA solution, respectively. As a result of the permeate flux performance, the concentration of PVDF-g-PHEA were increased, the difference of the final flux became larger at 20 ppm HA solution than at 6 ppm HA solution. The more the concentration of HA solution was added, the lower the final permeate flux was. Figure 8: Permeate flux of membranes during filtration of 6 ppm HA solution at 1 bar. Flux test was performed in order to demonstrate improvement of permeate flux during ultrafiltration. However, some researchers may believe that permeate fluxes are not accurate due to different initial flux. For that reason, authors conducted the normalized flux (\(J\)/\(J_{0}\)) test to confirm more reliable flux difference originated from different initial HA flux. Normalized flux measurement is an important method to evaluate performance of membranes without reference to the pore size. Normalized flux of humic acid (6 and 20 ppm) was evaluated by this equation:  
\[\text{Normalized flux}=J\text{/}J_{0} \tag{6}\]  
where \(J\) was the final flux of the humic acid permeated and \(J_{0}\) was the initial flux of the humic acid permeated. Figures 10 and 11 show normalized fluxes of membranes. As normalized fluxes were calculated, normalized flux values of A-1 and B-1 were 0.42 and 0.47, and normalized flux values of A-2 and B-2 were 0.44 and 0.51 at 6 ppm HA solution, respectively. When normalized fluxes were evaluated at 20 ppm HA solution, normalized flux values of A-1 and B-1 were 0.36 and 0.40, and normalized flux values of A-2 and B-2 were 0.40 and 0.45, respectively. The differences of normalized fluxes between PVDF membranes blended with PVDF-g-PHEA and PVDF pristine membranes were 0.02 and 0.04 at 6 ppm HA solution. Additionally, the differences of the normalized flux values between PVDF membranes blended with PVDF-g-PHEA and PVDF pristine membranes were 0.04 and 0.05 at 20 ppm HA solution, respectively. As a result of the normalized flux calculation, when the concentration of PVDF-g-PHEA was increased, the difference of the final flux became larger. Permeate flux performance and normalized flux calculation indicated the high concentration of PVDF-g-PHEA improved the permeate flux of PVDF membranes. Figure 12 represents rejection data of membranes during filtration of 6 ppm and 20 ppm humic acid solution. The initial rejections of A-1, A-2, B-1 and B-2 membranes were 71.7%, 73.0%, 79.3%, and 83.8% at 6 ppm humic acid solution, respectively. Rejection percents of A-1, A-2, B-1, and B-2 membranes were increased as time was passed. The final rejections of A-1, A-2, B-1, and B-2 membranes were 82.7%, 83.8%, 87.8%, and 88.8% at 6 ppm humic acid solution, respectively. The initial rejections of A-1, A-2, B-1, and B-2 membranes were 83.7%, 81.3%, 91.1%, and 90.9%,  
Figure 9: Permeate flux of membranes during filtration of 20 ppm HA solution at 1 bar. respectively. As time passed, rejection percentages of A-1, A-2, B-1, and B-2 membranes were increased and reversal phenomenon was occurred. After two hours, the final rejections of A-1, A-2, B-1, and B-2 membranes were 89.8%, 90.1%, 92.9%, and 93.4%. Based on pore size distribution, Figure 5, the reason that rejection of A-2 and B-2 membranes was higher than A-1 and B-1 membranes was because average pore size of A-2 and B-2 membranes was smaller than that of A-1 and B-1 membranes. Additionally, humic acid passed through larger pores easier. Therefore, the humic acid rejection percent of A-1 and B-1 membranes was lower than that of A-2 and B-2 membranes. More humic acid was stacked on the surface of A-1 and B-1 membranes and it caused membrane fouling [18]. Figure 11: Normalized flux of membranes during filtration of 20 ppm HA solution at 1 bar. Figure 10: Normalized flux of membranes during filtration of 6 ppm HA solution at 1 bar.