## 3 Results and Discussion
### Fouling Behavior on the PAN-NF Membrane after Backwashing
#### 3.2.2 CEB for Organic-Fouling Control  
The performance of chemical backwashing for the modified PAN-NF HF membrane was evaluated because physical backwashing did not help recover the flux performance of the organic-fouled membrane. Earlier studies found that NaClO solutions can negatively affect membrane properties or membrane aging [46, 47]. Therefore, in this study, NaClO solutions with low concentrations were used during chemical backwashing of the membrane to achieve optimal membrane permeability recovery with minimum impact on membrane performance [48, 49].  
Figure 4b shows the removal efficiency results of the experiments performed once a week using different NaClO concentrations. Increasing the concentration of NaClO to 20 and 30 mg/L increased the backwashing efficiency after 60 s of backwashing following seven days of filtration. Backwashing with 30 mg/L of NaClO exhibited the best flux recovery rate of the PAN-NF HF membrane; 95% of both NOM-fouled membranes were recovered. However, the HA and SA removal efficiencies significantly decreased after chemical backwashing at 30 mg/L of NaClO. The removal efficiencies were 87.6% for HA and 88.4% for SA. These results indicate that the properties of the membrane may have deteriorated because of the high concentration of NaClO. Thus, 20 mg/L NaClO was considered appropriate for backwashing the PAN-NF HF membrane. The flux recovery of the PAN-NF HF membrane was efficiently restored through backwashing at 20 mg/L NaClO. Figure 4a indicates that the water flux recoveries of the PAN-NF HF membrane were 93.3% for HA and 93.7% for SA, whereas the removal efficiencies were 92.7% for HA and 93.7% for SA. The PAN-NF HF membrane exhibited stable water permeability after backwashing with 20 mg/L NaClO. As NaClO comprises functional groups such as aldehyde (CHO), carboxyl (COOH), and keto (C=O), these functional groups can deteriorate the mechanical properties of the membrane [50]. As shown in Figure 5, NaClO backwashing of the HF membrane in this experiment provided relatively good recovery of water permeability due to the oxidation and removal of the organic-fouling layer. Although the NaClO solution increased the pore size of the membrane because of the oxidation of the membrane inner surface, it increased the negative charges on the membrane surface, thereby causing an increase in electrostatic repulsion between the negative charges on the membrane and NOM foulants [51]. Moreover, exposure to NaClO increased the hydrophilicity of the inner surface of the membrane. This may increase the interactions between the membrane and both NOM foulants, resulting in stable TOC removal efficiency after NaClO backwashing.  
Figure 4: (**a**) Water flux recovery of the HA- and SA-fouled PAN-NF HF membrane after backwashing with NaClO solutions of different concentrations compared to water flux recovery using nonchemical backwashing. (**b**) Removal efficiencies of HA and SA from the PAN-NF HF membrane after backwashing with different concentrations of NaClO.  
Case Study: Application of NaClO Backwashing in the PAN-NF HF Membrane Used in Domestic Drinking-Water Plants