## 3 Results and Discussion
### Flux Behavior during GDM Filtration  
Figure 2 shows the selected permeate flux behaviors of PES-UF, PVDF-UF, and PTFE-MF membranes at the beginning (cycle 1), middle (cycle 49 for PES-UF/cycle 65 for PVDF-UF and PTFE-MF), and end (cycle 96 for PES-UF/cycle 110 for PVDF-UF and PTFE-MF) of the GDM filtration cycle. The initial permeate fluxes were 53 LMH for PES-UF, 60 LMH for PVDF-UF, and 147 LMH for PTFE-MF (cycle 1). The different initial permeate fluxes were expected because of their clean membrane permeability, which was 120 LMH/bar for PES-UF, 921 LMH/bar for PVDF-UF, and 8923 LMH/bar for PTFE-MF. The higher flux of PTFE-MF compared to the other two UF membranes can be understood by its relatively larger pore size. Permeate fluxes dropped sharply to approximately 7 LMH due to both membrane fouling and the decreased hydrostatic pressure from 0.01 bar to 0.075 bar. In the middle of the GDM operation period (cycle 49 for PES-UF and cycle 65 for PVDF-UF and PTFE-MF), permeate fluxes were much lower compared to those at cycle 1. At cycle 65, the initial fluxes for PVDF-UF and PTFE-MF membranes were 11.8 and 11.1 LMH and decreased to 4.3 and 5.1 LMH, while the flux for PES-UF decreased from 3.3 LMH to 1.0 LMH at cycle 49. Similar trends were observed at the end of the operation (cycle 96 for PES-UF and cycle 110 for PVDF-UF and PTFE-MF). These results show that only PTFE-MF showed higher permeate flux at the beginning, and relatively higher fluxes were observed for PTFE-MF and PVDF-UF than PES-UF during the entire GDM operation period.  
The mean permeability of each cycle could be calculated from the average of the permeate fluxes divided by the corresponding hydraulic pressure during GDM filtration (Figure 3). The initial mean permeability (cycle 1) was 312 \(\pm\) 202 LMH/bar for PES-UF, 491 \(\pm\) 111 LMH/bar for PVDF-UF, and 1172 \(\pm\) 372 LMH/bar for the PTFE-MF membrane. The mean permeability for PES-UF gradually decreased and stabilized, while those for both PVDF-UF and PTFE-MF fluctuated. After cycle 5, a relatively higher mean stabilized permeability was observed for PVDF-UF and PTFE-MF, 192.9 \(\pm\) 170.4 and 180.6 \(\pm\) 120.9 LMH/bar, respectively, while PES-UF showed a much lower mean stabilized permeability at 46.1 \(\pm\) 31.5 LMH/bar. These results can be explained by the effect of membrane surface properties on the fouling layer characteristics. Particularly, membrane surface roughness has a strong relationship with a mean stabilized permeability. Membrane surface roughness can affect the attachment and accumulation of fouling layers or predation and movement of eukaryotes within the fouling layer. These impacts can change the fouling layer structure. Therefore, the presence of a biologically active fouling layer on PVDF-UF and PTFE-MF membrane surfaces increased the permeability (or permeate flux) but also caused the highly fluctuating permeability. The membranes with higher roughness (i.e., PTFE-MF and PVDF-UF) showed higher fluxes than the smooth membrane  
Figure 2: Permeate flux development depending on pressure drop from 0.075 bar to 0.01 bar during gravity-driven membrane (GDM) filtration with different membranes. The permeate flux was identified based on the operation cycle and time: cycle 1 (day 1), cycle 49 (day 170) = PES-UF and cycle 65 (day 229) = PVDF-UF/FTFE-MF, and cycle 96 (day 331) = PES-UF and cycle 110 (day 390) = PVDF-UF/PTFE-MF.  
(i.e., PES-UF) in our GDM operation system. This was contrary to the typical flux behaviors of conventional MF/UF systems where rough membranes show lower permeability due to more severe membrane fouling [33; 34; 35; 36; 37]. The reason for this phenomenon is further discussed in Section 3.6.