## 3 Results and Discussion
### Fouling Layer Characteristics  
The morphology of the fouling layer on membrane surfaces was monitored at macro- and micro-scales using a digital camera (top-view images) and a scanning electron microscope (SEM). Figure 4 shows the top view images of the fouling layers at different operation cycles of GDM filtration. During the operation, the fouling layer gradually covered the membrane surfaces and became darker and heterogeneous. It was clear that the fouling of PVDF-UF and PTFE-MF was much more severe than that of PES-UF. The morphology of the PES-UF fouling layer barely changed with increasing operation cycles. The fouling layer on PVDF-UF and PTFE-MF showed dark brown mounds and heterogeneous structures. Coverage of the PVDF-UF and PTFE-MF membrane increased from 32-46% on day 23 to ~100% at the end of the operation, while that of PES-UF remained significantly lower (1-24%). Figure 3: Mean permeability during GDM filtration using (**a**) PES-UF, (**b**) PVDF-UF, and (**c**) PTFE-MF membranes with an input of lake water as a function of operation cycles and time (mean permeability = average permeate flux divided by hydraulic pressure at each cycle, error bar represents the standard deviation during several measurements at each cycle). Figure 5 shows top-view and cross-sectional SEM images of PES-UF, PVDF-UF, and PTFE-MF at the end of the filtration. The fouling layers developed on PVDF-UF and PTFE-MF had highly thick and complex structures (Figure 5e,f), while PES-UF showed a less developed fouling layer (Figure 5b,c). The thicknesses of the fouling layer, obtained from cross-sectional SEM images, were \(1.1\pm 0.5\)\(\upmu\)m (PES-UF), \(72.6\pm 24.5\)\(\upmu\)m (PVDF-UF), and \(73.8\pm 26.1\)\(\upmu\)m (PTFE-MF). The green arrows indicate the interface between the membrane and the fouling layer. Unlike the heterogeneous and thick fouling layers on the PVDF-UF and PTFE-MF membranes, PES-UF showed a homogeneous and thin fouling layer. Although PVDF-UF and PTFE-MF showed increased thickness and coverage of the fouling layer, the stabilized permeability values were much higher compared to PES-UF (Figure 3), indicating that the thicker fouling layers of PVDF-UF and PTFE-MF were loosely formed and enabled higher permeate flux. Figure 4: Top-view images of the fouling layer on the membrane surface as a function of operation cycles and days. Figure 5: Top-view and cross-sectional SEM images of the membrane surfaces: (**a,d**) PES-UF, (**b,e**) PVDF-UF, and (**c,f**) PTFE-MF. Images (**a–c**) show clean membranes, and (**d–f**) are images of the fouled membranes taken at the end of the GDM filtration operation. The green arrows indicate the boundary between the membrane surface and the fouling layer. Figure 6 shows the results of Student's \(t\)-test among the membranes in terms of mean permeability, roughness, and biomass concentration. ATP concentrations of the fouling layers were measured as the indicator of active biomass in the fouling layer. Measured biomass concentrations were \(0.16\pm 0.01\) gATP/m\({}^{2}\) for PES-UF, \(1.13\pm 0.63\) gATP/m\({}^{2}\) for PVDF-UF, and \(0.96\pm 0.63\) gATP/m\({}^{2}\) for PTFE-MF. Note that PES-UF had a much lower ATP concentration than PVDF-UF and PTFE-MF; the amount of active biomass of the fouling layer was 70% higher in PVDF-UF and PTFE-MF than in PES-UF. The fouling layer of PES-UF showed low microbial activity and may produce a lower amount of soluble microbial products [38]. More biomass accumulated onto the rougher surfaces of the PVDF-UF and PTFE-MF membranes. There was no significant difference in the stabilized permeability, membrane roughness, or biomass concentration between PVDF-UF and PTFE-MF (\(p>0.05\)), whereas PES-UF showed statistically significant differences compared to both PVDF-UF and PTFE-MF (\(p<0.05\)).