## Materials and Methods
### Analysis of Membrane Fouling Layer and Dissolved Organic Matter  
The membrane surfaces during GDM filtration were periodically recorded using a digital camera (Canon EOD 100D) to obtain macroscopic observation of membrane fouling layers. To quantify the membrane coverage by the fouling layer, the 'Image J' software ([[http://rsb.info.nih.gov/ij/](http://rsb.info.nih.gov/ij/)]([http://rsb.info.nih.gov/ij/](http://rsb.info.nih.gov/ij/))) was used. The automatic triangle algorithm was then applied to distinguish between the fouling layer and the uncovered membrane surface. The membrane coverage was obtained by calculating the selective surface area of the membrane [10; 27]. The microscopic structure of the fouled and clean membrane was characterized using a Scanning Electron microscope (SEM, Hitachi S-4800, Tokyo, Japan) after the long-term operation. Sample preparation was done using the freeze-drying method (at low temperature (-55 \({}^{\circ}\)C) and vacuum for 24 h) to remove the water content and preserve the structure of the fouling layer. The samples were then coated with platinum using an ion sputter coater (E-1030, Hitachi, Tokyo, Japan). Top-view and cross-sectional SEM images were visualized at an accelerating voltage of 10 kV.  
The biological activity of the fouling layer was determined by adenosine triphosphate (ATP) measurement using BacTiter-Glo reagent (Promega Corporation, Madison, WI, USA). A small fraction of the membrane fouling layers were carefully detached by a sterile cell scraper and transferred to DI water, and then homogenized. The samples of 500 \(\upmu\)L and ATP reagent of 50 \(\upmu\)L were warmed for 1 min at 38 \({}^{\circ}\)C in separate sterile tubes followed by combining them for the reaction for 20 s incubation time at 38 \({}^{\circ}\)C under dark conditions [28].  
Size exclusion chromatography coupled to an organic carbon detector (SEC-OCD) was used to determine the chromatographically separable organic matter fraction according to molecular weight and charge. There were four fractions of raw water: (1) biopolymer (>10 kDa), (2) humic and building block (0.5-1 kDa), (3) low molecular organic acids (<0.5 kDa), and (4) low molecular neutral compounds (<0.5 kDa). The SEC-OCD includes high-performance liquid chromatography (Agilent 1260, Palo Alto, CA, USA) with a size exclusion column (TSK HW-50s, 3000 theoretical plates, Tosoh, Tokyo, Japan) and an organic carbon detector (Sievers Portable turbo total organic carbon analyzer 9000, Boulder, CO, USA) [29; 30]. The online mobile phase (Phosphate buffer, pH 6.85, 2.5 g KH\({}_{2}\)PO\({}_{4}\) + 1.5 g Na\({}_{2}\)HPO\({}_{4}\) \(\times\) 2H\({}_{2}\)O to 1 L) is delivered with an HPLC pump at a flow rate 1 mL/min to a size-exclusion column. The molecular weight calibration standard was comprised of polyethylene glycols (PEGs: 600-8000).  
Figure 1: Experimental set-up of the submerged gravity-driven membrane (GDM) filtration system in the intermittent operation mode. Each cycle of the GDM operation consisted of feed water filling and discharging. The next cycle was started by emptying the remaining water and refilling with the feed water. One cycle indicates the changes in permeability according to the water level (from 0.075 bar to 0.01 bar corresponding to the height of 97.5 cm and 32.5 cm).  
OCD and UVD calibration was based on potassium hydrogen phthalate. Customized software (SEC-OCD software, Young-in, Seoul, Korea) was used to quantify the organic carbon concentration.