## 1 Introduction  
Contaminated water sources are becoming a tremendous problem in developing and transient countries [1]. Since the installation of a centralized drinking water treatment system requires a high energy input and a large footprint, a decentralized drinking water treatment system at the household or community level is an alternative option for a sustainable drinking water supply [2]. Gravity-driven membrane (GDM) filtration is one of the promising decentralized drinking water treatment systems due to its low cost, simple operation, and convenient maintenance [3; 4; 5; 6]. The GDM filtration system usually operates in a dead-end mode at ultralow pressure (<0.1 bar), which leads to a relatively low but stable permeate flux (<10 L/m\({}^{2}\)/h (LMH)) over several months [6]. Although membrane fouling phenomena decrease the filtration performance in water treatment systems [7], the ultralow transmembrane pressure in GDM filtration systems favors the formation of a loosely bound fouling layer on the membrane, which creates a pathway for the treated water. [PERSON] et al. [4] found that over 250 days a thicker but porous fouling layer with less accumulation of organic substances improved the permeate flux in GDM filtration. [PERSON] et al. [8] reported that the composition of extracellular polymeric substances (EPS) from biofilm formation determined the physical structure of the fouling layer and eventually affected the hydraulic resistance during GDM filtration. The presence of a diverse microbial community, including eukaryotic organisms in the fouling layer, could affect flux behavior during GDM filtration. For example, the movement and predation activity of metazoans made the fouling layer more porous and heterogeneous, resulting in temporary flux recovery [9; 10]. The surface properties of membranes can play an important role in membrane fouling phenomena [11; 12]. It is generally known that membrane properties, including hydrophilicity and roughness, are closely related to membrane fouling; more hydrophobic and rougher surface membranes typically show decreased permeate fluxes due to facile attachment and accumulation of foulants on the membrane surface. In terms of the effect of surface charge on the membrane fouling phenomenon, negatively charged membranes usually exhibit fouling resistance, although some literature reported that positively charged membranes or zwitterionic membranes were more effective against membrane fouling [13]. In GDM filtration systems, a more hydrophilic membrane could increase permeate flux in the initial period of operation, but contributes less to the stabilized permeate flux in the extended operation period [14]. Thus, by using surface membrane modification, it has been tried to raise the low permeate flux in the GDM process to the level of conventional membrane filtration systems, whose permeate flux ranges from 50 to 100 LMH at 0.2-1.0 bar [15]. The modified membranes with amphiphilic multi-arms polymer, zwitterion polymer, and powdered activated carbon (PAC)/zeolite particles enhanced the hydrophilicity of the membrane surface, resulting in 50-70% increased permeate fluxes in the GDM system [16; 17; 18]. However, the effect of membrane surface properties on the eukaryotic community composition within the fouling layer was not investigated. The objective of this study was to investigate the effects of surface properties on permeability behavior related to the nematode population, as a representative metazoan species, on the fouling layer. A submerged GDM filtration system with an intermittent operation mode was used to simulate the practical process for approximately 400 days (~100 cycles in GDM operation). Two ultrafiltration membranes and one microfiltration membrane with different surface properties were tested using lake water as the input.