This research explores initial organic membrane fouling of element-scale spiral-wound forward osmosis (SWFO) operated at a wastewater plant using real secondary wastewater effluent (SWWE) as feed solution (FS) and synthetic seawater as draw solution (DS). The SWFO was operated long-term and the average water flux reduced from 6.0 to 3.2 L/m(2).h. After the operation, membrane samples were taken from three locations in the SWFO element (i.e., inlet, middle, and outlet) to study the structural effects of organic membrane fouling. The membrane foulants were extracted in sodium hydroxide and deionized water to characterize irreversible and reversible organic foulants, respectively. Organic matter (OM) contained in the FS was dominated by hydrophilic (HPI) aromatic proteins (molecular weights (MWs) of 30,000 Da) and soluble microbial byproducts (which are associated with humic substances and have MWs of 920-2000 Da). The highest organic content (2.67 mg-C/cm(2)) was found in the inlet of the SWFO element and, interestingly, was mostly irreversible. In the inlet, humic-or fulvic-like organics and aromatic proteins were dominant, while higher protein-like organics were detected in the middle and the outlet of the element. The organic fouling behavior of the SWFO membrane can be explained as follows: HPI OM formed the initial, irreversible fouling on the membrane surface, after which the hydrophobic (HPO) OM, which is reversible, attached to the initially formed HPI fouling layer by electric interaction dominantly. Therefore, HPI organic foulants should be controlled to reduce irreversible fouling and subsequently guarantee the sustainable operation of SWFO in the SWWE treatment.