In this study, the origin and extent of membrane fouling on spiral wound module (SWM) elements from different pressure vessels and the distribution of membrane foulants inside the SWM elements were systematically investigated using the membrane autopsy methods. Three fouled SWM elements from different pressure vessels were examined for a quantitative and qualitative comparison of membrane foulants. Results obtained indicate that a major contributor to membrane fouling were fulvic acid components, consisting of small, hydrophilic compounds from the raw groundwater. A small amount of inorganic components such as Fe and Al, which can form a complex structure with fulvic acid, also partially contributed to the membrane fouling of lead elements in first pass RO vessels. In particular, through the elemental comparisons of two sheets (both sides) of fouled membranes obtained from identical positions of a membrane leaf, the distribution of membrane foulants inside the SWM elements was fully evaluated. Based on the data obtained, much more organic and inorganic foulants were accumulated on the back side of the membrane leaf than on the front side. Overall, these results would help improve understanding of fouling behavior inside membrane modules and pressure vessels. (C) 2015 Elsevier B.V. All rights reserved.