The overall objective of this research is to investigate natural organic matter (NOM) fouling in low-pressure membranes (MF/UF) by morphological analyses, with comparison before and after membrane filtration. Morphological analyses are powerful techniques, which provide spatial imaging generated by direct probing or indirect scanning of the membrane surface and make it feasible to analyze depth distribution, topographical changes and the profile of the membrane surface. Dissolved organic matter, defined as organic matter passing through 0.45 mu m filter, contains major foulants that cause significant fouling as revealed by parallel experiments with 1 and 0.45 mu m prefiltration. Atomic force microscopy (AFM) image analyses demonstrated that UF membranes are relatively smooth while MF membranes are rough (over 90 nm as average roughness). Surface coverage was a dominant fouling mechanism for UF membranes while pore blockage was dominant with MF membranes due to a different interaction between the size of organic matter components and membrane pore size. While morphological differences were observed in stirred and non-stirred cell simulations of membrane filtration, fouling propensity was not significantly affected by stirring versus non-stirring conditions. Image analysis by scanning electron microscopy (SEM) or field emission scanning electron microscopy (FESEM) was also applied to visualize membrane structures providing explanations of fouling mechanisms associated with porosity, mean pore radius, and pore size distribution as they relate to pore blockage and/or surface (gel layer) coverage, by image processing software. (c) 2005 Elsevier B.V. All rights reserved.