In order to provide a better understanding of the impact of flocculant used on membrane properties in drinking water production, a complete structural characterization of membranes was carried out from microscopic to macroscopic scale. New flat-sheet PES membranes with 10, 30 and 100 kDa MWCO were characterized by SEM, ellipsometry of angle resolved scattering (EARS), white light interferometry (WLI) and atomic force microscopy (AFM). It was shown that AFM is able to differentiate between membranes according to their MWCO and their manufacturing processes. The impact of flocculant filtration (PAX-XL 7A and Aqualenc F1) on flat-sheet PES 100 kDa membrane was studied. SEM and AFM characterizations revealed a modification of membrane surface state after flocculant filtration and cleaning step. AFM was finally used to characterize hollow fiber membranes after 1 and 2 years of water production. The results showed that AFM is a very interesting tool to investigate membrane ageing.