The surface characteristics of the high cut-off UF-PS-100H membrane were similar to that of the UF-PES-050H membrane with the pores being less clear on the fouled membrane (Fig. 5a and c). The pore openings were larger and had a wider spread in size (∼10–50 nm) in this instance compared to that of the UF-PES-050H membrane (∼15–25 nm). The pristine membrane cross-section indicates particulate deposits with a dimension of around 50 nm within the pores (Fig. 5b). Such polymer particles are a typical feature in membranes prepared from unstable/inhomogeneous polymer solutions. The fouled membrane (Fig. 5d) also displayed deposits, but of a larger size (around 100 nm). In the figures, the deposits appear to be similar in size as the images are recorded at different magnification (50 K for the pristine membrane and 25 K for the fouled sample). These deposits within the pores after fouling are expected since a partial permeation of the HMW component was reported earlier