This work investigated the effects of two types of algogenic organic matter (AOM), namely Anabaena AOM and Chlorella AOM, on the removal of naproxen using three different nanofiltration (NF) membranes. The effects of AOM membrane fouling on the rejection of naproxen were interpreted with respect to the AOM molecular weights (MW) and the fouling characteristics. Organics with medium and low molecular weights, e.g., less than the 10 kDa of the Anabaena AOM and Chlorella AOM, significantly affected the naproxen removal and resulting membrane fouling. Anabaena AOM and Chlorella AOM with large and medium MW (>1.5 kDa) adhered well to the NF membranes to form a fouling layer, which enhanced the hydrophobicity and negative charge of the membrane surface. The hydrophobicity and electronegativity of the adhered AOM fouling layer was responsible for the decreased membrane flux and the increased naproxen removal. During the filtration process, AOMs with low MW could enter the membrane pores, which also affected the naproxen removal and resultant membrane fouling. Anabaena AOM contained more hydrophobic organics than Chlorella AOM, whose cohesion free energy and adhesion free energy between the membrane and foulants were more negative and caused more severe membrane fouling and a higher naproxen removal effect, or, more specifically, the degree of membrane flux declined from 7.0% to 18.8% and naproxen retention increased from 7.3% to 15.1% in the presence of Anabaena AOM, while which from 3.0% to 15.4% and from 3.9% to 12.3% in the presence of Chlorella AOM, respectively. The surface free energies of the membranes and foulants is a useful parameter for membrane fouling analysis, and a mechanism for the enhanced naproxen removal in the presence of AOM was also proposed. (C) 2014 Elsevier B.V. All rights reserved.