Nanofiltration membranes used for treating wastewater streams can be modified to change their adsorption and solute rejection properties and their susceptibility to fouling. NF200 membranes modified by hydrophilic monomer such as PEGMA showed greatly reduced adsorption of certain organic compounds typical of paper mill streams. Further testing of these membranes and NF270 in flat sheet and spiral element form showed indications that the modification gave modest increases in resistance to fouling by acid clear filtrate from paper mills. In tests on tertiary effluent from wastewater treatment plants, modification of NF membranes (NF70, NF270) by HEMA lead to significantly reduced salt passage, especially of monovalent anions (chloride). The effects of modification on solute retention were much more pronounced on the tertiary effluent. These effects can be partly explained in terms of the relative role of ordered water and secondary membranes formed by deposits on the membrane. (C) 2004 Elsevier B.V. All rights reserved.