Total recycle (TR) and batch (13) recovery of ammonium fumarate from aqueous model solutions was studied in a pilot plant equipped with a spiral-wound thin-composite reverse osmosis (RO) and nanofiltration (NF) membranes at 40 degreesC and feed input absolute pressure (P-i) of about 60 and 20-40 bar, respectively. Modelling of both membrane processes under total recycle and batch testing was successfully carried out on the basis of the Spiegler-Kedem model, thus allowing the negative effect of solute concentration on solvent permeation flux (J(v)) to be reconstructed with average percentage errors between experimental and calculated J(v) data of 8 and 13%, respectively. Such models allowed the most appropriate membrane process for a given solute concentration ratio (phi) and recovery efficiency to be selected. If no reduction in the water intrinsic membrane hydraulic permeability (L-pWo) was accounted for, the performance of the RO module would overcome that of the NF module for any phi value in the range 1-4. In the case of a 30% reduction in L-pWo, the performance of the NF module would become remunerative with respect to the RO module for phi values ranging from 1 to 2.45 on condition that the solute recovery efficiency was of the order of 90%.