The performance of spiral-wound (SPW) and capillary nanofiltration (NF) modules can be improved by optimizing the module configuration and operating conditions. To this effect, two mathematical models for spiral wound and capillary modules have been developed. Both models are based on the homogeneous solution model and take into account concentration polarization, difference in rejection of mono- and bivalent ions, and the pressure drop in the feed and permeate channel. In this paper the influence of the operating conditions (feed pressure, feed flow) and the module configuration (spiral wound, capillary, feed channel height, permeate channel height, porosity, capillary diameter) on the performance of the modules are presented. Compared to the optimized spiral-wound module, a further increase in performance of 100% can be achieved by optimizing the configuration and operation conditions of a capillary module.