The modelling of membrane filtration processes is often conducted by applying black-box models or short-cut methods because of its complexity due to molecular interactions on and inside the membrane. The assumptions made for the short-cut-methods are applicable for reverse osmosis, whereas the simulation of nanofiltration processes can lead to unreliable results, which sometimes deviate greatly from real conditions. A steady-state process simulation NF-PROJECT, which is based on input information from membrane characterisation, was developed (isothermal operation). The individual separation characteristics of every membrane element are calculated in iteration, which leads to reduction of permeability and rejection for every further element arranged in series inside the pressure vessel. The simulation provides information on the increasing feed concentration and osmotic pressure, the hydraulic pressure loss, the deterioration of the flow conditions along the feed side in the vessel and the combined performance of the membrane elements to be analysed. Serving as an example from practical applications, a two-stage nanofiltration pilot plant was simulated, the results of which will be presented. Examples of design and optimisation potentials will be illustrated for the target criteria of economic efficiency (specific energy costs), permeate quality and flow.