A steady-state simulation toot was developed for a two-stage nanofiltration (NF) process with recycling of the retentate from the second filtration stage ("feed and bleed" operation). The simulation tool calculates the NF process parameters in dependence on the partial recoveries in both filtration stages. The input for the simulation tool contains six parameters for modelling the filtration performance of NF membranes, process configuration parameters and the specification of a separation problem, which consists of system feed flow and concentration, system recovery, maximum permeate concentration as well as maximum partial recoveries. The definition of a separation problem leads to a non-linear optimisation problem, which is solved numerically. By comparing different operation points, which fulfil the requirements, the specific costs of power consumption are minimised by optimising partial recoveries in both filtration stages. The power consumption is calculated with a modified equation with variable efficiencies of the high-pressure pumps. An operating two-stage NF plant was used as a case study for process simulation. Isothermal operation,vas assumed at 293 K.