The stepwise process design approach, proposed in a previous study, to generally model membrane unit operations in combination with experimental model parameter determination is transferred to a reverse-osmosis unit. In order to predict the purification a priori by simulations, the changes of the rigorous ultrafiltration model to gain a rigorous reverse-osmosis model are described. The modular process model, combining the most limiting factors within one mathematical model, is transmitted to reverse osmosis and various filtrations are carried out. Together with the new standard laboratory equipment, practicability of the proposed buildup is proven in order to design and integrate reverse-osmosis membranes into biotechnology processes.