In this paper, we present a control relevant rigorous dynamic model for an ultrafiltration membrane unit in a whey separation process. The model consists of a set of differential algebraic equations and is developed for online model based applications such as model based control and process monitoring. In this model, membrane resistance concept is adjusted to describe the membrane fouling. Based on the observations regarding the permeate flux, we propose a membrane resistance expression consisting of static and dynamic resistances. The empirical expressions for the membrane resistances are identified by solving a parameter estimation problem. The dynamic model is investigated for its predictive capabilities and is further utilised for the study of optimal operation strategies. (C) 2017 Elsevier Ltd. All rights reserved.