In this paper, optimal strategy based on Pontryagin's minimum principle is presented for batch membrane separation of lactose-salt water solution, whilst minimizing costs. Optimal control problem comprises a weighted combination of the processing time and diluant consumption, i.e., the overall processing cost of separation. Process model is derived from mass balance and its parameters are estimated using dynamic least-squares procedure from process data. Nanofiltration plant is used to verify and critically review the proposed optimal, and standard industrial strategies. In comparison to the standard strategies, optimal strategies reduce the batch time up to 8% and the diluant consumption up to 25%. (C) 2019 Elsevier Ltd. All rights reserved.