Nanofiltration as state-of-the-art technology was used for the separation of lactic acid (LA) and amino acids (AA) in a 'Green Biorefinery' pilot plant. For this process, the performances of six different nanofiltration membranes were compared by experiments in lab scale. In this work the focus was on the separation of the two products, LA and AA. Enhanced differences in the retentions were required to produce two purified process streams, LA enriched permeate and amino acid enriched retentate. In the reference experiment, performed with original solution from the 'Green Biorefinery' pilot plant, the retention values were about 60% for LA, and about 88% for AA, this hindered good performance in the separation of the main components. Process optimization with pH value variations and different diafiltration-modes were investigated; one experiment was done with original solution, two tests dealt with varying pH-values, two with different diafiltration rates. A pH-variation from 3.9 (reference solution) down to 2.5 transferred the chemical structure of LA, which reduced the retention of the LA significantly from 67% to 42% for the membrane DL (Osmonics). Beside the separation, further attention was given to the flux behaviour. All screening scenarios were compared with a reference experiment done with original solution and standard process parameters as used in the plant itself to evaluate the efficiency trends shown in the tests. It was shown that a nanofiltration unit allowed the separation of sufficient degree for further treatment technologies between AA and LA, a membrane screening for the optimization of this process ensured best performance in practice. (C) 2011 Elsevier B.V. All rights reserved.