Abstract: A difference in pressure is generated between the two sides of a filter medium (1) so that the liquid (2) charged with biopolymers (3), for instance xanthan, that is fed through the inlet (5) goes through the filter medium (1), wherein the biopolymers (3) are retained by the filter medium (1). As opposed to crossflow filtration, the main direction of movement (4) of the liquid (2) is determined by the difference in transmembrane pressure. A cathode (7) is arranged under the filter membrane (1). A membrane serving as anode (8) is arranged on the opposite side of the process chamber (9). An electrical field is built up between the electrodes (7, 8).