Novel approach for membrane modification to improve separation performance and antifouling stability was proposed. It involves using of 0.1-0.3 wt.% aqueous solutions of Praestol 859 (cationic polyelectrolyte based on copolymer of acrylamide and 2-acryloxyethyltrimethylammonium chloride) as coagulants upon membrane preparation via non-solvent induced phase separation. It was shown that the addition of small amounts (0.1-0.3 wt.%) of Praestol 859 into the coagulation bath leads to an increase in membrane pure water flux from 51 up to 68 L m(-2) h(-1) without decreasing membrane retention. Contact angle for modified membranes decreased from 64 degrees down to 55 degrees. Immobilization of Praestol 859 on the surface of a selective membrane layer was confirmed by FTIR spectroscopy. It was found that the addition of Praestol 859 into the coagulation bath suppressed the macrovoid formation in the membranes supporting layer due to the decrease of "solvent-non-solvent" exchange rate which is attributed to the significant increase of viscosity of the coagulation bath. The separation performance of modified membranes for fractionation of thermomechanical pulp mill process water, for concentration and purification of hemicellulose for further processing was studied. It was found that membrane modification by Praestol 859 leads to 2-6 times increase of flux, increase of fouling recovery ratio and improvement of cleaning efficiency without decreasing membrane rejection with regard to hemicelluloses (91.5-93%) and lignin (21-22%) as reference components. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.