Modelling of a forward osmosis-nanofiltration (FO-NF) integrated treatment system for treating hazardous wastewater has been done with model validation against experimental data of a semi-pilot unit using live wastewater from a leather industry. The flat sheet cross flow forward osmosis (FO) membrane module ensured near 100% rejection of chromium while total pollution load of the wastewater in terms of chemical oxygen demand (COD), chlorides and sulphates came down by 98%. A reasonably high water flux of about 48 L/m(2) h (LMH) could be sustained in the FO module. The coupled downstream NF system recovered the draw solute NaCl by more than 98% for recycling in the FO loop. That the developed model is capable of predicting system performance quite accurately is reflected in low relative error (<0.1), high overall correlation coefficient (R-2 > 0.98) and high Wilmot d-index (>0.95). The model will help assess operating conditions, system design and hence industrial scale up. (C) 2019 Elsevier Ltd. All rights reserved.