This study investigated long-term stability of forward osmosis (FO) membranes against biodegradation during prolonged exposure to activated sludge. Results show that cellulose triacetate (CTA) membranes were more resistant against biodegradation than polyamide thin film composite (TFC) ones. Nevertheless, CTA membrane biodegradation was discernible after seven months of exposure to activated sludge as manifested by an increase in the membrane average pore size, water and salt permeability, and membrane structural parameter. Asaresult, of prolonged exposure to activated sludge, the water and reverse salt fluxes of the CTA membrane increased; and concomitantly the rejection of a range of trace organic contaminants decreased significantly. The impact of prolonged exposure to activated sludge on the polyamide active layer of TFC FO membranes was even more severe. Our results indicate that currently available commercial CTA and polyamide TFC FO membranes may not be readily compatible for practical osmotic membrane bioreactor (OMBR) operation. Thus, the development of new and robust FO membrane materials specifically designed for membrane bioreactor operation is essential for commercial OMBR applications.