Effect of lithium chloride additive on forward osmosis membranes performance

•Thin film composite forward osmosis (FO) membranes were synthesized.•Addition of Lithium chloride to the casting solution increases water flux and reverse salt flux.•PSU and PESU have the highest water flux when the active layer facing the feed solution (AL-FS).•All membranes possessed highly asymmetric porous structures with atop dense upper layers and finger-like porous substrate.•Higher water flux was observed under AL-DS orientation compared to AL-FS orientation for all types of membranes.