The feasibility of Forward Osmosis (FO) as an alternative treatment technology to current membrane processes is believed to hinge on its reported lower fouling propensity. In this study, the impacts of constant osmotic pressure and hydraulic pressure driving forces on membrane fouling were investigated using a novel approach. In each case the cake layer was modelled accounting for all concentration polarisation effects and effective driving force. Compared to the widely employed method of using a non-constant osmotic pressure difference during bench-scale fouling experiments, maintaining a constant osmotic pressure led to 50% more alginate deposited on the same membrane surface (from 13.7 to 21.7 g/m2). This was attributed to a stronger osmotic driving force at the active layer interface and enhanced fouling due to a greater reverse flux of Na+ ions. An applied hydraulic pressure of 1 bar already changed fouling cake deposition and the cake structural parameter shrunk by 224 and 83 μm for the two thin-film composite membranes tested. A detailed analysis of the model however demonstrated that it needs further development, incorporating pore size, porosity and tortuosity of the foulant cake to enable drawing reliable conclusions on the causality of cake layer compaction.