A better understanding of large spiral wound forward osmosis (SW FO) module operation is needed to provide practical insight for a full-scale FO practical implementation desalination plant. Therefore, this study investigated two different 8″ SW FO modules (i.e. cellulose tri acetate, CTA and thin film composite, TFC) in terms of hydrodynamics, operating pressure, water and solute fluxes, fouling behaviour and cleaning strategy. For both modules, a significantly lower flow rate was required in the draw channel than in the feed channel due to important pressure-drop in the draw channel and was a particularly critical operating challenge in the CTA module when permeate spacers are used. Under FO and pressure assisted osmosis (PAO, up to 2.5 bar) operations, the TFC module featured higher water flux and lower reverse salt flux than the CTA module. For both modules, fouling tests demonstrated that feed inlet pressure was more sensitive to foulant deposition than the flux, thus confirming that FO fouling deposition occurs in the feed channel rather than on the membrane surface. Osmotic backwash combined with physical cleaning used in this study confirmed to be effective and adapted to large-scale FO module operation.