this paper evaluates the pore blocking mechanism of two ultrafiltration membranes with different geometries (tubular and spiral wound) when dairy wash water is filtered. the work evaluated the effect of transmembrane pressure and the cross-flow velocity with a 32 experimental design using classical hermia's models and the resistance-in-series model. the resistance-in-series analysis identified external fouling on the surface of both membranes, but the tubular membrane showed higher reversible fouling, while the reversible and irreversible resistances of the spiral wound membrane averaged 41% and 45%, respectively. cake formation is the model that best represents ultrafiltration in the spiral wound membrane at all transmembrane pressures and cross-flow velocities. in the tubular membrane, at the lowest cross-flow velocity (0.79 m.s-1), the cake formation model fitted the experimental data best. however, at higher cross-flow velocities (1.42 and 2.23 m.s-1), the best fit of flux data was obtained with the complete pore blocking model. in addition, the tubular membrane apparently had a two-step pore blocking: "pore blocking" as the initial governing mechanism followed by "cake formation".