Injecting gas to create a gas-liquid two-phase crossflow operation in ultrafiltration can significantly increase permeate flux and improve the membrane rejection characteristics. Previous study is largely based on the analysis of the experimental data and mass transfer corrections. In this work, an attempt is made to model the slug flow ultrafiltration process using the volume of fluid (VOF) method with the aim of process optimisation by understanding the mechanism behind the permeate flux enhancement. The commercial CFD package, Fluent, is used for this numerical study. Good agreement is obtained between theory and experiment.