In precoat filter aid filtration the precoat layer can be built up also by crossflow filtration. For the subsequent separation task. the resistance and separation properties of the precoat layer are important. Using the crossflow mode, the resistance and separation properties of the developed precoat layer are highly determined by the operating conditions during the cake build-up. In this study, the effects of operating parameters such as crossflow velocity (CFV) and filter aid concentration on the properties of precoat layers built up during crossflow filtration were investigated. The filtrate rate was found to increase with decreasing CFV and decreasing filter aid concentration. A specific asymmetrical precoat layer structure could be generated during the crossflow filtration. This structure could cause better separation efficiency and higher filtration flux or longer operating period. A theoretical model was established to simulate the precoat layer build-up process that can be used for later optimization calculation. The particle selective depositing effect during crossflow filtration was discussed in detail. With the process specific parameters of K and beta evaluated from experiments. the characteristics of precoat layers built up under crossflow condition could be predicted by the theoretical model. (C) 2002 Elsevier Science B.V. All rights reserved.