The hybrid process coagulation and low pressure membrane filtration for the direct treatment of surface water became a generally accepted rule of technology in recent years. The objective of this work was to contribute to the understanding of fouling layer formation by porous floc aggregates in inside-out driven capillaries. A computational fluid dynamics (CFD) model was developed for the description of a complete fluid flow field in capillaries of arbitrary cylindrical geometries and alignments. It covers user-defined physical properties of flocs and membrane materials and is valid for different chosen operation conditions. This was done by numerical calculations of the Navier-Stokes and continuity equations. Floc velocities and trajectories were derived by balancing the forces and torques acting on the flocs, determining places of preferential deposition, based on numerical calculations of the convection and diffusion equation and the influence of the membrane wall. The models were coupled and eventually used to account for the growing fouling layer height and its influence on the resulting fluid flow field and floc trajectories, delivering a dynamic fouling layer formation over time. New aspects could be derived qualitatively to understand the formation of fouling layers from which recommendations for appropriate operation conditions were concluded.