The effects of biofilm development on ultrafiltration membranes with regard to permeate stability and permeation rates were investigated using Gravity Driven Membrane (GDM) filtration. The first part of the study aimed at evaluating the influence of the biofilm on permeate flux quality and quantity with regard to Assimilable Organic Carbon (AOC) degradation. In addition, two types of biological pre-treatments were evaluated: slow sand filtration and packed bed bio-reactor, compared to a control (no treatment). Biofilm formation helped to decrease the AOC content of permeate water, compared to the influent. Both pre-treatments additionally reduced the AOC level in the permeate and thus increased its biological stability, however none of the systems were able to guarantee microbiologically stable water. Removal of AOC before the GDM filtration reduced the biofilm growth potential, which in turn influenced its physical structure and enhanced the permeation rates. Influence of inorganic particle removal by pre-sedimentation and its effect on biofilm structure were also studied. Pre-sedimentation of particle populations selected fine and homogeneous particle fractions, which led to the formation of a homogeneous biofilm structure characterised by an increased hydraulic resistance. This was clearly visible between horizontally and vertically installed membranes where the latter ones had a significantly reduced flux despite lower deposited particle mass. The presence of larger, heterogeneous particle fractions counterbalanced the negative effects of the fine particles, which overall resulted in enhanced permeation rates.