Removal of bacteria and reduction of the assimilable organic carbon (AOC) are two of the main goals in drinking water treatment. In a gravity‐driven membrane (GDM) system the bacteria are retained by an ultrafiltration membrane while the biofilm on top of the membrane reduces the AOC concentration. But with the ageing of the biofilm, the AOC concentration in the permeate increases and the drinking water quality deteriorates (Derlon et al., 2014). In this thesis, the influence of multiple GDMs in series on the permeate quality is investigated. An experiment with five GDMs in series, fed with a 5 mg C L‐1 acetate feed, is conducted. The permeate after each GDM was analyzed for its acetate concentration, total organic carbon (TOC) concentration and total cell count (TCC). The biofilm on each GDM was analyzed for its TOC, TCC and adenosine triphosphate (ATP) content as well as its biofilm resistance. The mean biofilm thickness was measured with optical coherence tomography. The results of the experiment showed a decrease of TCC in the permeate at the time of sampling, as well as after three days of incubation at 30 °C, over the number of GDMs. The TOC, TCC and ATP in the biofilm decreased over the number of GDMs. So did the biofilm resistance and the mean biofilm thickness. These results indicate a decrease of AOC over the number of GDMs. No flux stabilization over the five GDMs in series was observed. The flux varied between 3 L m‐2 h‐1 and 4 L m‐2 h‐1. This flux is higher than expected and can be explained by the low biofilm resistance of the GDM 2 to 5. Biofilm 1 displays by far the highest biofilm resistance to flux. Overall, multiple GDMs in series result in a decrease of TCC in the permeate and therefore in an increase of the permeate quality. The flux obtained was higher than expected. Hence, multiple GDMs in series should be further investigated for drinking water treatment.