The most distinctive characteristic of the Mediterranean climate is its strong seasonality, dry summers and intense rainfall episodes concentrated in spring and autumn. This changeability is translated into a wide variability of the water resources in terms of quantity and quality. Barcelona, located in the northeast extreme of the Iberian Peninsula, shows all the characteristics of the Mediterranean climate. The Llobregat River, which is the main conventional water source for drinking water production in Barcelona region, presents a poor and variable quality as well as high flow variability. This study is focused on testing ultrafiltration (UF) membranes, based on a submerged outside-in configuration, as direct pre-treatment of such water at demonstration scale. A prototype with Zeeweed® 500D membranes was optimised for the different scenarios tested during 2 years of continuous operation. It was demonstrated to be able to stand the extreme feed quality limits, obtaining water yields in the range of 89–92% in all conditions used. The prototype worked better in terms of water production yield at high turbidity episodes (>70 NTU), probably due to a layer formation which prevented organic species and particles reaching the membrane. Permeate water was shown, in spite of feed water quality fluctuations, to be constant and appropriate in quality terms. Water quality results obtained are compared to the ones obtained in a conventional pre-treatment. The preliminary results of a technical comparison between direct UF and conventional pre-treatment are also detailed in the study.