A filtration system for filtering a liquid includes a drum filter (2) having a primary inlet (4) and a primary outlet (6) in a housing (8), and a rotatable drum screen (10) arranged in the housing. A back flush element (12) is arranged on one side of the drum filter for back flushing filtered particles from the drum screen, and a gutter element (14) is arranged on an opposite side of the drum screen for collecting filtered particles, wherein the gutter element is connected to a secondary outlet (16) in the housing. The filtration system is furthermore provided with a buffer reservoir (18) having a reservoir inlet (20) which is connected to the secondary outlet so as to be closable. The filtration system further includes compression elements (34) for controlling a gas pressure (Pt) in the housing of the drum filter and a gas pressure (Pr) in the buffer reservoir.