An apparatus for carrying out a three-phase separation during treatment of wastewater and sludge, which includes a gas collector (1) which traps gases that rise in the settling basin during the treatment process. The gas collector has an inclined outer surface (2, 3) which ensures that sewage sludge can slide down unhindered. The angle of inclination () can be adjusted in order to adapt the apparatus to different sewage sludges. The gas collector has a flexible tube (4) which is guided around a plurality of reversing or bending surfaces (5, 6, 7, 8) so that sections (10, 11) of the tube form the inclined outer surfaces (2, 3). Another section (15) of the tube defines the interior volume (V) of the gas collector. The resulting gas collector is simple to produce and effectively prevents the separation apparatus from becoming clogged by particles of sewage sludge adhering to the outer surfaces of the gas collector.