A gravity separator unit, such as a dissolved air flotation (DAF) or a sedimentation unit, comprising a container (10) having sidewalls (11,12,13,14) and a floor (20), wherein the floor comprises one or more corrugated pile sheets (21,22,23) which define a plurality of alternating tops (32,34,36) and parallel gutters (31,33,35,37) facing the interior of the container, wherein the tops and lower portions of the gutters are vertically spaced apart by a distance (h) of between 3 and 30 cm from each other. The gravity separator unit is preferably provided with scraping elements with scraping blades which are arranged for pushing sludge collected in the gutters along the longitudinal direction of the gutters, and/or with jet nozzles for ejecting water into the gutters in order to push the sludge along the longitudinal direction of the gutters. A method for manufacturing a gravity separator unit with a floor comprising one or more corrugated pile sheets is disclosed as well.