An upflow reactor is provided that includes a reactor tank, conduits, a waste water distributor, at least one first flotation separator and an optional second flotation separator for separating reactor water, biomass, and biogas, a collection device, and a gas separator for separating biomass and biogas. The first flotation separator includes one or more gas domes that are connected to the collection device and have outlets, the cross-sectional area or geometry of which can be controlled using movable covers.