A device and a method are provided for blood treatment in a single-needle operating mode. The blood treatment device has an extracorporeal blood circuit comprising a blood supply line leading to the inlet of a blood treatment unit and a blood return line leading away from the outlet of the blood treatment unit. A blood collecting container is provided in the blood return line, and is connected to an air reservoir by a flow line. The flow connection comprises two line branches, one branch being used to transfer gas from the air reservoir into the blood collecting container and the other branch being used to transfer gas from the blood collecting container into the air reservoir. The connection branch for transferring gas from the air store into the blood collecting container can contain a compressor.
