A method for filling and/or emptying flexible containers, in particular bags, of, in particular hazardous, loose substances, preferably in powder form, the method being characterized by the method steps of: - folding a container edge over a radially deformable, disposable fastening ring, in particular from the radial inside to the outside; - inserting, in particular manually inserting, the disposable fastening ring into an end portion of a filling and/or emptying neck of a filling and/or emptying device, the disposable fastening ring thus being arranged substantially concentrically with the filling and/or emptying neck and a radially outer fold-over section of the container edge being arranged and/or clamped between an inner surface of the filling and/or emptying neck and a radially outer surface of the disposable fastening ring; - pressing, in particular mechanically pressing, the disposable fastening ring into a sealing section of the filling and/or emptying neck disposed downstream of the end portion in the axial direction and preferably having a smaller inner diameter than the end portion, the pressing of the disposable fastening ring into the inside of the filling and/or emptying neck leading to a radial deformation and to an opposite contact force, in particular an elastic restoring force, which presses the radially outer fold-over section of the container edge against the inner surface of the filling and/or emptying neck, wherein the radially outer fold-over section of the container edge is pressed against the sealing section of the filling and/or emptying neck and seals the container against the filling and/or emptying neck in the process; - filling or emptying the container by adding or discharging the substance through the filling and/or emptying neck; - sealing and detaching the filled or emptied container; - sealing the fold-over section, the disposable fastening ring preferably being completely enclosed by the fold-over section. - folding a container edge over a radially deformable, disposable fastening ring, in particular from the radial inside to the outside; - inserting, in particular manually inserting, the disposable fastening ring into an end portion of a filling and/or emptying neck of a filling and/or emptying device, the disposable fastening ring thus being arranged substantially concentrically with the filling and/or emptying neck and a radially outer fold-over section of the container edge being arranged and/or clamped between an inner surface of the filling and/or emptying neck and a radially outer surface of the disposable fastening ring; - pressing, in particular mechanically pressing, the disposable fastening ring into a sealing section of the filling and/or emptying neck disposed downstream of the end portion in the axial direction and preferably having a smaller inner diameter than the end portion, the pressing of the disposable fastening ring into the inside of the filling and/or emptying neck leading to a radial deformation and to an opposite contact force, in particular an elastic restoring force, which presses the radially outer fold-over section of the container edge against the inner surface of the filling and/or emptying neck, wherein the radially outer fold-over section of the container edge is pressed against the sealing section of the filling and/or emptying neck and seals the container against the filling and/or emptying neck in the process; - filling or emptying the container by adding or discharging the substance through the filling and/or emptying neck; - sealing and detaching the filled or emptied container; - sealing the fold-over section, the disposable fastening ring preferably being completely enclosed by the fold-over section.