An apparatus for mixing a first liquid and a second liquid, said apparatus comprising: (a) a static mixer having - a first inlet for receiving the first liquid, - a second inlet for receiving the second liquid, and - an outlet for discharging a third liquid that results from mixing the first liquid and the second liquid; (b) a first feed module (6, 14) for providing the first liquid to the first inlet; (c) a second feed module (6, 14) for providing the second liquid to the second inlet; wherein each of the first and the second feed module (6, 14) comprises: - a substrate chamber (7, 15) for holding a flexible container (8, 16, 40), said flexible container having an interior space for holding the first liquid or the second liquid, respectively; - a conduit (10, 18) for providing fluid communication between the interior space of the flexible container and the first or the second inlet of the static mixer, respectively; - a pressure reservoir chamber (11, 19) for holding a pressurised gas; - a connector (12, 20) for providing fluid communication between the pressure reservoir chamber (11, 19) and the substrate chamber (7, 15), wherein said fluid communication is for permitting a flow of pressurised gas from the pressure reservoir chamber (11, 19) to the substrate chamber (7, 15) such as to exert pressure on an external surface of the flexible container (8, 16, 40) and to force the first or second liquid from the container into the conduit; - a means for reversibly interrupting the fluid communication between the pressure reservoir chamber (11, 19) and the substrate chamber (7, 15), said means having an open state and a closed state; characterized in that the connector (12, 20) and said means are arranged for achieving immediate pressure equilibration in less than 2 seconds between the pressure reservoir chamber (11, 19) and the substrate chamber (7, 15) upon changing the state of said means from closed to open. (a) a static mixer having - a first inlet for receiving the first liquid, - a second inlet for receiving the second liquid, and - an outlet for discharging a third liquid that results from mixing the first liquid and the second liquid; - a first inlet for receiving the first liquid, - a second inlet for receiving the second liquid, and - an outlet for discharging a third liquid that results from mixing the first liquid and the second liquid; (b) a first feed module (6, 14) for providing the first liquid to the first inlet; (c) a second feed module (6, 14) for providing the second liquid to the second inlet; - a substrate chamber (7, 15) for holding a flexible container (8, 16, 40), said flexible container having an interior space for holding the first liquid or the second liquid, respectively; - a conduit (10, 18) for providing fluid communication between the interior space of the flexible container and the first or the second inlet of the static mixer, respectively; - a pressure reservoir chamber (11, 19) for holding a pressurised gas; - a connector (12, 20) for providing fluid communication between the pressure reservoir chamber (11, 19) and the substrate chamber (7, 15), wherein said fluid communication is for permitting a flow of pressurised gas from the pressure reservoir chamber (11, 19) to the substrate chamber (7, 15) such as to exert pressure on an external surface of the flexible container (8, 16, 40) and to force the first or second liquid from the container into the conduit; - a means for reversibly interrupting the fluid communication between the pressure reservoir chamber (11, 19) and the substrate chamber (7, 15), said means having an open state and a closed state; characterized in that the connector (12, 20) and said means are arranged for achieving immediate pressure equilibration in less than 2 seconds between the pressure reservoir chamber (11, 19) and the substrate chamber (7, 15) upon changing the state of said means from closed to open.