A bioreactor (100, 200, 400) for growing micro-organisms in a reaction mixture (102A, 102B) comprising a reaction medium and micro-organisms, the bioreactor comprising - a first reaction chamber for containing the reaction mixture when in operation, wherein the first reaction chamber has - a uniform cross-section and a first volume for containing a first number of micro-organisms, - a first input for providing the reaction mixture to the first reaction chamber, and - a first output for removing excess gases from the first reaction chamber; - a second reaction chamber, arranged downstream from the first reaction chamber, for containing the reaction mixture when in operation, wherein the second reaction chamber has - a uniform cross-section and a second volume for containing a second number of micro-organisms, the second number of micro-organisms being larger than the first number of micro-organisms, - a second input for providing gases to the second reaction chamber, and - a second output for removing the reaction mixture from the second reaction chamber; - a single means for connecting (116, 206, 208, 410) the first reaction chamber to the second reaction chamber, the means for connecting, when in operation, allowing the reaction mixture to flow from the first reaction chamber to the second reaction chamber and the gases to flow from the second reaction chamber to the first reaction chamber; wherein - the means for connecting is the sole input for the gases to the first reaction chamber and the sole input for the reaction mixture to the second reaction chamber, - a cross-sectional area of the means for connecting is at most 5 % of a cross-sectional area of one of the first reaction chamber or of the second reaction chamber, and - the bioreactor further comprises a collector arranged outside of and on the first reaction chamber and coupled to the first input and the first output, the collector comprises an inlet/outlet to allow the reaction mixture to flow into the first reaction chamber and the excess gases to exit from the first reaction chamber, and the collector is configured to be filled with the reaction mixture to a pre-defined level and with the excess gases on top of the reaction mixture. - a first reaction chamber for containing the reaction mixture when in operation, wherein the first reaction chamber has - a uniform cross-section and a first volume for containing a first number of micro-organisms, - a first input for providing the reaction mixture to the first reaction chamber, and - a first output for removing excess gases from the first reaction chamber; - a uniform cross-section and a first volume for containing a first number of micro-organisms, - a first input for providing the reaction mixture to the first reaction chamber, and - a first output for removing excess gases from the first reaction chamber; - a second reaction chamber, arranged downstream from the first reaction chamber, for containing the reaction mixture when in operation, wherein the second reaction chamber has - a uniform cross-section and a second volume for containing a second number of micro-organisms, the second number of micro-organisms being larger than the first number of micro-organisms, - a second input for providing gases to the second reaction chamber, and - a second output for removing the reaction mixture from the second reaction chamber; - a uniform cross-section and a second volume for containing a second number of micro-organisms, the second number of micro-organisms being larger than the first number of micro-organisms, - a second input for providing gases to the second reaction chamber, and - a second output for removing the reaction mixture from the second reaction chamber; - a single means for connecting (116, 206, 208, 410) the first reaction chamber to the second reaction chamber, the means for connecting, when in operation, allowing the reaction mixture to flow from the first reaction chamber to the second reaction chamber and the gases to flow from the second reaction chamber to the first reaction chamber; - the means for connecting is the sole input for the gases to the first reaction chamber and the sole input for the reaction mixture to the second reaction chamber, - a cross-sectional area of the means for connecting is at most 5 % of a cross-sectional area of one of the first reaction chamber or of the second reaction chamber, and - the bioreactor further comprises a collector arranged outside of and on the first reaction chamber and coupled to the first input and the first output, the collector comprises an inlet/outlet to allow the reaction mixture to flow into the first reaction chamber and the excess gases to exit from the first reaction chamber, and the collector is configured to be filled with the reaction mixture to a pre-defined level and with the excess gases on top of the reaction mixture.