Process for fermenting a free-flowing substrate with a given dry substance content, using a reactor, the process having at least the following process steps: - supplying the substrate via at least one feed inlet of the reactor, - conveying the substrate through the reactor, the reactor having a plurality of partitions which divide at least the internal volume of the reactor intended for the substrate into a plurality of compartments (7(i)-7(iv)) and each single compartment (7(i)-7(iv)) in each case into at least two chambers (8(i)-8(iv); 9(i)-9(iv)) passed through by the substrate in opposite directions, - drawing off the fermented substrate via at least one discharge outlet of the reactor, characterized in that • in order to increase or reduce the ratio - of the volume of the chambers (8(i)-8(iv)) passed through by the substrate in one direction - to the volume of the chambers (9(i)-9(iv)) passed through by the substrate in the other direction at least some of the partitions are moved with regard to their spatial location and/or position and/or extension, the movement and/or extension of the partitions being controlled depending on the dry substance content of the free-flowing substrate, • each compartment (7(i)-7(iv) has an internal volume intended for the substrate and an assigned gas space (5(i)-5(iv)), • at least one gas space of at least one compartment, selected from all the gas spaces (5(i)-5(iv)) of all the compartments (7(i)-7(iv)), is formed separately from the remaining gas space or the remaining gas spaces (5(i)-5(iv)) of the remaining compartments (7(i)-7(iv)), and the process having at least the following further process steps: • drawing off process gas from the at least one gas space, formed separately from the remaining gas space or the remaining gas spaces (5(i)-5(iv)) of the remaining compartments (7(i)-7(iv)), • supplying the previously drawn-off process gas and/or foreign gas into the internal volume of at least one of the formed compartments (7(i)-7(iv)), which is intended for the substrate. - supplying the substrate via at least one feed inlet of the reactor, - conveying the substrate through the reactor, the reactor having a plurality of partitions which divide at least the internal volume of the reactor intended for the substrate into a plurality of compartments (7(i)-7(iv)) and each single compartment (7(i)-7(iv)) in each case into at least two chambers (8(i)-8(iv); 9(i)-9(iv)) passed through by the substrate in opposite directions, - drawing off the fermented substrate via at least one discharge outlet of the reactor, • in order to increase or reduce the ratio - of the volume of the chambers (8(i)-8(iv)) passed through by the substrate in one direction - to the volume of the chambers (9(i)-9(iv)) passed through by the substrate in the other direction at least some of the partitions are moved with regard to their spatial location and/or position and/or extension, the movement and/or extension of the partitions being controlled depending on the dry substance content of the free-flowing substrate, - of the volume of the chambers (8(i)-8(iv)) passed through by the substrate in one direction - to the volume of the chambers (9(i)-9(iv)) passed through by the substrate in the other direction • each compartment (7(i)-7(iv) has an internal volume intended for the substrate and an assigned gas space (5(i)-5(iv)), • at least one gas space of at least one compartment, selected from all the gas spaces (5(i)-5(iv)) of all the compartments (7(i)-7(iv)), is formed separately from the remaining gas space or the remaining gas spaces (5(i)-5(iv)) of the remaining compartments (7(i)-7(iv)), • drawing off process gas from the at least one gas space, formed separately from the remaining gas space or the remaining gas spaces (5(i)-5(iv)) of the remaining compartments (7(i)-7(iv)), • supplying the previously drawn-off process gas and/or foreign gas into the internal volume of at least one of the formed compartments (7(i)-7(iv)), which is intended for the substrate.