Abstract: The invention relates to a process for generating biogas, electrical energy and heat starting from biological materials, more precisely a process for the anaerobic fermentation of a flowable substrate using a reactor including at least: an inlet (1), and outlet (3), a plurality of divider walls (6) which divide at least the internal reactor volume provided for the substrate into a plurality of compartments (7 (i)-7 (iv)) and divide each individual compartment (7 (i)-7(iv)) into at least two chambers (8 (i)-8 (iv); 9 (i)-9 (iv)) through which the substrate flows in opposite directions, wherein the process is characterized in that for increasing or reducing a ratio of a volume of the chambers (8 (i)-8 (iv)) through which substrate flows in one direction to a volume of the chamber (9 (i)-9 (iv)) through which substrate flows in the other direction at least some of the divider walls (6) are arranged moveable with respect of their spatial location and/or position and/or extension, wherein the movement and/or exten