Abstraet--A new concept of a biogas reactor for anaerobic waste water treatment is presented. The characteristic features of this type of reactor are the tower shape, its modular structure and the internal installations. Gas-collecting devices are installed at different levels along the height of the reactor to withdraw the gas produced and to avoid gas accumulation in the upper zones of the reactor. The remaining gas causes a fluid circulation along baffles similar to airlift-loop reactors. A settler is integrated at the top of the reactor. As the reactor is built in a tower shape, the mixing behaviour is strongly linked to two questions: (1) how to supplement the microorganisms in the upper zones of the reactor with substrate and (2) whether there is a toxic concentration due to insufficient mixing near the inlet of the substrate. The scale-up of the biogas tower reactor, as far as the liquid mixing is concerned, was based on the knowledge of the mixing within a module and the intermixing between two modules. Two mathematical models are proposed to describe liquid mixing within the reactor. Model A describes the intramixing within one module as well as the intermixing between neighbouring modules. Model B describes only the global intermixing between different modules. Therefore, model A can be used for investigations on the internal concentration profile of a module, e.g. over concentration in the feeding zone of the reactor, while model B is able to calculate the global mixing of a reactor. The experimental studies were performed in the laboratory-and in pilot-scale plants. An excellent agreement between the computer simulation and the experimental results was obtained with both models. The characteristic parameters of the system which include the axial dispersion coefficient Dax, the mean circulation velocity w,, and the exchange flow rate l~cxeha,g e were calculated by means of the mathematical model from the experimental data according to the least-squares method. The mean circulation velocity w,, increases with the enlargement in diameter since the drag coefficient for the circulating flow decreases. The exchange flow rate between two neighbouring modules Vexchange related to the connecting area remains constant during the scale-up.