A bioreactor having improved gas yield is described, wherein the necessary residence time of the biomass in the digestion tank is reduced. During the fermentation of dry, i.e. non-pumpable biomass, percolating juices, so-called percolate, is generated as a result of the moisture contained in the biomass, which percolate is withdrawn via a drainage system and, if necessary, recirculated from the top onto the biomass to be fermented. It has now been found that the biomass yield is significantly increased—in the region of between 10% and 40%—when the resultant percolate is not immediately withdrawn by way of the drainage system, but is accumulated in the digestion tank up to a specific level. This is achieved by a technical device wherein the digestion tank is liquid-tight, i.e. also the flap for loading and unloading the digestion tank must be liquid-tight and also be designed in a correspondingly solid manner in order to withstand the resultant liquid pressure. By linking the existing percolate drainage system with a percolate control system it is possible to adjust and control the liquid level of the percolate in the fermenting biomass in such a way that the biogas generation rate or the biogas yield is maximized.