TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: The chemical, biological and/or physical properties of the percolate: are controlled in the liquid fermenter in such a way that they are maintained at predetermined value and an optimal production of biogas production is reached in the liquid fermenter; are controlled by controlling the frequency, volume and/or duration of percolation of the organic solids in the solid fermenter. The percolation of the organic solids in the solid fermenter is raised when reaching a certain pH, a certain temperature, a certain concentration of the dry matter of the percolate in solid fermenter and/or in the presence of a certain organic acids/total inorganic carbon-ratio of the percolate in solid fermenter. The concentration (0.5-5 wt.%, preferably 2 wt.%) of the dry substance in the percolate in the liquid fermenter is maintained by a corresponding regulation of the percolation of the organic solids in the solid fermenter. The regulation of chemical, biological and/or physical properties of the percolate in liquid fermenter is controlled by adjusting the ratio of the volume of the percolate in liquid fermenter to the base of the active solid fermenter at the same time periodic percolation, where the ratio of the volume of liquid fermenter to the base of the active solid fermenter is 0.5-1.5, preferably 0.8-1.3. The solid fermenter is provided for the digestion of organic solids. A fermentation residue is squeezed out after the completion of fermentation process in the solid fermenter for recovering the percolate contained in the fermentation residue, and the squeezed-out percolate is supplied back to the liquid fermenter. The fermentation residue is blow through after the completion of the fermentation process in the solid fermenter for sanitizing and/or drying with blow hot air, preferably for producing a sanitized compost.    TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: The liquid fermenter is connected with an overflow tank (6), and liquid substrates are introduced in the overflow tank. The controlling unit comprises a sensor (7) to measure the biological, chemical and/or physical properties of the percolate in the liquid fermenter, and an adjustable percolate valve and/or a controllable percolate pump for controlling the percolation of organic solid absorbed in the solid fermenter such that the properties of the percolate in liquid fermenter for the production of biogas are optimized. A press unit is provided for pressing the fermentation residue such that the fermentation residue of 60-80% is pressed after completion of the fermentation containing the percolate. A sanitizing device is provided by which hot air is blown into the fermentation residue for sanitizing the fermentation residue.