NOVELTY - The process comprises creating a solid-liquid phase mixture (5) of organic solid and/or liquid substrates (2a, 2b) by adding water, performing biological treatment of the solid-liquid phase mixture within fermenters (3a, 3b) consecutively connected in series until the last fermenter obtains a stabilized digestate with a low residual gas potential, where the biological treatment includes anaerobic fermentation process and aerobic fermentation process using biologically active microorganisms, and separating a partial flow of volume flow discharged from the last fermenter.    USE - The process is useful for producing biogas from organic substrates.    ADVANTAGE - The process ensures effective, economical and simple production of biogas with reduced energy consumption and high yield.    DETAILED DESCRIPTION - The process comprises creating a solid-liquid phase mixture (5) of organic solid and/or liquid substrates (2a, 2b) by adding water, performing biological treatment of the solid-liquid phase mixture within fermenters (3a, 3b) consecutively connected in series until the last fermenter obtains a stabilized digestate with a low residual gas potential, where the biological treatment includes anaerobic fermentation process and aerobic fermentation process using biologically active microorganisms, separating a partial flow of volume flow discharged from the last fermenter and separating the partial flow of the stabilized digestate into a liquid and a solid phase (11), transferring a portion of the volume flow of the resulting liquid phase for the biological treatment of other untreated organic substrates directly or indirectly through one of the fermentation tanks of upstream slurry containers in one of the fermenters, intermittently transferring the resulting volume flow of the solid phase to a thermal treatment for thermal disintegration, transferring the thermally treated volume flow of the solid phase for renewed biological treatment through the fermentation tank of the upstream containers in one of the fermenters, and extracting non-heat-treated solid-phase from the system to avoid the accumulation of the thermal treatment-resistant fractions in the system. The step of transferring the thermally treated volume flow of the solid phase is carried out at time intervals, which are determined by a simulation model. An autoclave: is used for the thermal treatment; using residual and/or excess heat "Q" is operated from a cogeneration unit; and is operated at a temperature of 170 degrees C and a pressure of 0.8 MPa. An INDEPENDENT CLAIM is included for a device for producing biogas from organic substrates.    DESCRIPTION OF DRAWING(S) - The figure shows a flow chart of a method for producing biogas from organic substrates by biological treatment.    Biogas (1)    Organic substrates (2a, 2b)    Fermenters (3a, 3b)    Solid-liquid phase mixture (5)    Solid phase. (11)