NOVELTY - The method for treating fermentable organic substances by anaerobic fermentation of the substance in a fermenter (10) and an equipment for mixing the fermentation material, comprises supplying fresh fermentation material to a mixing tank (14) in which the fermentation material is mixed, optionally minced and mixed to a viscous pulp, and supplying the mixture first to a fermenter for further fermentation, where the fermentation material after a residual time, is conveyed out of the mixing tank and fed to the fermentation tank by means of a vacuum pressure pumping system (20).    USE - The method useful for treating fermentable organic substances by anaerobic fermentation of the substance in a fermenter.    ADVANTAGE - The method ensures simple treatment of fermentable organic substances by anaerobic fermentation without using highly explosive gases.    DETAILED DESCRIPTION - The method for treating fermentable organic substances by anaerobic fermentation of the substance in a fermenter (10) and an equipment for mixing the fermentation material, comprises supplying fresh fermentation material to a mixing tank (14) in which the fermentation material is mixed, optionally minced and mixed to a viscous pulp, and supplying the mixture first to a fermenter for further fermentation, where the fermentation material after a residual time, is conveyed out of the mixing tank and fed to the fermentation tank by means of a vacuum pressure pumping system (20). The mixing tank and the fermenter are separated by a container and no direct liquid compound from mixing tank is provided into the fermenter. The fermentation material in the ring-shaped mixing tank is hydrolyzed before pumping into the fermenter for further fermentation. The fermentation material is mixed with 20-35% with dry substance in the mixing tank to form the viscous pulp before pumping into the fermenter. Only a fraction of the mixed fermentation material present in the ring-shaped mixing tank is removed and pumped into the fermenter. The mixing tank is filled partially with the fermentation material during the operation of the arrangement. The mixing tank to be filled with fresh fermentation material is pumped from a stock in a tank and initially pumped from the tank into the mixing tank and/or the pre-treated fermentation material from the mixing tank is pumped into the tank and then pumped into the fermenter and/or the fermentation material is pumped from the fermenter to the tank and then pumped from the tank into a final disposal site. An INDEPENDENT CLAIM is included for a device for treating fermentable organic substances.    DESCRIPTION OF DRAWING(S) - The figure shows a top view of a device for treating fermentable organic substances.    Fermenter (10)    Cogeneration unit (13)    Mixing tank (14)    Vacuum pressure pumping system. (20)