NOVELTY - The method for producing methane from process water and biogenic material such as that occurring in the sugar and ethanol production comprises supplying the process water and biomass to a mixer/reservoir (2), producing a slurry optionally with washing and/or fresh water and/or substrate water, where the pH and the temperature of the slurry are adjusted to a suitable value, transferring the slurry into an anaerobic methane bacteria-containing bioreactor to form biogas, extracting the biogas and removing the biodegraded fluid. The biogas in the bioreactor is recirculated.    USE - The method is useful for producing methane from process water and biogenic material.    ADVANTAGE - The method ensures simple production of methane from process water and biogenic material with high quality and quantity.    DETAILED DESCRIPTION - The method for producing methane from process water and biogenic material such as that occurring in the sugar and ethanol production comprises supplying the process water and biomass to a mixer/reservoir (2), producing a slurry optionally with washing and/or fresh water and/or substrate water, where the pH and the temperature of the slurry are adjusted to a suitable value, transferring the slurry into an anaerobic methane bacteria-containing bioreactor to form biogas, extracting the biogas and removing the biodegraded fluid. The biogas in the bioreactor is recirculated. Alkali such as alkaline solution, alkaline wash water from the sugar-ethanol production, lime or ammonia is provided into the bioreactor for further pH-value-increase under the lowering of carbon dioxide portion in the formed biogas. The aerobic ratio is adjusted by aerating the mixer/reservoir and the carbon dioxide is expelled. Normal pressure prevails in the mixer/reservoir and the mixer/reservoir is aerobically operated. The fermentation is carried out by using aerated and/or immobilized bacteria. An INDEPENDENT CLAIM is included for a biogas plant (1) for carrying out the method for producing methane from process water and biogenic material.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic representation of a biogas plant for preparing biogas from sugarcane leaves.    Biogas plant (1)    Mixer/reservoir (2)    Shredders (3)    Conveying system (4)    Fresh air inlet. (6)