Abstract: A carbon dioxide-neutral biodigester plant includes a biodigester for carbon dioxide-neutral, single-stage or multi-stage production of biogas by fermentation of biomass in a fermentation liquid agitated in presence of elemental hydrogen, hydrogenotrophic and methanogenic archaea, and activated carbon masses, a source of ammonia connected to a first ammonia line, an ammonia cracker to produce hydrogen or a hydrogen-nitrogen mixture by catalytic cracking of ammonia, a first hydrogen line for feeding the produced hydrogen or the hydrogen-nitrogen mixture directly into the biodigester, and a treating device for treating activated coal masses with hydrogen at higher pressure. The treating device includes a pressure line and a treating spray lance. Hydrogen-containing activated coal masses configured as spray cones are injected into the fermentation liquid of the biodigester.