TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Method: The ammonium content or the ammonia content in the fermentation substrate is measured for determining the second ammonium-nitrogen concentration. The first ammonium-nitrogen concentration in the substrate (S1) is determined. The ammonium content or the ammonia content in the substrate (S1) is measured for determining the first ammonium-nitrogen concentration. The first ammonium-nitrogen concentration is reduced to a predetermined value by heating the substrate (S1) before adding to the fermentation substrate. The given value in dependence of the second ammonium nitrogen concentration as well as the quantity of the fermentation substrate and the quantity of the substrate (S1) is determined. Waste gases produced by combustion of biogas are used for heating the substrate. A third ammonium-nitrogen concentration of the further substrate (S2) is less than the first ammonium nitrogen concentration. The further substrate (S2) is essentially made from renewable plant raw materials and biogenic residues. A self-discharging sediment is suspended by a reel agitator (4) in the fermented substrate for producing the fermentation residue from the fermentation substrate. The fermentation residue is removed by a pump from a fermentation reactor. A solid material containing in the fermentation residue is separated by microfiltration, preferably ultrafiltration. The separated solid material is composted or pelletized. During the reverse osmosis procedure a first reverse osmosis and subsequently a second reverse osmosis are carried out. A part of a residual liquid remaining after the removal of solid materials is removed from the fermentation residue and is used as liquid fertilizer. The remaining liquid is cleaned by reverse osmosis procedure and a formed further residual fluid is used as liquid fertilizer. The volume of removed residual liquid and/or further residual liquid is added to the fermentation substrate. Water contained in the substrate (S1) or other substrate (S2) is used as liquid. The remaining difference-volume is adjusted by adding additional liquid, preferably fresh water, residual water or waste water. EXAMPLE - No suitable example given.