NOVELTY - The process comprises mixing a fermentation substrate having a dry matter content of 15% with reduced ammonia fermentation liquor having an amount of 2.5 g NH4+/l or less, subjecting the resultant fermentation substrate to a first fermentation during a time period of 1-4 days at a pH of below 6.5, and performing hydrolysis and acidogenesis processes for forming a usable, non-combustible weak first biogas stream with carbon dioxide having a proportion of 0-50% and with predominant fraction of hydrogen sulfide and ammonia.    USE - The process is useful for producing biogas from predominantly animal excrements such as liquid manure or chicken manure by multistage anaerobic conversion.    ADVANTAGE - The process is capable of economically producing biogas from predominantly animal excrements with higher yield and higher methane content and very low fraction of nitrogen.    DETAILED DESCRIPTION - The process comprises mixing a fermentation substrate having a dry matter content of 15% with reduced ammonia fermentation liquor having an amount of 2.5 g NH4+/l or less, subjecting the resultant fermentation substrate to a first fermentation during a time period of 1-4 days at a pH of below 6.5, performing hydrolysis and acidogenesis processes for forming a usable, non-combustible weak first biogas stream with carbon dioxide having a proportion of 0-50% and with predominant fraction of hydrogen sulfide and ammonia, pumping the fermented substrate into a fermenter and then subjecting the fermented substrate to a further fermentation for 20-80 days at a temperature of 35-60 degrees C and a pH value of 7-8, and performing acidogenesis, acetogenesis and methanogenesis processes for forming a second biogas stream with a methane content of 60-75 vol.% and a hydrogen sulfide content, which is lower than the first gas stream. The fermentation substrate is separated into a liquid and a viscous to solid phase, where the viscous phase is transported to the solid phase in the subsequent fermenter and the liquid phase is completely or partially transported to the initial substrate. The reduced ammonia fermentation liquor is: mixed with the fermentation substrate at the temperature of 90-120 degrees C and a pressure of 0.5-2 over the steam pressure; and derived from a different biogas production process. The generated biogas streams, separately or together, are subjected to a further purification and preparation process. The volume of the fermentation residues, used starting from the first fermenter continuously until the last fermenter or secondary fermenter, is reduced to one-half to one-fifth. The reduced ammonia fermentation liquor is stored over a predetermined period to improve the biology. The fermentation substrate is adjusted to the desired dry matter content of 2%. In an arrangement of series-connected fermenters, the last fermenter or secondary fermenter is driven with the highest temperatures. The fermentation substrate is added with an enzyme such as urease to decompose urea to ammonia and carbon dioxide, where the enzyme is separated within the recycle loop as a gaseous component during the treatment of the liquid fermentation substrate.    DESCRIPTION OF DRAWING(S) - The figure shows a flow diagram of a plant for producing biogas (drawing includes non-English language text).    Transport path of the fermentation residue (14)    Metering screw (20)    Line for removal of biogas (21)    Line for forwarding fermentation substrate (22)    Second container. (B2)