NOVELTY - The method useful for producing biogas, comprises concentrating a suspension with a low solid content to form a pumpable suspension having an increased solid content of 11 wt.%, introducing the concentrated suspension into a fermenter (5), heating the concentrated suspension at a fermenting temperature of 40-50 degrees C by a heat exchanger (3) with the fermentation residues, and fermenting the concentrated suspension in the fermenter to produce a biogas. The suspension with predetermined dry matter content is continuously pumped to the fermenter.    USE - The method is useful for producing biogas from a suspension including a liquid pig manure or other animal feces and a cow manure or brown water with a low solid content at a stable temperature (all claimed).    ADVANTAGE - The method is capable of producing the biogas from the suspension with high yield thus producing electricity with output of 100 kW.    DETAILED DESCRIPTION - The method useful for producing biogas, comprises concentrating a suspension with a low solid content to form a pumpable suspension having an increased solid content of 11 wt.%, introducing the concentrated suspension into a fermenter (5), heating the concentrated suspension at a fermenting temperature of 40-50 degrees C by a heat exchanger (3) with the fermentation residues, and fermenting the concentrated suspension in the fermenter to produce a biogas. The suspension with predetermined dry matter content is continuously pumped to the fermenter. The method further comprises separating liquid fraction from a device for concentrating the suspension to obtain volatile organic acids and then supplying into the fermenter. The volatile organic acids are obtained by reverse osmosis of the liquid fraction. The biogas is utilized to generate electricity and heat. The heat is used to heat the concentrated suspension, a space and/or hot water. The current is fed into an electric network and/or used to operate a plant. An INDEPENDENT CLAIM is included for a system for producing biogas from a liquid pig manure or other animal feces suspension.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a system for producing a biogas.    Heat exchanger (3)    Inlet (4)    Fermenter (5)    Manure storage (8)    Gas pipe. (10)