NOVELTY - The system for treating a fluid degradable-mass that is formed by organic substances, waste of a food processing industry e.g. fish waste and fluid degradable-additives e.g. water and is biodegradable for production of biogas, comprises a floating layer (35) placed at a fluid level (33) in the biodegradable process, and an extracting mechanism (31) for separation of a part of the floating layer at the fluid level and for removal of a part of the float layer of the fluid degradable-mass. The liquid displacement body lies concentrically to a cylindrical container (17).    USE - System for treating a fluid degradable-mass formed by organic substances, waste of a food processing industry e.g. fish waste and fluid degradable-additives e.g. water, useful in a plant for production of biogas (claimed).    ADVANTAGE - The system provides high yield production of biogas in less time and less energy consuming manner.    DETAILED DESCRIPTION - The system for treating a fluid degradable-mass that is formed by organic substances, waste of a food processing industry e.g. fish waste and fluid degradable-additives e.g. water and is biodegradable for production of biogas, comprises a floating layer (35) placed at a fluid level (33) in the biodegradable process, and an extracting mechanism (31) for separation of a part of the floating layer at the fluid level and for removal of a part of the float layer of the fluid degradable-mass. The liquid displacement body lies concentrically to a cylindrical container (17) for the admission of the degradable-mass, and has a ring cross section in its buoyancy direction. The extracting mechanism comprises an arrangement for partial rising of the fluid level of the degradable-mass. The arrangement causes a turbulence of a partial region of the degradable-mass below its fluid level and comprises a liquid displacement body and a mechanism for lowering the liquid displacement body. The density of the displacement body is set in such a manner that a volume of the liquid displacement body does not swim over the fluid level. A surface of the liquid displacement body protruding from the degradable-mass corresponds to a quarter of a free fluid level surface of the degradable-mass. The mechanism is adjusted in such a manner that the liquid displacement body remains constant during sinking partly over the fluid level. The mechanism comprises a valve (41). A line (39) flowing in a reception locks or opens in the admission below the fluid level for the substance and/or the degradable-additive and is rigidly coupled with the liquid displacement body in such a manner that it opens the line by sinking the liquid displacement body. The valve is implemented as check valve, where its locking force is provided by the sinking of the liquid displacement body, and freely unlocks during excess of a pre-determined internal pressure in the line. The arrangement comprises a metering pump for supplying additional substance and/or additional degradable-additive below the fluid level, where the pump causes a short term pressure increase in the line for releasing the valve. The extracting mechanism comprises a stripper, at which a part of the floating layer flows during the separation of the degradable-mass. The arrangement is coordinated on the stripper in such a manner that the fluid level is slightly raised on activation of the arrangement over an overflow of the stripper. The stripper is positionally fixed with respect to the reception. The stripper is the free upper rim of a circulating wall of the container that forms the reception. The container is concentrically surrounded by an external container, which forms a part of a housing of the system and is closed by a funnel-shaped roof section for collecting raising biogas. The container and the external container limit an outlet reception surface for the floating layer separated from the degradable-mass. The outlet reception surface comprises a discharge for the separated intermediate layer. The extracting mechanism comprises a suction mechanism, which extracts the floating layer to a pressure sink, and a movable stripper above the fluid level. INDEPENDENT CLAIMS are included for:    (1) a plant for the production of bio gas; and    (2) a method for treating a fluid degradable-mass.    DESCRIPTION OF DRAWING(S) - The diagram shows a cross-section of a biogas plant.    Container (17)    Extracting mechanism (31)    Fluid level (33)    Floating layer (35)    Line (39)    Valve. (41)