NOVELTY - The device (410) has a service section (436) that is sealed to a cover. A separation curtain (440) is extended downwards from the service section. A conduit is provided for fluid communication between the service section and a gas section (6). An isolation valve is provided in the conduit. A port is provided in the service section for withdrawing biogas from the gas section through the service section. The conduit is extended upwards through the elevation of a cover. The device main portion is provided with a vent valve and a purge valve.    USE - Servicing device for tank of anaerobic digester (claimed) such as fermenter and biogas plant for producing biogas.    ADVANTAGE - The ability of the retention wall to resist a differential pressure between the gas section and the atmosphere is enhanced. The bending resistance of the retention wall is enhanced. Since the agitator is rotated by turning the holding rod, the agitator is maintained effectively with the servicing device. Since the hatch in the service section of the power plant is closed, the escape of the produced biogas from the gas section is prevented. Since the inflatable curtain is made of the pliable material such as flexible plastic or rubber impregnated fabric, the degradation of the inflatable curtain in the biogas plant is prevented. The digester tank is inspected and maintained effectively.    DETAILED DESCRIPTION - The sealing device separates the servicing section from the gas section of the fermenter tank. The sealing device is provided with one retention wall of a flexible material that is movable at inoperative position and a sealing position. The lower edge of the retention wall is forced downwards. The mechanical structure is provided within the retention wall. The retention wall is provided with the inflatable pockets. The retention wall is biased upwards. A spring or pneumatic cylinder is adapted to bias the retention wall upwards. A non-moving structure is provided within the retention wall adapted to support the retention wall in the sealing position. The non-moving structure is provided with the opening. INDEPENDENT CLAIMS are included for the following:    (1) digester;    (2) method for servicing digester tank, involves raising the surface of the liquid to form the seal between the separation curtain and the gas. A hatch between the outside of the digester tank and the separation curtain is opened. The opening in the side of the separation curtain is closed;    (3) combination structure of service device and digester, has service device is supported on the tank. A pipe provides fluid communication between the gas sections inside of the servicing device. The pipe is selectively closed to prevent the communication. The pipe passes through a wall of the service device and then returns to a port in the service device. The shut off valve is located in the portion of pipe outside of the service device. The service device is provided with a purge line and a vent with the valve, and is provided with the biogas outlet. The servicing device is used to hold an upper end of an agitator supporting rod, and is provided with an arm preferably operable from outside of the servicing device for rotating the supporting rod. The servicing device is provided with a viewing port. The servicing device is provided with a platform to support a person working on the digester. The servicing device is provided with a door or hatch to allow a person to have access to the inside of the servicing device. The servicing device is provided with the unit for raising or lowering the agitator on the supporting rod; and    (4) method of operating digester with service device, involves closing the vent pipe to isolate the gas section from the inside of the service device. The liquid contents is lowered or allowed to drop to below the bottom of the separation curtain, after servicing the digester.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of the servicing device.    Gas section (6)    Service device (410)    Service section (436)    Separation curtain (440)    Stack (462)