NOVELTY - An anaerobic digester comprises two covered fluid containers (12, 26), circulating mechanism, gas processor, and liquid processor. The first covered fluid containment mechanism comprises first two sections, and aqueous slurry containing anaerobically digestible fiber. The second covered fluid containment mechanism is in the first covered containment mechanism. The second covered fluid containment mechanism comprises second two sections.    USE - For processing organic materials, e.g. organic waste used in livestock operations, e.g. dairies, feedlots, slaughterhouses, and food processing plants. It is also used in digesting animal excrement, food wastes, and other organic wastes material used for fertilizer or animal nutrients.    ADVANTAGE - The invention increases digester efficiency without requiring complex digester vessels. It does not require entire processing system to be offline while repairs are made to the digester vessel and/or to its components parts. It allows operators to perform maintenance on single module without shutting in the entire digester system. It allows the system to grow as required by the needs of the facility.    DETAILED DESCRIPTION - An anaerobic digester (10) comprises two covered fluid containers, circulating mechanism, gas processor, and liquid processor. The first covered fluid containment mechanism comprises first two sections (18, 20), and aqueous slurry containing anaerobically digestible fiber. The second covered fluid containment mechanism is in the first covered containment mechanism. The second covered fluid containment mechanism comprises second two sections (28, 30). The second upper section comprises first fluid inlet for receiving fluid flow from the first upper section of the first covered fluid containment mechanism. The first fluid inlet has screen disposed at the interface of the first covered fluid container. The first fluid inlet has screen (40) disposed at the interface of the first covered fluid containment mechanism. The screen is used for formation of filter cake (42) upon fluid flow from the first covered fluid containment mechanism through the first fluid inlet into the second covered containment mechanism. The filter cake comprises fiber. The second upper section comprises first gas outlet. The second lower section comprises first liquid outlet. The circulating mechanism circulates the aqueous slurry through the first covered fluid containment mechanism. The gas processor receives and processes gas from the first gas outlet. The liquid processor receives liquid from the first liquid outlet.    An INDEPENDENT CLAIM is included for processing aqueous slurry having anaerobically digestible fiber comprising disposing aqueous slurry into anaerobically digester, receiving gas from the first outlet mechanism into gas processor for receiving and processing gas, and receiving liquid from the first liquid outlet into a liquid processor.    DESCRIPTION OF DRAWING(S) - The figure shows a side view of the digester.    Anaerobic digester (10)    Two covered fluid containers (12, 26)    Side walls (14)    First two sections (18, 20)    Cover (22)    Slurry inlet (24)    Two sections (28, 30)    Covered top (32)    Third covered fluid container (39)    Screen (40)    Filter cake (42)