NOVELTY - The anaerobic digester bladder tank comprises an upper chamber (5), a lower chamber (6), and a flexible wall disposed between the chambers. The upper chamber contains biodegradable waste slurry from which microbes anaerobically produce biogas. The lower chamber is able to have water or pass the water. The tank is made of a flexible material. The upper chamber has a volume of greater than the lower chamber. The lower chamber has an inlet and an outlet. The inlet and the outlet are connected to a water circuit (3). A pressure of the water within the lower chamber is modulated.    USE - The bladder tank is useful in an aerobic digester and a system for anaerobic digestion of biodegradable waste slurry including swine waste, food processing waste or sewage to produce biogas (all claimed).    ADVANTAGE - The bladder tank ensures economical and efficient anaerobic digestion of the biodegradable waste slurry, can be operated in cold climate environments and warm climates, and is easy to handle and maintain.    DETAILED DESCRIPTION - The anaerobic digester bladder tank comprises an upper chamber (5), a lower chamber (6), and a flexible wall disposed between the chambers. The upper chamber contains biodegradable waste slurry from which microbes anaerobically produce biogas. The lower chamber is able to have water or pass the water. The tank is made of a flexible material. The upper chamber has a volume of greater than the lower chamber. The lower chamber has an inlet and an outlet. The inlet and the outlet are connected to a water circuit (3). A pressure of the water within the lower chamber is modulated. The water within the lower chamber is heated water. The water is heated by a gas engine (2) powered by biogas produced in the upper chamber. The upper chamber has a water-lock biogas outlet (4). INDEPENDENT CLAIMS are included for:    (1) an aerobic digester;    (2) a method for anaerobic digestion; and    (3) a system for anaerobic digestion.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic front perspective view of an anaerobic digester tank system.    Gas engine (2)    Water circuit (3)    Biogas outlet (4)    Upper chamber (5)    Lower chamber. (6)