NOVELTY - Digestion system (100) for converting manure mixture comprising liquid effluent layer having liquid level and sludge layer having solids level, comprises: digestion tank (130) including first end and second end, headspace, and height adjustable valve; heat exchange system (150) coupled to digestion tank; water recirculation system coupled to heat exchange system; biogas collection system (170); biogas conditioning system; and main controller (190) for controlling height adjustable valve, heat exchange system, water recirculation system, biogas collection system and biogas conditioning system.    USE - The digestion system is useful for: converting a manure mixture comprising liquid effluent layer having liquid level and sludge layer having solids level; and converting agricultural waste to biogas (all claimed) or animal waste into methane gas.    DETAILED DESCRIPTION - Digestion system (100) for converting a manure mixture comprising a liquid effluent layer having a liquid level and a sludge layer having a solids level, comprises: a digestion tank (130) including a first end and a second end, a headspace defined above the liquid level within the tank, and a height adjustable valve including an intake end positioned within the liquid effluent layer and configured to withdraw liquid effluent from the liquid effluent layer and out of the digestion tank; a heat exchange system (150) fluidly coupled to the digestion tank, where liquid effluent withdrawn from the liquid layer is passed through the heat exchange system for heating before being returned to the digestion tank; a water recirculation system fluidly coupled to the heat exchange system for providing heat to the heat exchange system; a biogas collection system (170) for regulating a level of biogas collected in the headspace of the digestion tank; a biogas conditioning system for processing the biogas; and a main controller (190) for controlling the height adjustable valve, the heat exchange system, the water recirculation system, the biogas collection system, and the biogas conditioning system. INDEPENDENT CLAIMS are also included for:    (1) a digestion tank assembly for converting a manure mixture including a liquid effluent layer having a liquid level and a sludge layer having a solids level to biogas comprising a digestion tank including the first end, the second end and the headspace defined above the liquid level where biogas collects in the headspace, and a height adjustable valve including an intake end configured to withdraw liquid effluent out of the digestion tank and adapted to move in a vertical direction between at least a first position and a second position so that the intake end is maintained within the liquid effluent layer and above the sludge layer within the digestion tank; and    (2) converting agricultural waste to biogas, comprising transferring fresh waste from a waste reception area to an anaerobic digestion tank including a sufficient quantity of anaerobic bacteria to digest the waste to produce a manure mixture having a predetermined level and including a liquid effluent layer having a liquid level and a sludge layer having a solid level and biogas, determining the solid level of the sludge layer, maintaining an intake end of a valve configured to withdraw liquid effluent from the digestion tank within the liquid effluent layer and above the sludge layer, withdrawing liquid effluent from the liquid effluent layer, recirculating the liquid effluent by passing the liquid effluent through a heat exchange system fluidly coupled to the digestion tank and returning the liquid effluent to the digestion tank, and spraying the returning liquid effluent in an upwards direction to mix the manure mixture within the tank.    DESCRIPTION OF DRAWING(S) - The figure shows a flow chart of the digester system.    Digestion system (100)    Digestion tank (130)    Heat exchange system (150)    Biogas collection system (170)    Main controller (190)