NOVELTY - A digestion system comprises a digestion tank having a height adjustable valve including an intake end within a liquid effluent layer; a heat exchange system fluidly coupled to the digestion tank; a water recirculation system fluidly coupled to the heat exchange system; a biogas collection system for regulating a level of biogas collected in the headspace; a biogas conditioning system; and a main controller for the height adjustable valve, heat exchange system, water recirculation system, biogas collection system, and biogas conditioning system.    USE - A digestion system for converting a manure mixture comprising a liquid effluent layer and a sludge layer to biogas (claimed).    DETAILED DESCRIPTION - A digestion system comprises a digestion tank having a headspace above a liquid level within the tank and a height adjustable valve including an intake end within a liquid effluent layer for withdrawing liquid effluent from the liquid effluent layer and out of the digestion tank; a heat exchange system fluidly coupled to the digestion tank, where the withdrawn liquid effluent is passed through the heat exchange system before being returned to the digestion tank; a water recirculation system fluidly coupled to the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 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 for the height adjustable valve, heat exchange system, water recirculation system, biogas collection system, and biogas conditioning system. An INDEPENDENT CLAIM is included for a process for converting agricultural waste to biogas comprising    (A) transferring fresh waste from a waste reception area to an anaerobic digestion tank;    (B) determining the solids level of the sludge layer;    (C) maintaining an intake end of a valve for withdrawing liquid effluent from the digestion tank within the liquid effluent layer and above the sludge layer;    (D) withdrawing liquid effluent from the liquid effluent layer;    (E) 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    (F) spraying the returning liquid effluent in an upwards direction to mix the manure mixture within the tank.    DESCRIPTION OF DRAWING(S) - The drawing shows a flow chart of the major steps and components of a digester system.