NOVELTY - A bioenergy production apparatus (10) comprises initial digester (12) for digestion of digestible material; collector (14) formed at or near base of initial digester for collecting leachate product of digestible material digestion; secondary digester (16) operatively connected to collector to digest leachate product; burner (18) connected to initial digester for receiving biogas output from initial digester, where burner is adapted to combust biogas; and heat exchanger for transferring heat from combustion of biogas in burner to secondary digester and/or to energy conversion device(s).    USE - A bioenergy production apparatus for use in processing cattle feedlot by-products, or other solid organic materials including by-products from poultry stalls and crop residues.    ADVANTAGE - The initial digester is able to be constructed at low cost, due to use of earthen walls, earthen base and plastic film cover, thus making it economically feasible for apparatus to process input digestible material over longer time frames than are typically required for higher cost prior art systems.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) production of bioenergy; and    (2) operation of bioenergy production apparatus.    The heat from combustion of biogas in burner is used to heat secondary digester or to facilitate steam flaking process. The digestible material in initial digester is moistened. The leachate product output from secondary digester is returned to initial digester. The collection of leachate product is ceased when bioenergy produced per unit volume of digestible material placed in initial digester is at or below threshold value. At least some of leachate product from initial digester is treated to extract struvite. The treatment comprises adding magnesium hydroxide to leachate product from initial digester.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a bioenergy production apparatus.    Bioenergy production apparatus (10)    Initial digester (12)    Collector (14)    Secondary digester (16)    Burner (18)