NOVELTY - Converting biodegradable organic material, where a mixture of non-contaminated water and non-toxic biodegradable feed stock is introduced into a thermophilic anerobic digester (200), is new. The mixture is anerobically digested to generate biogas, which is withdrawn from the head space of the digester. A portion of biogas is transferred to a methanogenic digester (300) at a mesophilic reaction temperature. Biogas is generated, withdrawn and used.    USE - To produce biogas, especially methane which is used in boilers for generating steam, in internal combustion engines, in energy producing turbines, in gas storage tanks and gas burners.    ADVANTAGE - Biodegradable organic material is converted into product gas without leaving waste. Maximum amount of biogas is produced at low cost.    DETAILED DESCRIPTION - A metered stream of slurry comprising a mixture of non-contaminated water and a non-toxic biodegradable feed stock containing at least 10 % biodegradable solids, of which at least 95 % of the solids are converted to gas, is introduced into a completely mixed thermophilic anaerobic digester (200). The digester is at a thermophilic reaction temperature, and has an active, microbial anaerobic digestion population comprising hydrolysis fermentation organisms and methanogenic organisms suspended in fluids. The biodegradable feed stock is chosen from glucose, beet sugar, sugar beet molasses, sugar beet syrup, sugar beet juice, sugar cane molasses, cane sugar, sugar cane syrup, sugar cane juice, corn syrup, cereal grain, oat flour, rice flour, corn flour, wheat flour, potato juice, potato pulp and/or sorghum. The mixture is anaerobically digested in the digester, to generate biogas. The biogas is withdrawn from the head space of the digester and a portion of the gas is transferred to a completely mixed methanogenic digester (300) at a mesophilic reaction temperature. The methanogenic digester (300) has the same microbial population as that of the anaerobic digester (200). Biogas is generated by anaerobic digestion of the mixture. The biogas is withdrawn from the head space of the digester, and is used.    INDEPENDENT CLAIMS are included for:    (1) an apparatus for converting biodegradable organic material into product gas, which comprises:    (a) a heated drop tank (10) for receiving biodegradable material;    (b) a heated line in communication with the drop tank having a pump, for transferring the material to a holding tank;    (c) a thermophilic anaerobic digester containing a population of hydrolyzing anaerobic fermentation organisms at thermophilic temperature, in communication with the holding tank;    (d) a methanogenic anaerobic digester containing the same population as that of thermophilic digester, in communication with the thermophilic digester;    (e) several pipelines and pumps for introducing and re-circulating feed stock into and between digesters;    (f) a conical opening and splash plate inside the digesters in the upper portion for reducing scum and spreading re-circulated fluid inside the digester; and    (g) removal pipelines connected to digester tops for removing biogas; and    (2) an apparatus for converting biodegradable feed stock into product gas, which comprises:    (a) a drop tank for receiving a non-toxic biodegradable carbohydrate material;    (b) a storage tank in communication with the drop tank;    (c) a viscosity pump (95) for transferring the material from the drop tank to the storage tank;    (d) a mixing tank for adding water to the material to form a slurry;    (e) an adjustable valve (15) for variably controlling the amount of slurry fed out of the mixing tank; and    (f) a thermophilic anaerobic digester with output for gas and non-gas materials and a methanogenic anaerobic digester.    DESCRIPTION OF DRAWING(S) - The drawing shows the stand-alone portion of flow scheme of waste water plant.    Drop tank (10)    Valve (15)    Viscosity pump (95)    Thermophilic digester (200)    Methanogenic digester (300).