NOVELTY - Organic materials are digestion-treated to produce biogas and compost using anaerobic fermentation of anaerobic microorganisms. Anaerobic fermentation is completed so as to provide a total solid content of 18% or more. The digested sludge solids dehydrated by a sludge dehydration unit (3) are returned to be mixed with a freshly incoming raw material. The drying anaerobic digestion process is improved by adding waste vegetable oils (1.5-2.5 tons) as a carbon source to the organic materials charged into the anaerobic fermentation.    USE - Drying anaerobic digestion process of organic waste, such as livestock excretion, food waste, waste paper, wood waste, and domestic waste, for producing biogas.    ADVANTAGE - The process provides enhanced digestion efficiency of organic materials, increases the generation amount of biogas, and improves the quality of compost.    DETAILED DESCRIPTION - Organic materials are digestion-treated to produce biogas and compost using anaerobic fermentation of anaerobic microorganisms. Anaerobic fermentation is completed so as to provide a total solid content of 18% or more, based on the organic materials, during anaerobic fermentation. The digested sludge solids dehydrated by a sludge dehydration unit are returned to be mixed with a freshly incoming raw material. Only 55-56% of the digested sludge solids are returned and mixed. The drying anaerobic digestion process is improved by adding waste vegetable oils as a carbon source to the organic materials charged into the anaerobic fermentation. The waste vegetable oils are added in an amount of 1.5-2.5 tons based on 100 tons of organic materials. An INDEPENDENT CLAIM is included for method for improving digestion efficiency, which involves adding ferric chloride (200-300 kg with respect to 100 tons of organic materials) to organic materials during anaerobic fermentation.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the digestion efficiency improving method. (Drawing includes non-English language text)    Anaerobic digester (1)    Digestion tank feeder (2)    Sludge dehydration unit (3)    Polymer interfuse unit (4)