NOVELTY - The biomass (3) in e.g. a bale, heaps, pressed sheet bale (4), silo or off-roll container is contained in an air tight envelope (5), and dosed (7, 8) once only with inoculant. Fermentation proceeds anaerobically with no further addition.    USE - To produce methane whilst composting or digesting biowastes.    ADVANTAGE - The process aims at minimal cost and simplest possible execution, using readily available waste materials with little special preparation. Small charges may be treated. The normal containment is used in situ, at the location where the waste arises. The reaction mass itself is not made substantially liquid, to be pumped round at high cost. Only percolate is recirculated.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for the equipment used. Preferred features: Before methanization, aerobic heating takes place. Inoculant is made and added, to suit the particular biomass. It may be liquid from a biogas plant or e.g. cattle slurry. Inoculant is made from structured wastes, grass cuttings and/or coppice wood and/or straw and/or waste solids and slurry. Structured waste is mixed with liquid to a dry mass content of 20-40%, preferably 30%, at a pH value of 7-9, preferably 8. The ratio of dry masses of inoculant to biomass exceeds 0.6. Various industrial wastes are added. Composting takes place at about 50degreesC for 2-5 days without turning. Septic liquid is recycled as inoculant. Inoculant is mixed in before filling the vessel. Storage conditions (aerobic at 20degreesC) exclude spontaneous methanization. Fluid inoculant is used with biomass having high fluid absorption. It may be introduced between biomass layers during or before pressing bales by e.g. spraying. It is embedded locally in the biomass. Liquid inoculant is introduced into the heating biomass, displacing air. Digestion continues at no more than 35degreesC. The inoculant is preheated. The methanizer has external thermal insulation. Conventional and/or solar heating is applied. Percolate is recirculated from above, controlling the dry matter content and adding nutrients. Some is stored. Biomass layering or other structured introduction optimizes methanization.    DESCRIPTION OF DRAWING(S) - biomass (3)    pressed sheet bale (4)    air tight envelope or casing (5)    dosing probe and openings (7, 8)