NOVELTY - A self pressurizing bioreactor (4) connected to a positive displacement pump (14), having several microbes for converting biomass into digested material and biogas which includes methane, is new. A self purifying tank (6) receives biogas from the reactor at pressure above ambient pressure and separates biogas into methane gas and non methane gas.    USE - For producing methane used as fuel in vehicles, from biomass by anaerobic digestion.    ADVANTAGE - The system needs less or no energy input and is usable to produce methane equivalent to liquid fuel in terms of energy, density and purity. Produces optimally pressurized and pure methane.    DETAILED DESCRIPTION - A self pressurizing bioreactor (4) connected to a positive displacement pump (14), having several microbes for converting biomass into digested material and biogas which includes methane, is new. A self purifying tank (6) receives biogas from the reactor at pressure above ambient pressure and separates biogas into methane gas and non methane gas. The pump has chambers separated by a plunger (15). The self purifying tank has water. Water transfer vessel is connected to the tank. The bioreactor is maintained at fixed pressure of 1000 psig above ambient pressure. An effluent container is connected to the bioreactor for receiving digested material. The pump is connected to a feed chamber (8) through a feed pipe and connected to the bioreactor through a positive displacement rector pipe and digested material pipe. The feed material (10) including biomass is added to the bioreactor through the reactor pipe simultaneously with the removal of the digested material through the digested material pipe. The feed material includes 75 % of water. The self-purifying tank is maintained at a pressure of 1000 psig.    An INDEPENDENT CLAIM is also included for the method of producing methane by anaerobic digestion.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the self-pressurizing and self purifying system.    self pressurizing and self purifying system (2)    bioreactor (4)    self purifying tank (6)    feed chamber (8)    feed material (10)    plunger (15)