NOVELTY - Anaerobic digester comprises support, drum having one end and opposed end mounted for rotation on the support, where the drum includes at least one fixed internal matter displacing formation mounted on the internal drum surface that projects from the internal drum surface into interior space, and feed/discharge arrangement on the drum towards the one end of drum through which to-be-treated raw organic matter is introduced into the interior space and through which treated solid or liquid organic matter is discharged from the interior space.    USE - The anaerobic digester and apparatus are useful for digesting organic matter and producing biogas (all claimed). The organic matter is animal waste obtained from livestock on farms such as cow, and green waste and sewage. The biogas is useful for driving generator that is used to drive rotation of drum, as distributed energy source, renewable and sustainable energy source, plant nutrient and fertilizer.    ADVANTAGE - The anaerobic digester is transportable, can be easily maintained and accessed, removes organic waste from environment, promotes favorable rate of digestion of organic matter, reduces usage of water needed to displace organic matter into digester, and improves biogas yield.    DETAILED DESCRIPTION - Anaerobic digester (12) comprises (a) support (14), (b) drum (16) having one end and opposed end mounted for rotation on the support, where the drum having internal surface (19) and defining interior space (20) is closed off from external environment outside of the drum to resist oxygen from entering the drum and to facilitate anaerobic digestion of organic matter within the drum, drum includes at least one fixed internal matter displacing formation mounted on the internal drum surface that projects from the internal drum surface into interior space, and each internal matter displacing formation action to mix organic matter within the drum when the drum rotates, and (c) feed/discharge arrangement (40) on the drum towards the one end of drum through which to-be-treated raw organic matter is introduced into the interior space and through which treated solid or liquid organic matter is discharged from the interior space. INDEPENDENT CLAIMS are included for the following:    (1) apparatus comprising anaerobic digester that comprises the above components, and outer container defining container interior space in which the anaerobic digester is received, and enables the apparatus to be lifted and moved around; and    (2) method for anaerobically digesting organic matter to generate biogas, involves (a) carrying out anaerobic digestion of organic matter within drum which has interior space sealed off from external environment, and at least one fixed blade (32, 34) mounted on the drum that projects into the interior space of the drum, (b) rotating the drum at least periodically to cause the blades to mix the organic matter in the interior space, (c) removing biogas generated in the drum, and (d) discharging treated solid or liquid organic wastes from the drum.    DESCRIPTION OF DRAWING(S) - The drawing shows front view of anaerobic digester.    Anaerobic digester (12)    Support (14)    Drum (16)    Drum internal surface (19)    Drum interior space (20)    Blades (32,34)    Feed/discharge arrangement (40)    Screw formation (42,44)