NOVELTY - A pretreatment unit (3) includes a tube (10) fed at one end (18), with an outlet (19) at the other, connected to the reactor (2) directly or indirectly. The jacketed (13) tube is supplied with hot water (14), for a thermal hygiene treatment of the feedstock. A conveyor screw (16) extends along the tube, optionally also for comminution and/or mixing.    USE - A co fermentation unit, and especially the pretreatment section, producing biogas from animal waste and other mixed organic materials of industrial or domestic origin.    ADVANTAGE - The arrangement is simple to construct, and is operable at low cost. Fully automatic operation is feasible. Hygiene is maintained. The system is suitable for smaller agriculturally-based units. The pre-storage tank is heated to keep fatty materials fluid. It is suitable for tanker deliveries of e.g. dairy wastes, liquid foods and fatty materials.    DETAILED DESCRIPTION - Preferred features: The vertical feed tube (11) operates continuously, or includes e.g. a slide- or flap valve (12) for periodic introduction of feedstock to the screw. The pretreatment tube may have a similar continuous or intermittent feed arrangement at its outlet end, feeding the reactor. The otherwise horizontal pretreatment tube, optionally rises slightly towards its outlet. Screw and its external drive, are coaxial. The screw periphery, and/or tube internal wall is/are structured. A variant design, operating on similar principles and based on a chain conveyor, is also described. The tube is thermally insulated, and the pumped hot water system raises the feedstock to at least 70 degrees C. The water is heated directly or indirectly by combustion of biogas produced. Screw speed and/or a feed valve is/are controlled for a feedstock residence time of 1 hr at least, at 70 degrees C. A heated feedstock storage tank is provided for fluid, fat-containing co substrate. At least part of the preheated pretreatment tube is located in this tank. The inlet of the supply tube is at the liquid level in the tank and the outlet is adjusted correspondingly as an overflow.    DESCRIPTION OF DRAWING(S) - A schematic cross section of the pretreatment and fermentation plant is presented.    reactor (2)    pretreatment unit (3)    tube (10)    slide- or flap valve (12)    heating jacket (13)    hot water (14)    conveyor screw (16)    feed end (18)    outlet (19)