NOVELTY - The method comprises pumping a substrate quantity to a container (1), separating a part of the fluid and introducing the concentrated substrate into a collection container. The substrate quantity is guided through an external housing to a filter device (10) that surrounds by a filter tube (14), which is equipped in its tube mantle with fluid permeable reception and is surrounded by a second filter tube and external chamber (17). A part flowed into the filter tube as a head flow and a part of the substrate quantity flowed through the external chamber as partial flow are drawn-off.    USE - Method for thickening sludge- and/or fiber material containing fermentation substrate of biogas plant.    ADVANTAGE - The method ensures efficient thickening of sludge- and/or fiber material containing fermentation substrate with good filtering effect.    DETAILED DESCRIPTION - The method comprises pumping a substrate quantity to a container (1), separating a part of the fluid and introducing the concentrated substrate into a collection container. The substrate quantity is guided through an external housing to a filter device (10) that surrounds by a filter tube (14), which is equipped in its tube mantle with fluid permeable reception and is surrounded by a second filter tube and external chamber (17) forming filter housing. A part flowed into the filter tube as a head flow with a low dry material portion and a part of the substrate quantity flowed through the external chamber as partial flow with a high dry material portion are drawn-off. The drawn-off fermentation substrate is pressed or sucked as a base volume flow over a first pump (21) in the filter device. The feeding performance of the pumps is regulated dependent upon the difference pressure adjusted between the inside and outside of the filter tube. The difference pressure is measured over a first pressure receiver in the filter tube and a second pressure receiver in the area of the second pump (22). The partial flow direction guiding from the filter tube is locked in flow direction behind the second pressure receiver. The partial flow direction is closed in an initial phase of the dewatering process up to reach a nominal pressure measured at the second pressure receiver. The partial flow of 30-100% and main flow of 70-0% are substrate quantity to be removed. The ratio of the main flow and the partial flow is such a way that the partial flow with a dry material content of less than or equal to 30% and the main flow with a dry material content of less than or equal to 4% are removed from the filter device in a dry material content of the substrate supplied to the filter device. The main flow is conveyed over a reversible rotary piston pump and the partial flow is conveyed over a rotary piston pump. An INDEPENDENT CLAIM is included for a filter device for thickening sludge- and/or fiber material containing fermentation substrate of biogas plant.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic block view of a filter system.    Container (1)    Filter device (10)    Filter tube (14)    External chamber (17)    Pump. (21, 22)