NOVELTY - The method comprises supplying aqueous substrate with microorganisms contained in the aqueous substrate in a reactor container (3, 9) of two- or multi-stage biogas plant with subsequent hydrolysis and fermentation of the substrate in a defined temperature range, transferring hydrolyzed and fermented, aqueous substrate in a separation module connected with the reactor container, separating the aqueous substrate in a concentrate and a permeate in a separation module (5) and recirculating the concentrated concentrate into the reactor container.    USE - Method for concentrating microorganisms in aqueous substrate in wet fermentation process in two- or multi-stage biogas plant.    ADVANTAGE - The method ensures simple and efficient concentrating of microorganisms in aqueous substrate.    DETAILED DESCRIPTION - The method comprises supplying aqueous substrate with microorganisms contained in the aqueous substrate in a reactor container (3, 9) of two- or multi-stage biogas plant with subsequent hydrolysis and fermentation of the substrate in a defined temperature range, transferring hydrolyzed and fermented, aqueous substrate in a separation module connected with the reactor container, separating the aqueous substrate in a concentrate and a permeate in a separation module (5), recirculating the concentrated concentrate into the reactor container for enriching the aqueous substrate with microorganisms and controlling dry substance content, separating the permeates from the reactor container and/or storing the permeate, fermenting the substrate in the reactor container, and separating the biogas obtained by the fermentation. A plate-, tube- or cross-flow filter or a gravitation thickener, a centrifuge and/or decanter are used as separation module. The permeate is introduced to the reactor container for controlling the dry substance content, which is adjusted to 12-15%. The temperature of 37-90 degrees C is adjusted for the fermentation in the reactor container. The separation of the aqueous substrate is carried out in temporal intervals during the fermentation process. The transferring of the aqueous substrate is carried out in the separation module by a circulation pump. The aqueous substrate is a co-substrate such as cereal or grasses, corn or organic remnant such as food scraps. The aqueous substrate is concentrated to a dry substance content of 12-15% before supplying into the reactor container. The separation of the aqueous substrate begins in separation module in a particle size of the dry substance of 0.01-1.5 mm. The substrate consists of vegetable and/or animal biomass. Enzymes and/or further additives such as trace elements are added to the aqueous substrate. The microorganisms are methane bacteria. The biogas is withdrawn over a biogas pipeline and is stored in a gas storage. An INDEPENDENT CLAIM is included for a biogas plant for concentrating microorganisms in aqueous substrate.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic representation of a method for concentrating microorganisms in aqueous substrate in biogas plant.    Reactor container (3, 9)    Circulation pump (4)    Separation module (5)    Concentrate line (6)    Gas storage. (11)