NOVELTY - The method comprises adding an aqueous substrate with microorganisms contained in a reactor vessel (3) of a biogas plant (100) with subsequent hydrolysis and fermentation of the substrate in a defined temperature of 39-60 degrees C, transferring the hydrolyzed and fermented aqueous substrate into a separation module (5) connected with the reactor vessel using a circulation pump (4), separating the aqueous substrate in a concentrate and a permeate in the separation module, and returning the concentrate in the reactor vessel for enriching the aqueous substrate with microorganisms.    USE - The method is useful for concentration of microorganisms in aqueous substrates in wet fermentation process for two-or multi-level biogas plants, where the aqueous substrates are renewable raw materials such as food wastes and animal by-products or substrates from the processing and/or organic waste.    ADVANTAGE - The method ensures economic, energy efficient and rapid concentration of the microorganisms in the aqueous substrates with reduced material consumption.    DETAILED DESCRIPTION - The method comprises adding an aqueous substrate with microorganisms contained in a reactor vessel (3) of a biogas plant (100) with subsequent hydrolysis and fermentation of the substrate in a defined temperature of 39-60 degrees C, transferring the hydrolyzed and fermented aqueous substrate into a separation module (5) connected with the reactor vessel using a circulation pump (4), separating the aqueous substrate in a concentrate and a permeate in the separation module, returning the concentrate in the reactor vessel for enriching the aqueous substrate with microorganisms and controlling the dry matter content, precipitating the permeate from the reactor vessel and the separation module and discharging the permeate into a repository for storing and/or disposal of the permeate, refermentation of the substrate in the reactor vessel, and separating biogas obtained by the fermentation. The solid materials separated from the aqueous substrate are deposited in the separation model, where the solid materials have a particle size of 0.01-0.5 mm. Enrichment of methanogenic archaea and syntrophic acetate-oxidizing bacteria is effected in the fermenter. A plate, a tube or a cross-flow filter, or a gravity thickener and/or a centrifuge or a decanter is used as separation modules. The permeate is added in the reactor vessel to control the dry matter content. The dry matter content of the substrate is adjusted to 10-15% before the addition into the reactor vessel. The separation of the aqueous substrate is carried out at time intervals during the fermentation process. An INDEPENDENT CLAIM is included for a biogas plant.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a method of concentrating microorganisms in a biogas plant.    Biogas plant (100)    Substrate conduction (2)    Reactor vessel (3)    Circulation pump (4)    Separation module. (5)