NOVELTY - The arrangement for the concentration of biomass in biogas fermenters (9) using micro-bubbles for flotation, comprises a device control (15) for formation of micro-bubbles, a booster pump (5) and an agitator (10) for mixing the bio-sludge. The micro-bubbles are generated by an aerated liquid, which is removed from the system after the separation of floatable components. A negative pressure is produced for suction of the gas by a valve in the inlet of the liquid before the booster pump and the formation of the micro-bubbles takes place through a flow-intensive expansion.    USE - For concentration of biomass in biogas fermenter for the production of fermentation gas by using micro bubbles.    ADVANTAGE - The arrangement has low operating cost and yields more fermentation gas in short duration as well as increases the efficiency of the plant by producing the concentration of biomass.    DETAILED DESCRIPTION - The arrangement for the concentration of biomass in biogas fermenters (9) using micro-bubbles for flotation, comprises a device control (15) for formation of micro-bubbles, a booster pump (5) and an agitator (10) for mixing the bio-sludge. The micro-bubbles are generated by an aerated liquid, which is removed from the system after the separation of floatable components. A negative pressure is produced for suction of the gas by a valve in the inlet of the liquid before the booster pump and the formation of the micro-bubbles takes place through a flow-intensive expansion in a cavitation region of the bubbles. The biogas produced in the fermenter is sucked over a line (17) for the device control and a space is enclosed in the fermenter by a wall. The liquid is clarified over the line. The liquid loaded with micro-bubbles is introduced into the space, while the quantity of the liquid phase, which is introduced into the fermenter, is purified against the flow of micro-bubbles, leaves the plant as a purified and an essentially smell-free liquid over a overflow mechanism and over the line of the clarified liquid. From the clarified liquid, the necessary quantity of the micro-bubbles is removed, while the floating bio-sludge is led back into the region of the biogas fermenter and is mixed down by the agitator. The wall terminates 0.5-1 m below the fluid level that adjusts itself through the height of the overflow mechanism. The bio-sludge is sucked in by the agitator with the help of a conduit tube, which is led from the self-forming floating layer and spans beyond the agitator and is mixed with the contents of the fermenter.    DESCRIPTION OF DRAWING(S) - The diagram shows a biogas plant:    Booster pump; (5)    Biogas fermenter; (9)    Agitator; (10)    Device control; (15)    Line. (17)