NOVELTY - Ultrasonic treatment device for a biogas plants comprises a wet crusher (1) for crushing fibers and solid particles in the biomass suspension, a suspension pump (2) for guiding biomass suspension through the treatment device, an ultrasonic reactor (8), which is arranged in volume flow after the wet crusher and the suspension pump for the ultrasonic treatment, and a volume flow meter (9) for measuring the volume flow of the biomass suspension. The ultrasonic reactor comprises a cavitation chamber, which flows successively through the biomass suspension.    USE - The ultrasonic treatment device is useful for a biogas plant.    ADVANTAGE - The ultrasonic treatment device requires low-maintenance operation over longer periods. The ultrasonic reactor is automatically flushed without the requirement of an operator.    DETAILED DESCRIPTION - Ultrasonic treatment device for a biogas plants comprises a wet crusher (1) for crushing fibers and solid particles in the biomass suspension, a suspension pump (2) for guiding biomass suspension through the treatment device, an ultrasonic reactor (8), which is arranged in volume flow after the wet crusher and the suspension pump for the ultrasonic treatment, and a volume flow meter (9) for measuring the volume flow of the biomass suspension. The ultrasonic reactor comprises a cavitation chamber, which flows successively through the biomass suspension. The cavitation chambers are provided with an ultrasonic transducer, and a flushing lance for removing biomass deposits to the ultrasonic transducers. An INDEPENDENT CLAIM is also included for an ultrasonic reactor for the ultrasonic treatment of biomass suspensions in biogas plants comprising many cavitation chambers, which are provided in the ultrasonic transducer, where (a) the cavitation chambers are arranged in series in volume flow direction of the biomass, and (b) at least one cavitation chambers comprise flushing lance with outlet nozzle, which is designed such that the flushing liquid strikes to the ultrasonic transducer during flushing    DESCRIPTION OF DRAWING(S) - The figure shows a schematic representation of the treatment device.    Wet crusher (1)    Suspension pump (2)    Ultrasonic reactor (8)    Volume flow meter (9)    Open frame construction (11)