NOVELTY - Wastewater sludge treatment involves collecting wastewater sludge from a primary and/or secondary settling unit; and further directing a specified portion of the sludge to a cavitation treatment in units, where cavitation treatment of the sludge is lead through a pipe channel (11) having a cross section of regular polygon, and piezoceramic ultrasonic transducers (12-17) arranged side-by-side and uniformly bonded to the wall on each side of pipe channel. The transducers provide a mode of vibration, where all side walls of pipe channel vibrate with a uniform forth and back movement.    USE - For wastewater sludge treatment in a wastewater sludge handling process (Claimed).    ADVANTAGE - The wastewater sludge treatment arrangement produces better and more even energy transfer from the side walls to the wastewater sludge; has more effective power penetration in sludge having high viscosity; also has lower cavitation erosion and a longer life time of use. With the help of the effectiveness of the present wastewater sludge treatment arrangement only a portion of the total wastewater sludge needs to be directed to the cavitation treatment. When implemented in a wastewater treatment plant the present wastewater sludge treatment arrangement boosts the biogas production and reduces the volume of the residual sludge. Particularly, the implementation of the wastewater sludge treatment arrangement in the wastewater treatment plant boosts the biogas production of the digestion units 33, 34, or up to 40%. The wastewater sludge treatment arrangement reduces the volume of the residual sludge by 20%. The impedance adapter gives better efficiency to the whole wastewater sludge treatment arrangement. When the reactive power with good impedance match is close to zero, the generator, connecting cables and reactor are not heated by reactive power, which results in better durability and fewer failures.    DETAILED DESCRIPTION - Wastewater sludge treatment in a wastewater sludge handling process involves collecting wastewater sludge from a primary settling unit and/or a secondary settling unit; further directing a specified portion of the wastewater sludge to a cavitation treatment in at least one cavitation treatment unit in which cavitation treatment the wastewater sludge is lead through a pipe channel (11) having a cross section of an essentially regular polygon, and piezoceramic ultrasonic transducers (12-17) arranged essentially side-by-side and uniformly bonded to the wall on each side of the pipe channel. The transducers provide a mode of vibration, where essentially all side walls of the pipe channel vibrate with a uniform forth and back movement. The cavitation treated sludge is returned back to the wastewater sludge mainstream pipeline from the cavitation treatment units via a collector tank. An INDEPENDENT CLAIM is included for an arrangement for wastewater sludge treatment in a wastewater sludge handling process, comprising a pipe channel through which a specified portion of the wastewater sludge is lead, where the pipe channel has a cross section of an essentially regular polygon, and piezoceramic ultrasonic transducers arranged essentially side-by-side and uniformly bonded to the wall on each side of the pipe channel; and further an ultrasonic generator, an vibration reactor comprising the vibration elements, and an impedance adapter for adapting and optimizing the impedances of the ultrasonic generator output and vibration reactor input in commissioning stage, where impedance adapter is connected between the ultrasonic generator and vibration reactor.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a wastewater sludge treatment arrangement.    Wastewater sludge pipe channel (11)    Ultrasonic transducers (12-17)