A method for controlling a treatment apparatus arrangement in a treatment plant, the treatment plant being arranged to treat liquid comprising solid matter such as waste water, the treatment plant comprising: - a basin housing a basin content constituted by a liquid comprising solid matter, the operating condition in said basin being anoxic and/or anaerobic, and - said treatment apparatus arrangement that comprises: - at least one mixer machine located in said basin, the mixer machine having a treatment plant depending predetermined suspension speed adapted to keep solid matter suspended in the liquid, and - a variable speed drive operatively connected to said mixer machine, the method being characterized in that the variable speed drive operates the mixer machine alternately in - a Normal Operation Mode, in which an operational speed of the mixer machine is controlled in real-time to be the highest of said suspension speed and a basin throughput rate depending process speed, said basin throughput rate being determined continuously and is based on at least a basin input, a basin output and adequate blending of the basin input and a basin content, said process speed is predetermined for each combination of different process load situations and different process criteria in the specific basin to be as low as possible without adversely affecting the treatment results and still providing adequate mixing of the liquid comprising solid matter, wherein the process load is constituted by a hydraulic load and a waste load, and - a Reduced Operation Mode, in which the operational speed of the mixer machine is below said suspension speed. - a basin housing a basin content constituted by a liquid comprising solid matter, the operating condition in said basin being anoxic and/or anaerobic, and - said treatment apparatus arrangement that comprises: - at least one mixer machine located in said basin, the mixer machine having a treatment plant depending predetermined suspension speed adapted to keep solid matter suspended in the liquid, and - a variable speed drive operatively connected to said mixer machine, the method being characterized in that the variable speed drive operates the mixer machine alternately in - at least one mixer machine located in said basin, the mixer machine having a treatment plant depending predetermined suspension speed adapted to keep solid matter suspended in the liquid, and - a variable speed drive operatively connected to said mixer machine, - a Normal Operation Mode, in which an operational speed of the mixer machine is controlled in real-time to be the highest of said suspension speed and a basin throughput rate depending process speed, said basin throughput rate being determined continuously and is based on at least a basin input, a basin output and adequate blending of the basin input and a basin content, said process speed is predetermined for each combination of different process load situations and different process criteria in the specific basin to be as low as possible without adversely affecting the treatment results and still providing adequate mixing of the liquid comprising solid matter, wherein the process load is constituted by a hydraulic load and a waste load, and - a Reduced Operation Mode, in which the operational speed of the mixer machine is below said suspension speed.