According to a first aspect the invention relates to a method for controlling a treatment apparatus arrangement (3) in a treatment plant (1), the treatment plant (1) comprising a basin (2) housing a liquid comprising solid matter, the operating condition in said basin (2) being anoxic and/or anaerobic, and said treatment apparatus arrangement (3) that comprises at least one mixer machine (4) located in said basin (2) and a variable speed drive (5) operatively connected to said mixer machine (4 ). The variable speed drive (5) operates the mixer machine (4) alternately in a Normal Operation Mode, in which an operational speed of the mixer machine (4) is at least the highest of a suspension speed and a process speed, and a Reduced Operation Mode, in which the operational speed of the mixer machine (4) is below said suspension speed. According to a second aspect the invention also relates to a treatment apparatus arrangement and a treatment plant, respectively.
