The invention provides a liquid treatment apparatus such as a reverse osmosis apparatus wherein a portion of an inlet liquid permeates through a filter or a membrane e.g. to provide freshwater from saltwater. The apparatus comprises a pump which provides the necessary pressure of the liquid to drive the permeation process, and a recovery unit which transfers pressure of a residue quid to the inlet liquid. The pump and the recovery unit are driven at synchronous and variable speed to control the output and thereby e.g. to adjust for fouling of the filter or membrane. The invention further provides methods of controlling the synchronous speed, e.g. based on a pressure or based on the consumption of the produced liquid.