The combined pump and energy recovery turbine includes at least one fluid flow pressurizing a sliding vane pump and a sliding vane energy recovery turbine that recovers energy from a second fluid flow, such as the brine discharge from an RO seawater desalination system. A cylindrical rotor has two sliding vanes in respective slots, the rotor being concentrically disposed within an oval-shaped enclosure defining two mirror image crescent-shaped chambers, each chamber having inlet and outlet passageways. The first chamber pressurizes the first fluid flow, and the second chamber functions as a second outflow-driven energy recovery turbine, thus enabling the single rotor device to operate as a pressurizing pump on the first fluid flow, and second outflow-driven energy recovery turbine recovering energy from the pressure drop in the second fluid flow.