Energy recovery operates on the principle of thermodynamic conservation by recycling output waste streams to offset primary energy input requirements. In fluid-based systems like reverse osmosis, high-pressure brine exiting the membrane is directed into a positive displacement pressure exchanger. Inside this device, the high-pressure waste liquid directly interfaces with low-pressure incoming raw feedwater, transferring its pressure energy hydraulically without requiring intensive electrical pumps. This mechanical energy transfer drastically reduces the external electrical power needed to pressurize the system.