A new pressure exchanger (PX) device transfers the energy from the concentrate stream directly to the feed stream. This direct, positive displacement approach results in a net transfer efficiency of over 95%. This efficiency advantage makes it possible to dramatically improve the performance of existing SWRO plants by reducing their energy consumption by as much as 75% or by expanding their capacity as much as 300%. Detailed system designs, parameters, and recommendations are provided in this paper for several retro-fit configurations and will be accompanied with case data from operating plants that have been retro-fitted using these new devices. There has been a recent proliferation of commercially available energy recovery devices based on the positive displacement direct pressure exchange approach. This increased interest is driven by the fact that the technology can significantly reduce the energy consumption of new and existing SWRO systems. Since energy costs are rising and can consume, as much as 75% of the total operating costs of an SWRO plant, it is important that the technology be encouraged and disseminated throughout the industry. Although the author of this paper is directly associated with Energy Recovery, Inc, a leading supplier of pressure exchanger technology, the principles and theories presented in this paper will be applicable to all devices that are based on the positive displacement, isobaric chamber approach.