By defining the combination of high pressure seawater feed pump and energy recovery system as the core hydraulic module of SWRO plants it becomes obvious that this subsystem mainly contributes to the specific water costs by its operational costs and has to be optimized by a complete approach. In the previous paper published at the IDA Conference 2003 [1], different energy recovery systems were analyzed with respect to theire overall life cycle costs. Identifying the core hydraulic module using pressure exchangers as the advantageous ERS-concept the question of appropriate component optimization was discussed for the high pressure pump design. In the current paper, this approach is extended to the pressure exchanger itself, contributing to an overall optimized core hydraulic module. Therefore, inadequacies and limits of conventional pressure exchanger operation are explored and discussed with respect to their impact on reliability. Based on these results, a new pressure exchanger design is presented. This design was realized in a laboratory prototype and tested on a special test rig. The arrangements of the test rig as well as the results of first prototype tests are described.