The introduction of low pressure, high rejection ESPA membranes enables the achievement of one of the goals of commercial RO technology: the ability to operate a given RO system at the minimum value of feed pressure, which is the pressure equal to the osmotic pressure of the concentrate plus the pressure losses along the system. However, at the operating conditions of high feed water temperature, high feed water salinity or high permeate recovery rate, conditions could be created of excessive permeate flux rate from the lead elements and negligible NDP at the end of the system. This is due to a very high specific permeate flux value of the ESPA membranes. Such operating conditions result in higher permeate salinity and the possibility of an increasing fouling rate. Corrective measures could include the use of an interstage pump or a hybrid membrane system design. The hybrid design consists of the use of membranes with lower specific permeate flux in the lead position (first stage), followed by the high flux membranes. The hybrid design does not utilize the full extent of energy savings possible with the ESPA membranes but provides more uniform nux distribution and improved permeate quality. Due to the low values of NDP required by the ESPA membranes, a high permeate flux system design becomes more economically attractive. There is a possibility of improving the efficiency of a RO system operation by changing the configuration of array. These new system configurations, however, require design optimization to assure long-term stable performance.