Abstract: The retrofit technology utilizes pressurized brine of convention RO to feed a Closed Circuit Desalination (CCD) unit; wherein, further desalination takes place to a desired recovery level. The application exemplified in FIG. 4 is of a retrofit unit comprising a Booster Pump (BP2) for raising pressure of inlet feed; a Circulation Pump (CP) for creating cross flow over membranes (E) in the pressure vessel (M), thereby enable efficient RO desalination; an Actuated Valve (AV) in line with a partially open Manual Valve (MV) to enable periodic replacement of high salinity concentrates with fresh feed without stopping desalination; No Return (NR) valve means to control the direction of flow in the system; and monitoring means such as of electric conductivity (CM) and flow (FM). Periodic replacement of high salinity concentrates by fresh feed initiated at desired high system electric conductivity and terminated at a desired low system electric conductivity, while desalination continued.