An integrated system comprising: a Brackish Water Reverse Osmosis unit having an inlet, a pressurized brine outlet and a permeate outlet; and a non-autonomous closed circuit desalination retrofit unit for the further desalination of pressurized brine feed received from said Brackish Water Reverse Osmosis unit, the retrofit unit comprising: a closed circuit comprising one or more desalination modules, the or each desalination module having a respective desalination module inlet and a desalination module outlet connected in parallel by respective first and second conducting lines, the or each desalination module comprising one or more membrane elements; a third conducting line with circulation means for recycling concentrate from desalination module outlets to desalination module inlets of the or each desalination module; a fourth conducting line for supply of said pressurized brine feed from said Brackish Water Reverse Osmosis unit to said closed circuit of said retrofit unit; a fifth conducting line of permeate from said retrofit unit to the inlet of said Brackish Water Reverse Osmosis unit or to the permeate outlet of said Brackish Water Reverse Osmosis unit; a first valve means positioned in the third conducting line, wherein the first valve means is a two-way valve configured to enable flow from desalination module outlets to desalination module inlets of the or each desalination module and for discharge of brine from desalination module outlets of the or each desalination module in said retrofit unit without stopping desalination; a sixth conducting line at outlet of said first valve means to enable discharge of brine from said closed circuit of said retrofit unit; a sensor for measuring electric conductivity of recycled concentrate in said closed circuit of said retrofit unit to enable a follow up of desalination recovery in said closed circuit; and monitoring and control systems configured to enable continuous closed circuit desalination of desired recovery in said retrofit unit, wherein the integrated system includes a second valve means which is positioned in the fourth conducting line and is configurable between a first state in which said pressurized brine feed is fully diverted from said Brackish Water Reverse Osmosis unit to said closed circuit of said retrofit unit thereby activating operation of said retrofit unit and a second state in which said pressurized brine feed is stopped from flowing from said Brackish Water Reverse Osmosis unit to said closed circuit of said retrofit unit thereby terminating operation of said retrofit unit, wherein the retrofit unit is configured to perform a two-step consecutive sequential process, wherein a first step includes closed circuit desalination of recycled concentrate mixed with fresh pressurized feed through the or each desalination module experienced most of the time and wherein a second step includes a plug flow desalination process whereby brine in the closed circuit is replaced with fresh pressurized feed, and wherein brine discharge and fresh feed recharge take place at a desired desalination recovery level. a closed circuit comprising one or more desalination modules, the or each desalination module having a respective desalination module inlet and a desalination module outlet connected in parallel by respective first and second conducting lines, the or each desalination module comprising one or more membrane elements; a third conducting line with circulation means for recycling concentrate from desalination module outlets to desalination module inlets of the or each desalination module; a fourth conducting line for supply of said pressurized brine feed from said Brackish Water Reverse Osmosis unit to said closed circuit of said retrofit unit; a fifth conducting line of permeate from said retrofit unit to the inlet of said Brackish Water Reverse Osmosis unit or to the permeate outlet of said Brackish Water Reverse Osmosis unit; a first valve means positioned in the third conducting line, wherein the first valve means is a two-way valve configured to enable flow from desalination module outlets to desalination module inlets of the or each desalination module and for discharge of brine from desalination module outlets of the or each desalination module in said retrofit unit without stopping desalination; a sixth conducting line at outlet of said first valve means to enable discharge of brine from said closed circuit of said retrofit unit; a sensor for measuring electric conductivity of recycled concentrate in said closed circuit of said retrofit unit to enable a follow up of desalination recovery in said closed circuit; and monitoring and control systems configured to enable continuous closed circuit desalination of desired recovery in said retrofit unit,