An apparatus for continuous closed-circuit consecutive sequential desalination of a salt water solution by reverse osmosis, the apparatus comprising: a closed circuit system comprising one desalination module having an inlet, a concentrate outlet, a permeate outlet, or a plurality of desalination modules (M 1,..., M n) having their respective inlets and outlets connected in parallel, and a concentrate recycling line connecting each concentrate outlet to each inlet of the one or more desalination modules (M 1,..., M n), each of said desalination modules (M 1,..., M n) comprising one or more membrane elements; a feed line for supplying fresh feed of salt water solution to the closed circuit system, a pressurizing pump in said feed line for pressurizing the fresh feed of salt water solution into the closed circuit system and for creating counter pressure to enable reverse osmosis desalination in the closed circuit system and replacement of released permeate by fresh feed; a circulation pump in said concentrate recycling line for recycling pressurized concentrate from each concentrate outlet to each inlet of the one or more desalination modules (M 1,..., M n); a permeate conducting line for permeate collection from the at least one desalination module of said closed circuit system; an effluent conducting line connected to the concentrate recycling line for removing brine effluent from the closed circuit system; a valve system in the concentrate recycling line downstream of the circulation pump and configured to divert concentrate flow from the concentrate recycling line into the effluent conducting line to enable periodic discharge of brine from the closed circuit system at a desired recovery level without stopping desalination; and a monitoring and control system including a first solution conductivity monitoring device (CM) arranged in the concentrate recycling line upstream of the module inlet(s) for monitoring the reached recovery level, a second solution conductivity monitoring device (CM) arranged in the effluent conducting line for determining brine rejection completion, a first pressure monitoring device (PM) arranged in the concentrate recycling line upstream of the first solution conductivity monitoring device (CM) and downstream of the valve system as well as downstream of the pressurizing pump, and a second pressure monitoring device (PM) arranged in the concentrate recycling line downstream of the circulation pump and upstream of the valve system to enable continuous closed circuit desalination of desired recovery proceed in consecutive sequential steps under variable pressure conditions by switching from a concentrate recycling mode to a concentrate discharge mode upon reaching the desired recovery level and by reactivating the concentrate recycling mode once brine rejection completion is determined. a closed circuit system comprising one desalination module having an inlet, a concentrate outlet, a permeate outlet, or a plurality of desalination modules (M 1,..., M n) having their respective inlets and outlets connected in parallel, and a concentrate recycling line connecting each concentrate outlet to each inlet of the one or more desalination modules (M 1,..., M n), each of said desalination modules (M 1,..., M n) comprising one or more membrane elements; a feed line for supplying fresh feed of salt water solution to the closed circuit system, a pressurizing pump in said feed line for pressurizing the fresh feed of salt water solution into the closed circuit system and for creating counter pressure to enable reverse osmosis desalination in the closed circuit system and replacement of released permeate by fresh feed; a circulation pump in said concentrate recycling line for recycling pressurized concentrate from each concentrate outlet to each inlet of the one or more desalination modules (M 1,..., M n); a permeate conducting line for permeate collection from the at least one desalination module of said closed circuit system; an effluent conducting line connected to the concentrate recycling line for removing brine effluent from the closed circuit system; a valve system in the concentrate recycling line downstream of the circulation pump and configured to divert concentrate flow from the concentrate recycling line into the effluent conducting line to enable periodic discharge of brine from the closed circuit system at a desired recovery level without stopping desalination; and a monitoring and control system including a first solution conductivity monitoring device (CM) arranged in the concentrate recycling line upstream of the module inlet(s) for monitoring the reached recovery level, a second solution conductivity monitoring device (CM) arranged in the effluent conducting line for determining brine rejection completion, a first pressure monitoring device (PM) arranged in the concentrate recycling line upstream of the first solution conductivity monitoring device (CM) and downstream of the valve system as well as downstream of the pressurizing pump, and a second pressure monitoring device (PM) arranged in the concentrate recycling line downstream of the circulation pump and upstream of the valve system to enable continuous closed circuit desalination of desired recovery proceed in consecutive sequential steps under variable pressure conditions by switching from a concentrate recycling mode to a concentrate discharge mode upon reaching the desired recovery level and by reactivating the concentrate recycling mode once brine rejection completion is determined.