A system for treating a feed solution by closed circuit desalination, said system comprising:
- a module containing a selected number of membrane elements and comprising a module inlet, a first module outlet for discharging a pressurized concentrate stream, and a second module outlet for discharging a permeate stream to a permeate outlet line;
- a closed circuit line connecting said first module outlet and said module inlet, said closed circuit line suitable to convey the discharged pressurized concentrate stream from said first module outlet to said module inlet;
- a recirculation pump located within the closed circuit line, said recirculation pump having a variable frequency drive;
- a high-pressure pump (HP) comprising a HP inlet for receiving the feed solution from a first path and a HP outlet for providing a pressurized feed solution to the closed circuit line;
- a pressure exchange unit (PE) comprising:
- a first PE inlet for receiving the feed solution from a second path;
- a first PE outlet suitable for providing a pressurized feed solution to the closed circuit line at a first junction point within the closed circuit line;
- a second PE inlet suitable for receiving the pressurized concentrate stream from a second junction point within the closed circuit line, said second junction point is located between the first junction point and the first module outlet;
- a second PE outlet for providing a depressurized concentrate stream to a brine effluent line;
- a plurality of valves suitable to be configured in a first mode of operation wherein
- flow is enabled to pass between the second junction point and the first junction point, and
- the pressurized concentrate stream discharged from the first module outlet is mixed with the pressurized feed solution from the high-pressure pump and conveyed to the module inlet;
- and suitable to be configured in a second mode of operation wherein
- flow is prevented between the second junction point and the first junction point, and
- the pressurized concentrate stream discharged from the first module outlet is passed through the second junction point, the second PE inlet, the second PE outlet, and into the brine effluent line, and
- feed solution passes from the second path through the first PE inlet, the first PE out, the first junction point, and into the closed circuit line;
- and
- a control system suitable to alternate between the first and second modes of operation.