Abstract: An inventive apparatus for closed circuit batch-RO desalination comprising a RO-skid with membrane elements and circulation means for the recycling of RO concentrate through membranes and a designed hydraulic-arm with a disc separating between a section of pressurized hydraulic fluid created by a high pressure hydraulic pump under fixed flow and variable pressure conditions and a section of RO recycled concentrates. The batch desalination sequence is completed when the entire hydraulic-arm volume is filled with hydraulic fluid and thereafter, desalination is stopped, said apparatus decompressed, brine removed and hydraulic-arm recharged with fresh feed before the initiation of a new batch sequence. The inventive apparatus enables RO desalination under the lowest energy and highest recovery prospects not possible by any other RO technique.