Motor-pumping method and apparatus for recovery of energy from spent brine discharged from a reverse osmosis desalination system are disclosed. Energy of spent brine discharged from the reverse osmosis system is used to pump a portion of the fresh brine input to the system. The novel apparatus includes a pair of closed cylinders having pistons reciprocable therein which are interconnected for simultaneous piston movement. First valve means movable between first and second conditions are provided for atternatively connecting a source of fresh brine to the reduced piston area side of one of said cylinders while connecting the reduced area piston side of the other cylinder to the inlet of the reverse osmosis canister included in the reverse osmosis system. Second valve means movable between first and second conditions are provided for atternatively connecting the spent brine outlet from the reverse osmosis system canister to the large piston area side of the other cylinder while venting spent brine from the large piston area side of the one cylinder. Because of the differential opposite surface areas of the motor-pump pistons, fresh brine is pumped from the cylinders at a pressure higher than the pressure of spent brine operating liquid supplied to the motor-pump means from the reverse osmosis system. The first and second valve means operate under control of the position of the pistons in the cylinders for reciprocal piston operation.
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