fig1 and 2 are views illustrating the structure and operation of a self reciprocating energy recovery device according to the first embodiment of the present invention the present invention relates to a self reciprocating energy recovery device utilized in driving of a seawater pump by recovering hydraulic power of high pressure concentrated water from a power recovery chamber in discharging treated water and concentrated water obtained by removing salt from sea water using reverse osmosis referring to fig1 and 2 the self reciprocating energy recovery device includes a pair of power recovery chambers 1 a and 1 b a low pressure seawater supply pipe 2 a high pressure seawater discharge pipe 3 a high pressure concentrated water supply pipe 4 a low pressure concentrated water discharge pipe 5 a concentrated water control value block 6 a pair of high pressure concentrated water pilots 73 a and 73 b a pair of high pressure concentrated water pilot valves 74 a and 74 b a pair of low pressure concentrated water pilots 75 a and 75 b and a pair of low pressure concentrated water pilot valves 76 a and 76 b the pair of power recovery chambers 1 a and 1 b are a first power recovery chamber 1 a and a second power recovery chamber 1 b having pistons 11 a and 11 b therein respectively.