A subsea system for injecting seawater into a subterranean formation comprising: a sub-micron filter module configured to accept incoming seawater from an inlet and use cross flow filtration to separate the incoming seawater into rejected seawater and sub-micron filtered seawater, wherein the sub-micron filter module comprises a particle filter, a micro-filter downstream of the particle filter and a nano filter downstream of the micro-filter; an injection pump configured to accept the sub-micro filtered seawater and increase its pressure for injection into the subterranean formation via a wellbore penetrating therethrough; and an auxiliary pump positioned downstream of the nano filter, the auxiliary pump having an inlet connected to accept the rejected seawater; a choke valve disposed between an outlet of the auxiliary pump and the sea, the choke valve configured to be selectively opened so as to provide a first flow path for the rejected seawater to the sea; a first ejector coupled to an inlet to the particle filter by a first valve; a second ejector coupled to an inlet of the micro filter by a second valve; a third valve disposed between the outlet of the auxiliary pump and the first ejector, the first and third valves (284, 280) being configured to be selectively opened while the choke valve is closed to provide a second flow path for the rejected seawater through the first ejector to backflush the particle filter; and a fourth valve disposed between the outlet of the auxiliary pump and the second ejector, the second and fourth valves (286, 282) being configured to be selectively opened while the choke valve is closed to provide a third flow path for the rejected seawater through the second ejector to backflush the micro filter. a sub-micron filter module configured to accept incoming seawater from an inlet and use cross flow filtration to separate the incoming seawater into rejected seawater and sub-micron filtered seawater, wherein the sub-micron filter module comprises a particle filter, a micro-filter downstream of the particle filter and a nano filter downstream of the micro-filter; an injection pump configured to accept the sub-micro filtered seawater and increase its pressure for injection into the subterranean formation via a wellbore penetrating therethrough; and an auxiliary pump positioned downstream of the nano filter, the auxiliary pump having an inlet connected to accept the rejected seawater; a choke valve disposed between an outlet of the auxiliary pump and the sea, the choke valve configured to be selectively opened so as to provide a first flow path for the rejected seawater to the sea; a first ejector coupled to an inlet to the particle filter by a first valve; a second ejector coupled to an inlet of the micro filter by a second valve; a third valve disposed between the outlet of the auxiliary pump and the first ejector, the first and third valves (284, 280) being configured to be selectively opened while the choke valve is closed to provide a second flow path for the rejected seawater through the first ejector to backflush the particle filter; and a fourth valve disposed between the outlet of the auxiliary pump and the second ejector, the second and fourth valves (286, 282) being configured to be selectively opened while the choke valve is closed to provide a third flow path for the rejected seawater through the second ejector to backflush the micro filter.