A method for improved oil recovery, comprising: introducing seawater to an electrically-driven separation system (200, 500); treating the seawater with the electrically-driven separation system to produce treated water (280; 580) characterized in that treating the seawater comprises passing a first portion (220; 520) of the seawater through a first electrodialysis apparatus (240; 540) to produce a first product water (260; 560) and passing a second portion (230; 530) of the seawater through a second electrodialysis apparatus (250; 550) to produce a second product water (270; 570), wherein the first electrodialysis apparatus (240; 540) and the second electrodialysis apparatus (250; 550) are fluidly connected in parallel, and wherein the first electrodialysis apparatus (240; 540) has standard cation and anion exchange membranes which remove divalent ions, including sulfate, to a much greater extent than monovalent ions and produce the first product water (260; 560) that has a reduced concentration of divalent anions and cations, and the second electrodialysis apparatus (250; 550) has standard anion exchange membranes, which preferentially remove divalent anions, and monovalent selective cation exchange membranes which do not efficiently remove divalent ions and thus a concentration of hardness in the second portion (230; 530) of the seawater is not substantially reduced, whereby the second product water (270; 570) treated by the second electrodialysis apparatus (250; 550) contains significantly more hardness relative to the first product water (260; 560), and further comprising tuning the quality of the treated water (280; 580) by adjusting the operating parameters of the first electrodialysis apparatus (240; 540), and/or the second electrodialysis apparatus (250; 550), and blending the first product water (260; 560) and the second product water (270; 570) to produce the treated water (280; 580) having less than about 200 mg/l of sulfate, a total dissolved solids content in the range of from 1000 mg/l to 5000 mg/l, and a hardness concentration in the range of from 100 ppm to 1000 ppm; and flooding at least a portion of an oil reservoir with the treated water (280; 580). introducing seawater to an electrically-driven separation system (200, 500); treating the seawater with the electrically-driven separation system to produce treated water (280; 580) characterized in that treating the seawater comprises passing a first portion (220; 520) of the seawater through a first electrodialysis apparatus (240; 540) to produce a first product water (260; 560) and passing a second portion (230; 530) of the seawater through a second electrodialysis apparatus (250; 550) to produce a second product water (270; 570), wherein the first electrodialysis apparatus (240; 540) and the second electrodialysis apparatus (250; 550) are fluidly connected in parallel, and wherein the first electrodialysis apparatus (240; 540) has standard cation and anion exchange membranes which remove divalent ions, including sulfate, to a much greater extent than monovalent ions and produce the first product water (260; 560) that has a reduced concentration of divalent anions and cations, and the second electrodialysis apparatus (250; 550) has standard anion exchange membranes, which preferentially remove divalent anions, and monovalent selective cation exchange membranes which do not efficiently remove divalent ions and thus a concentration of hardness in the second portion (230; 530) of the seawater is not substantially reduced, whereby the second product water (270; 570) treated by the second electrodialysis apparatus (250; 550) contains significantly more hardness relative to the first product water (260; 560), and further comprising tuning the quality of the treated water (280; 580) by adjusting the operating parameters of the first electrodialysis apparatus (240; 540), and/or the second electrodialysis apparatus (250; 550), and blending the first product water (260; 560) and the second product water (270; 570) to produce the treated water (280; 580) having less than about 200 mg/l of sulfate, a total dissolved solids content in the range of from 1000 mg/l to 5000 mg/l, and a hardness concentration in the range of from 100 ppm to 1000 ppm; and flooding at least a portion of an oil reservoir with the treated water (280; 580).