A crude oil storage tank comprising an electrostatic separator section including at least one set of electrostatic internals arranged to provide a treatment flow path oblique to a longitudinal centerline of the crude oil storage tank, the set of electrostatic internals arranged for communication with an oil-and-water inlet section of the crude oil storage tank, a water outlet section of the crude oil storage tank, and an oil outlet section of the crude oil storage tank; wherein an oil-and-water stream entering the set of electrostatic internals travels along the treatment flow path and is subjected to an electric field, wherein the set of electric internals includes at least two legs, a first leg and a second leg located downstream of the first leg, arranged parallel to each other and at an incline, wherein each leg has an upflow end or portion and a downflow end or portion, wherein the downflow end or portion of both legs is open at the bottom and passes a water dominant flow to the water outlet section, wherein the upflow end of the first leg is a closed end, whereas the upflow end of the second leg is an open end, wherein the upper ends of the legs (72A, 72B) lie closer to the oil outlet section of the crude oil storage tank than the bottom ends, wherein the oil-and-water stream entering the inlet section is distributed to an inlet of the first leg and initial separation occurs as the stream is subjected to a treatment field, with an oil dominant flow flowing upwardly and a water dominant flow flowing downwardly through the open bottom end of the first leg and the oil dominant flow exits an outlet of the first leg, which serves as an inlet to the second leg, where the stream is subjected to a treatment field within the second leg, where another water dominant flow exits the open bottom end and a now-drier oil dominant flow exits the open end of the second leg, wherein the oil dominant flow enters the oil outlet section of the tank where it is removed as outlet stream, wherein electrodes are placed in the inlets, the upflow ends or portions (75, 85) of the legs, or in both the inlets and upflow ends or portions (75, 85) and are connected to a voltage source to establish the treatment fields within the legs. an electrostatic separator section including at least one set of electrostatic internals arranged to provide a treatment flow path oblique to a longitudinal centerline of the crude oil storage tank, the set of electrostatic internals arranged for communication with an oil-and-water inlet section of the crude oil storage tank, a water outlet section of the crude oil storage tank, and an oil outlet section of the crude oil storage tank; wherein an oil-and-water stream entering the set of electrostatic internals travels along the treatment flow path and is subjected to an electric field, wherein the set of electric internals includes at least two legs, a first leg and a second leg located downstream of the first leg, arranged parallel to each other and at an incline, wherein each leg has an upflow end or portion and a downflow end or portion, wherein the downflow end or portion of both legs is open at the bottom and passes a water dominant flow to the water outlet section, wherein the upflow end of the first leg is a closed end, whereas the upflow end of the second leg is an open end, wherein the upper ends of the legs (72A, 72B) lie closer to the oil outlet section of the crude oil storage tank than the bottom ends, wherein the oil-and-water stream entering the inlet section is distributed to an inlet of the first leg and initial separation occurs as the stream is subjected to a treatment field, with an oil dominant flow flowing upwardly and a water dominant flow flowing downwardly through the open bottom end of the first leg and the oil dominant flow exits an outlet of the first leg, which serves as an inlet to the second leg, where the stream is subjected to a treatment field within the second leg, where another water dominant flow exits the open bottom end and a now-drier oil dominant flow exits the open end of the second leg, wherein the oil dominant flow enters the oil outlet section of the tank where it is removed as outlet stream, wherein electrodes are placed in the inlets, the upflow ends or portions (75, 85) of the legs, or in both the inlets and upflow ends or portions (75, 85) and are connected to a voltage source to establish the treatment fields within the legs.