Membranes with controlled porosity for serial filtration

A serial filtration system for liquid purification, comprising:

• a preliminary-stage reverse-osmosis module, comprising:
• (a) a preliminary-stage chamber including a preliminary-stage inlet;
• a preliminary-stage retentate outlet;
• and a preliminary-stage permeate outlet;
• and
• (b) at least one preliminary-stage reverse-osmosis membrane mounted in the preliminary-stage chamber and separating the preliminary-stage chamber into a retentate side, including the preliminary-stage inlet and the preliminary-stage retentate outlet, on an upstream side of the at least one preliminary-stage reverse-osmosis membrane and a permeate side, including the preliminary-stage permeate outlet, on a downstream side of the at least one preliminary-stage reverse-osmosis membrane;
• a first-stage reverse-osmosis module, comprising:
• (a) a first-stage chamber including a first-stage inlet;
• a first-stage retentate outlet;
• and a first-stage permeate outlet;
• and
• (b) at least one first-stage oxidized membrane, comprising an oxidized polyamide active layer coated on a porous support, wherein the oxidized polyamide active layer has an atomic oxygen/nitrogen ratio of at least 1.5:1 and includes chlorine at a concentration of 1-10 atomic percent, and wherein the at least one first-stage oxidized membrane is mounted in the first-stage chamber and separates the first-stage chamber into a retentate side, including the first-stage inlet and the first-stage retentate outlet, on an upstream side of at least one first-stage oxidized membrane and a permeate side, including the first-stage permeate outlet, on a downstream side of the at least one first-stage oxidized membrane;
• and
• a first-stage-inlet passage providing fluid communication between the preliminary-stage retentate outlet and the first-stage inlet and configured to direct retentate from the retentate side of the preliminary-stage chamber through the at least one first-stage oxidized membrane in the first-stage chamber.