Water purification system and method using reverse osmosis reject stream in an electrodeionization unit

1. A method for purifying water, comprising the steps of: separating a supply stream into an RO permeate stream and an RO reject stream with a reverse osmosis unit; providing the RO reject stream to a nanofiltration membrane; removing non-monovalent salt impurities from the RO reject stream with said nanofiltration membrane unit to produce a treated stream; providing the treated stream to a concentrating compartment of an electrodeionization unit; outputting a concentrate effluent stream from said concentrating compartment; providing a diluting feed stream to a diluting compartment of said electrodeionization unit, feeding said RO permeate stream to said diluting feed stream; and, producing an EDI product stream of purified water with said electrodeionization unit, said method producing said EDI product stream without using a recycle pump for recycling the concentrate effluent stream back to said concentrating compartment or said reverse osmosis unit.

2. The method of claim 1 , wherein the supply stream has a conductivity of about 3 μS/cm.

3. The method of claim 1 , wherein the EDI product stream has a conductivity of about 0.055 μS/cm.

4. The method of claim 1 , wherein said electrodeionization unit consumes less than about 2 megajoules of electrical energy per cubic meter of purified water in the EDI product stream produced.

5. The method of claim 1 , further comprising the step of discharging the concentrate effluent stream to a waste unit.

6. The method of claim 1 , further comprising the step of maintaining the RO reject stream at a pressure greater than atmospheric pressure between said reverse osmosis unit and said treatment unit.

7. The method of claim 1 , further comprising the step of providing a supplemental feed stream to said treatment unit.