Subsurface water purification method

1. A subsurface water purification method, comprising:

introducing ambient subsurface source water into and through a first ultrafiltration membrane unit and a second ultrafiltration membrane unit during a forward operating cycle using a system pump, thereby simultaneously producing an ultrafiltrate substantially free of solid particulates having a largest dimension greater than 0.1 microns;

preparing an aqueous solution comprising one or more hypohalous acid species in an electrochemical unit in fluid communication with the first ultrafiltration membrane unit; and

using the system pump to deliver at least a portion of the ultrafiltrate from one of the first and second ultrafiltration membrane units and at least a portion of the aqueous solution to the other one of the first and second ultrafiltration membrane units in an opposite direction of flow as the source water during a backwash cycle;

wherein a flux of the source water through each of the first and second ultrafiltration membrane units is less than thirty gallons per square foot per day.

2. The method according to claim 1 , wherein the backwash cycle occurs at a frequency of less than five backwash cycles per day.

3. The method according to claim 2 , wherein each backwash cycle is characterized by a duration of less than twenty minutes.

4. The method according to claim 1 , wherein the backwash cycle is characterized by a duration of less than twenty minutes.

5. The method according to claim 1 , further comprising subjecting the ultrafiltrate to a nanofiltration separation step in a nanofiltration unit, thereby producing a nanofiltrate and a retentate rejected by the nanofiltration unit.

6. The method according to claim 5 , further comprising subjecting the ultrafiltrate to a reverse osmosis separation step in a reverse osmosis membrane unit, thereby producing a permeate and a retentate rejected by the reverse osmosis membrane unit.

7. The method according to claim 6 , wherein a purified water stream comprises at least a portion of the nanofiltrate and at least a portion of the reverse osmosis membrane permeate.

8. The method according to claim 7 , wherein the purified water stream contains less than two percent dissolved solids and less than one hundred parts per million sulfate species.

9. The method according to claim 7 , wherein at least a portion of the retentate rejected by the reverse osmosis membrane unit is used as a source fluid for the aqueous solution comprising one or more hypohalous acid species.

10. The method according to claim 1 , wherein the ambient subsurface source water is seawater.

11. A subsurface water purification method for producing purified water, comprising:

introducing ambient subsurface source water into and through a first ultrafiltration membrane unit and a second ultrafiltration membrane unit during a forward operating cycle of a subsurface water treatment system using a system pump, thereby producing an ultrafiltrate substantially free of solid particulates having a largest dimension greater than 0.1 microns;

preparing an aqueous solution comprising one or more hypohalous acid species in an electrochemical unit in fluid communication with the first ultrafiltration membrane unit;

performing one of:

(1) using the system pump to deliver at least a portion of the ultrafiltrate from one of the first and second ultrafiltration membrane units to the other one of the ultrafiltration membrane units in an opposite direction of flow as the source water during a backwash cycle; and

(2) using the system pump to deliver at least a portion of the ultrafiltrate from one of the first and second ultrafiltration units and at least a portion of the aqueous solution comprising one or more hypohalous acid species to the other one of the first and second ultrafiltration membrane units during a chemical backwash cycle of the subsurface water treatment system;

subjecting a first portion of the ultrafiltrate to a nanofiltration separation step in a nanofiltration unit of the subsurface water treatment system to produce a nanofiltrate and a retentate rejected by the nanofiltration unit; and

subjecting a second portion of the ultrafiltrate to a reverse osmosis separation step in a reverse osmosis membrane unit of the subsurface water treatment system to produce a permeate and a retentate rejected by the reverse osmosis membrane unit;

wherein a flux of the source water through each of the first and second ultrafiltration membrane units is less than thirty gallons per square foot per day.

12. The method according to claim 11 , wherein a purified water stream comprises at least a portion of the nanofiltrate and at least a portion of the reverse osmosis membrane permeate.

13. The method according to claim 12 , wherein the purified water stream contains less than two percent dissolved solids.

14. The method according to claim 12 , wherein the purified water stream contains less than 100 parts per million sulfate species.

15. The method according to claim 11 , wherein the ambient subsurface source water is seawater.

16. The method according to claim 11 , wherein the electrochemical unit is configured to receive the reverse osmosis membrane unit retentate as a source fluid and to produce therefrom an aqueous solution comprising one or more hypohalous acid species.

17. The method according to claim 11 , wherein the subsurface water treatment system is configured to produce at least one product stream not delivered to the the other one of the first and second ultrafiltration membrane units during the backwash cycle.

18. The method according to claim 11 , wherein at least one of the first and second ultrafiltration membrane units comprises hollow fiber membrane structures.

19. A method of producing a hydrocarbon comprising:

injecting purified water derived from an ambient subsurface source fluid into a hydrocarbon reservoir to stimulate flow of a hydrocarbon fluid from the reservoir;

receiving the hydrocarbon fluid in a hydrocarbon production well; and

transporting the hydrocarbon fluid from the production well to a storage facility;

wherein the purified water is produced in a subsurface water treatment system comprising;

a first ultrafiltration membrane unit and a second ultrafiltration membrane unit configured to produce from the source fluid an ultrafiltrate substantially free of solid particulates having a largest dimension greater than 0.1 microns during a forward operating cycle using a system pump of the subsurface water treatment system;

a backwash unit comprising an electrochemical unit and the system pump, the electrochemical unit in fluid communication with at least one of the first and second ultrafiltration membrane units and capable of producing an aqueous solution comprising one or more hypohalous acid species, wherein the system pump is capable of delivering the ultrafiltrate from one of the first and second ultrafiltration membrane units to the other one of the ultrafiltration membrane units in an opposite direction of flow as the source fluid during a backwash cycle, and wherein the system pump is capable of delivering the ultrafiltrate from one of the first and second ultrafiltration membrane units and at least a portion of the aqueous solution from the electrochemical unit to the other one of the first and second ultrafiltration membrane units in an opposite direction of flow as the source fluid during a chemical backwash cycle;

a nanofiltration membrane unit configured to receive the ultrafiltrate and produce therefrom a nanofiltrate containing less than 100 parts per million sulfate species; and

a reverse osmosis membrane unit configured to receive the ultrafiltrate and produce therefrom a reverse osmosis membrane permeate and a reverse osmosis membrane retentate;

wherein a flux of the source fluid through each of the first and second ultrafiltration membrane units is less than 30 gallons per square foot per day, and wherein the reverse osmosis membrane retentate is used as a source fluid for the electrochemical unit.

20. The method according to claim 19 , wherein the purified water comprises at least a portion of the nanofiltrate and at least a portion of the reverse osmosis membrane permeate.