Systems and methods of atmospheric carbon dioxide enrichment and delivery to photobioreactors via membrane carbonation

Abstract

The present application focuses on systems and methods that utilize one or more carbon dioxide (CO.sub.2) sorbent substrates and a swing cycle, e.g., a moisture swing cycle, to increase the partial pressure of the CO.sub.2 in a gaseous feedstock, which is delivered through a membrane to a bioreactor, such as a membrane carbonation photobioreactor. Such systems and processes offer an effective means for concentrating and capturing CO.sub.2 obtained from air and delivering the concentrated CO.sub.2 to a photobioreactor through a membrane.

Claims

1. A system for growing phototrophic microorganisms comprising: a bioreactor comprising a microorganism-containing liquid and a membrane system; a collector comprising a sorbent for capturing CO.sub.2 from air; a regeneration unit configured to: receive the collector within an enclosure, regenerate the sorbent by causing it to release sorbed CO.sub.2, and capture the released CO.sub.2; a storage tank coupled to the regeneration unit, the storage tank comprising a headspace, the storage tank being configured to receive the released CO.sub.2 from the regeneration unit and containing at least a portion of the released CO.sub.2 from the regeneration unit as a storage solution and at least a portion of the released CO.sub.2 from the regeneration unit as a CO.sub.2 gas, the headspace of the storage tank containing the CO.sub.2 gas; and a pump coupled to the headspace and configured to pump the CO.sub.2 gas from the headspace to a bioreactor via membrane system, the membrane system comprising one or more membranes configured for diffusion-driven delivery of a CO.sub.2 gas across the one or more membranes to the microorganism-containing liquid of the bioreactor, a first side of a membrane of the one or more membranes configured to interface with a CO.sub.2 gas stream from the storage tank and a second side of the membrane of the one or more membranes configured to interface with a portion of the microorganism-containing liquid in the bioreactor, the second side different than the first side.

2. The system of claim 1, where the collector comprises a composite material comprising the sorbent and having a hydrophobic surface.

3. The system of claim 1, where: the sorbent comprises a composite material having a surface, and a porous hydrophobic material is disposed on the surface.

4. The system of claim 3, where the porous hydrophobic material is one or more of the following: a polyolefin, a fluoropolymer, and a fluoropolymeric membrane.

5. The system of claim 1, where the pH of the liquid in the bioreactor is maintained between 7 to 10, between 8.5 to 9.5, or between 8.8 to 9.2.

6. The system of claim 1, where the pH of the liquid in the bioreactor is maintained at a pH between 10 to 11 or between 0 to 7.

7. The system of claim 1, where the membrane system is disposed in a lower light area within the bioreactor.

8. The system of claim 1, where the one or more membranes have a membrane specific surface area at or less than 25 m.sup.−1, less than 20 m.sup.−1, less than 18 m.sup.−1, less than 15 m.sup.−1, less than 12 m.sup.−1, less than 5 m.sup.−1, less than 1 m.sup.−1 or less than 0.5 m.sup.−1.

9. The system of claim 1, where a surface area of the one or more membranes interfacing with the liquid is between 0.005 to 0.025 m.sup.2 per L of algae-containing liquid.

10. The system of claim 1, where a gaseous flow of the CO.sub.2 gas received by the membrane system comprises between 3% and 5% CO.sub.2, between 5% and 10% CO.sub.2, between 10% and 30% CO.sub.2, between 30% and 50% CO.sub.2, between 50% and 80% CO.sub.2, or between 80% and 100% CO.sub.2.

11. The system of claim 1, wherein the regeneration unit is configured to regenerate the sorbent and release the CO2 from the sorbent by causing one or more of a humidity increase, a temperature increase, and a pressure decrease within the regeneration unit.

12. The system of claim 1, wherein the one or more membranes comprise a plurality of hollow fiber membranes through which the CO.sub.2 from the storage tank passes.

13. The system of claim 1, where the sorbent is disposed on a sheet that is configured to be rolled and disposed in the regeneration unit and unrolled and exposed to the air.

14. The system of claim 1, where the sorbent is disposed on a sheet that is configured to be folded and disposed in the regeneration unit and unfolded and exposed to the air.

15. The system of claim 1, where the storage solution comprises bicarbonate and carbonate.

16. The system of claim 15, where the pump is configured to extract CO.sub.2 from the storage solution by causing a shift in pressure, temperature, or a combination thereof.

17. The system of claim 16, where the pump is configured to cause a shift in a temperature of the storage solution.

18. The system of claim 16, where a gaseous flow passes through the headspace having a CO.sub.2 partial pressure that is lower than a vapor pressure of CO.sub.2 for the storage solution.

19. The system of claim 1, wherein the sorbent comprises a plurality of quaternary amines or carbonate perfused activated carbon.

20. The system of claim 1, further comprising a compressor configured to compress CO.sub.2, either extracted from the storage solution or released from the sorbent during regeneration, to gas pressures at or greater than 1, 2, 3, 4, or 5 atm.

21. The system of claim 1, wherein the one or more membranes comprise a plurality of hollow fiber membranes configured to receive CO.sub.2 from the storage tank.

22. The system of claim 1, wherein the bioreactor is configured to send gas, after the gas has passed through the membrane, to the storage tank to increase a CO.sub.2 concentration and the storage tank is configured to receive the gas from the bioreactor and to increase its CO.sub.2 concentration.

23. A method of growing an algae culture comprising: providing the system of claim 1; exposing a sorbing substrate comprising the sorbent to an atmosphere comprising CO.sub.2 to capture a portion of the CO.sub.2; causing a release, inside the enclosure, of the captured CO.sub.2 from the sorbing substrate by altering pressure, humidity, temperature, or a combination thereof within the enclosure; and transferring a gas flow containing the released CO.sub.2 to the one or more membranes disposed in the bioreactor comprising a culture of phototrophic microorganisms, wherein the concentration of CO.sub.2 within the gas flow is higher than atmospheric concentration by at least 10-fold, wherein the released CO.sub.2 disposed within the one or more membranes diffuses across the one or more membranes and into the culture of phototrophic microorganisms.

24. The method of claim 23, where the pH of the liquid in the bioreactor is maintained at about a pH of 9.

25. The method of claim 23, where causing the release of the captured CO.sub.2 comprises applying an aqueous solution to the sorbing substrate.

26. The method of claim 23, comprising dissolving the CO.sub.2 released inside the enclosure in an aqueous solution.

27. The method of claim 26, comprising storing the aqueous solution with the dissolved CO.sub.2 in the storage tank.

28. The method of claim 26, comprising driving the CO.sub.2 out of the aqueous solution and into the gas flow.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The following drawings illustrate by way of example and not limitation. For the sake of brevity and clarity, every feature of a given structure may not be labeled in every figure in which that structure appears. Identical reference numbers do not necessarily indicate an identical structure. Rather, the same reference number may be used to indicate a similar feature or a feature with similar functionality, as may non-identical reference numbers.

(2) FIG. 1A illustrates a schematic of an embodiment in accordance with the present disclosure.

(3) FIG. 1B illustrates a schematic of a collector and a regeneration unit in accordance with the present disclosure.

(4) FIG. 2 illustrates the moisture-swing process with a quaternary amine sorbent. As shown in A), a wet resin binds a single CO.sub.2 as a carbonate ion (CO.sub.3.sup.2−) for every two quaternary amines. B) As the resin dries, some quaternary amines bind with a hydroxide and others with a bicarbonate ion (HCO.sub.3.sup.−). C) If the dried resin is exposed to CO.sub.2, a bicarbonate ion can form by the hydroxide ion reacting with CO.sub.2. D) Exposing the dried resin to moisture after it has been exposed to CO.sub.2, the water causes a release of CO.sub.2 and regenerates the wet resin to (A).

(5) FIG. 3 shows isotherms of the equilibrium partial pressure (P) of CO.sub.2 over a resin as function of the fraction (S) of the CO.sub.2 binding sites occupied. P is 500 times larger for the wet resin than the dry resin. Data are fit to Langmuir isotherms, i.e., P=P.sub.0S/(1−S), where P.sub.0 is a material property.

(6) FIG. 4 illustrates a schematic of a second embodiment in accordance with the present disclosure.

(7) FIG. 5 illustrates schematic of two 5-L membrane carbonation photobioreactor co-systems set-up at the bench scale. The components include as follows: (1) CO.sub.2 gas cylinder, (2) Pressure gauge, (3) Peristaltic pumps, (4) Magnetic stirrers, (5) Reservoir bottle for medium solution, (6 and 7) Photobioreactor vessels, (8) Light source, (9) Reservoir bottles for effluent, (10 and 11) Membrane Carbonation modules.

DETAILED DESCRIPTION OF THE INVENTION

(8) Referring now to the drawings and more particularly to FIG. 1A, shown there and designated by the reference numeral 100 is a first embodiment of an atmospheric CO.sub.2 enrichment and delivery (ACED) system configured to concentrate CO.sub.2 from air and transfer the CO.sub.2 to an algae culture through a membrane system configured for diffusion-driven delivery of the CO.sub.2. In the embodiment shown, system 100 comprises: (i) a collector 10 configured to capture CO.sub.2 from air; (ii) a regeneration unit 20 configured to receive collector 10 within an enclosure, to regenerate collector 10 by causing it to release the captured CO.sub.2; and to retain the released CO.sub.2 in the form of dissolved CO.sub.2; (iii) a storage-extraction unit 30 comprising a storage tank to store at least a portion of the released CO.sub.2 from the regeneration unit 20 and configured to selectively extract the CO.sub.2 from the storage tank for delivery to a bioreactor 40; (iv) a bioreactor 40 containing an algae-containing liquid; and (v) a membrane system 50 comprising one or more membranes configured for diffusion-driven delivery of a gas across the membranes to the liquid. Membrane system 50 is configured so that one side of each membrane interfaces with a gas flow comprising CO.sub.2 from regeneration unit 20 and/or storage-extraction unit 30 and the other side of the membrane interfaces with a portion of the algae-containing liquid. In some embodiments, such a configuration can allow higher than ambient concentrations of CO.sub.2 be delivered to an algae culture to boost productivity and do so in a more cost-effective way than delivery through traditional methods, such as gas sparging.

(9) In some embodiments, collector 10 comprises a sorbent configured to selectively (or preferentially) capture CO.sub.2 from air through the binding action of the sorbent, and regeneration unit 20 is configured to cause the captured CO.sub.2 to be released from the sorbent thereby regenerating collector 10. In some embodiments, collector 10 comprises a sorbent that is attached to or coated on a substrate.

(10) Collector 10 and/or regeneration unit 20 can be configured such that the collector can alternate between two environments, a first being one where collector 10 is exposed to outdoor/ambient air and the second being one where collector 10 is isolated from outdoor/ambient air (e.g., disposed within an enclosure of regeneration unit 20 which can be sealed). Collector 10 can be configured to move between at least two positions, namely, out of and into the regeneration unit 20, or an enclosure of regeneration unit 20 can be configured to move between at least two positions. For example, as shown in FIG. 1B, collector 10 can be coupled to a hoist 11 comprising a motor 12. Hoist 11 is configured to move collector 10 between a position that is within the enclosure 22 of regeneration unit 20 and a position where collector 10 is exposed to ambient/outdoor air, external to the enclosure 22. Enclosure 22 can comprise a cover 24 that is configured to move between two positions so that the enclosure can be opened to receive collector 10 and closed to seal enclosure and recover the absorbed CO.sub.2. As shown, cover 24 can also be coupled to hoist 11 and configured to move with collector 10.

(11) In other embodiments, collector 10 does not move between two positions but the enclosure of regeneration unit 20 is configured to move between two positions, such that the collector is disposed within the enclosure or disposed outside of the enclosure. And yet still in other embodiments, regeneration unit 20 can be configured to have one or more walls or sections of walls that form the enclosure and that move between two positions such that collector 10 disposed therein is exposed to outdoor/ambient air when the walls or section of walls are in a first position and the collector is isolated from outdoor/ambient air (e.g., the enclosure is sealed) when the walls are in a second position.

(12) In some embodiments, the released CO.sub.2, released in the regeneration unit 20, is transferred to membrane system 50. In some embodiments, the released CO.sub.2 is transferred to the storage tank of storage-extraction module 30. Storage-extraction unit 30 is configured to produce a gas stream comprising a CO.sub.2 concentration higher than ambient concentration with at least a portion of the CO.sub.2 obtained from the solution in the storage tank. This gas stream is then fed to bioreactor 40 through membrane system 50. Through membrane system 50, the CO.sub.2 is delivered by diffusion to the algae-containing liquid as needed. Of the CO.sub.2 delivered to the microalgae, some fraction may be directly taken from the regeneration unit 20, storage-extraction unit 30, air, and/or other sources that were available at a specific site, such as an exhaust gas stream.

(13) In some embodiments, the storage tank of extraction-storage unit 30 is included to account for discrepancies between the CO.sub.2 pressure delivered by regeneration unit 20 and the CO.sub.2 demand of the microalgal growth system. The CO.sub.2 delivered by the regeneration subsystem 20 will fluctuate with temperature and humidity conditions, whereas the CO.sub.2 demand of the bioreactor 40 varies with the seasons, temperature, and light levels. The extraction-storage unit 30 ensures the CO.sub.2 released from regeneration unit 20 is always actively taken up, even if the algae demand is not sufficient, and there is always a reliable CO.sub.2 supply for the microalgal growth system, even if atmospheric conditions were not conducive for capturing sufficient amounts of CO.sub.2.

(14) As mentioned above, collector 10 is exposed to air and preferentially sorbs CO2. Collector 10 is able to capture CO2 through the action of a sorbent. For example, in some embodiments, polystyrene anion exchange resins, which are functionalized with quaternary ammonium ions, are used as the sorbent to capture atmospheric CO2 and to selectively release the CO2 using a wet-dry cycle, as shown in FIG. 2. This wet-dry cycle is in the following referred to as a moisture swing. Functionalized polystyrene anion exchange resins can contain about 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4 moles of quaternary ammonium ions per kilogram, or any value therebetween. In the loaded state, the charge of the quaternary ammonium ion is balanced by a bicarbonate ion (HCO3—). In the unloaded state the charge of two quaternary ammonium ions is balanced by a carbonate ion (CO3 2—). As shown in FIG. 2 the unloaded state still binds CO2, and, when compared to the state where all ions have been exchanged with hydroxide, it could still be considered half full. However, these states which contain less inorganic carbon than the carbonate state cannot be reached with a moisture swing. When an unloaded resin (FIG. 2), holding one carbon per two quaternary ammonium ions, is exposed to ambient air with low relative humidity, it dries and loads up with atmospheric CO2 until the anions are nearly all bicarbonates (FIG. 2). When the loaded resin is wetted or exposed to sufficient humidity within regeneration unit 20, the state holding one carbon for every two cations is preferred and the equilibrium shifts from bicarbonate to carbonate releasing CO2 from the sorbent (e.g., 2HCO3—.fwdarw.CO2—+CO2+H2O) where it is captured into a contained environment for collecting the CO2. In some embodiments, a sweep gas flows through the regeneration unit 20 to carry the released CO2 to the storage-extraction unit and/or bioreactor 40. In some embodiments, the sweep gas is air.

(15) The sorbent of collector 10 can be a porous material For example, the sorbent can be a membrane-type or sheet-like material that contains small active sorbent materials in its pores. In some embodiments, the sorbent of collector 10 comprises a felt-like material that comprises an ion-exchange material in a powder or fibrous form. In some embodiments, the felt-like material is disposed within a plurality of air-permeable baffles akin to a down blanket or quilt.

(16) FIG. 3 shows that, at fixed temperature and loading, the equilibrium partial pressure of CO.sub.2 over the resin increases 500 fold as the relative humidity changes from 21% to 100%, using moisture to swing the CO.sub.2 partial pressure over the resin. In some embodiments, a plurality of collectors 10 can be used in system 100. In some embodiments, the regeneration unit 20 is configured to have a counter flow design where, a counter flow of a sweep gas advances from a first collector disposed in unit 20 to a second collector disposed in unit 20 and so on. The first collector holds less sorbed CO.sub.2 than the second collector, and, similarly, when present, the second collector holds less sorbed CO.sub.2 than a third collector. With a counter flow design, the last collector encountered by the sweep gas before the sweep gas exits the regeneration unit 20 is at the start of its regenerating process and thus holds the highest concentration of CO.sub.2. The first collector encountered by the sweep gas after entering the regeneration unit 20 is at the end of its regenerating process and thus holds the lowest concentration of CO.sub.2. The air in contact with each unit contains less CO.sub.2 than the equilibrium partial pressure over the unit and thus CO.sub.2 is released from each unit. However, the exit partial pressure of CO.sub.2 will exceed the equilibrium partial pressure over the first nearly empty unit. In some embodiments, each collector 10 can advance through the regeneration process by way of a conveyor that moves opposite from the direction of the sweep gas. In some embodiments, the CO.sub.2 partial pressure in a continuous gas stream can be amplified to 5000 Pa, or more than 100 times higher than in ambient air.

(17) Regeneration unit 20 comprises one or more enclosures configured to receive one or more collector 10 and can be configured to regenerate the sorbent and cause release of CO.sub.2 from the sorbent by causing one or more of a humidity increase, a temperature increase, and a pressure decrease at the surface of the sorbing substrate while within an enclosure of the regeneration unit. In some embodiments, regeneration unit 20 can comprise a buffer tank that comprises an aqueous solution that is applied to the sorbing substrate of collector 10, thereby regenerating the substrate and yielding a storage solution that can have a higher ratio of bicarbonate to carbonate than the initial solution. The storage solution is transferred to storage tank of storage-extraction unit 30. In some embodiments, CO.sub.2 from the sweep gas is transferred to the storage solution, such as through a gas exchange membrane or a trickle bed exchanger or similar device.

(18) In some embodiments, the storage solution has a lower pH than that of the initial aqueous solution. In some embodiments, the aqueous solution can comprise carbonate to bicarbonate ratio greater than or about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, or 100 or any value therebetween. In some embodiments, the aqueous solution can comprise bicarbonate to carbonate ratio greater than or about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, or 100 or any value therebetween. In embodiments, regeneration unit 20 can regenerate the sorbing substrate by applying water at about neutral pH to release bicarbonate and cause a decrease in the pH of the aqueous solution.

(19) In some embodiments, storage-extraction unit 30 comprises at least one pump and one or more storage tanks for holding the storage solution and is configured to extract CO.sub.2 from the storage tank solution for release into a gas stream either via a shift in pressure, temperature, or a combination thereof. The CO.sub.2 concentration in the gas stream can be varied by the degree of the shift in pressure, temperature, or a combination thereof. In some embodiments, a gas stream can comprise CO.sub.2 at 3% to 5%. Once the storage solution is effectively spent as a source of CO.sub.2, it can be transferred to the buffer tank to be used as the aqueous solution to regenerate the sorbent. In some embodiments, storage-extraction unit 30 is configured to maintain a headspace above the bicarbonate/carbonate solution and to pump the gas from the headspace. A gaseous flow passes through the headspace having a CO.sub.2 partial pressure that is lower than the vapor pressure of CO.sub.2 for the carbonate/bicarbonate solution.

(20) In some embodiments, collector 10 comprises a composite material comprising a sorbing surface. The sorbing surface can be adapted to impede salt build-up on the surface, which may occur as a result of repeat regeneration cycles. For example, the composite material can comprise a hydrophobic surface. In particular, a composite material comprising the sorbent can have a porous hydrophobic material disposed on the surface. The porous hydrophobic coating can comprise a polyolefin (e.g., a flashspun high-density polyethylene fibers such as Tyvek®), a fluoropolymer (e.g., polytetrafluoroethylene (PTFE) such as Teflon®), or a fluoropolymeric membrane (e.g., an expanded PTFE). The hydrophobic coating can be applied by vapor deposition, brush, dip or spray coating, or any method used to apply these hydrophobic coatings.

(21) The surface area, shape, and dimensions of the sorbing surface of each collector 10 and the number of collectors 10 can depend on the CO.sub.2 requirements of the algae culture being fed. In some embodiments, collector 10 comprises a wind-facing area that is at or less than 1 m.sup.2. In some embodiments, collector 10 comprises a wind-facing area that is at or less than 10 m.sup.2.

(22) Bioreactor 40 comprises a reservoir configured to retain a culture of phototrophic microorganisms (such as an algae culture) in the reservoir and a membrane carbonation system 50 comprising one or more membranes configured for diffusion-driven delivery of CO.sub.2. Bioreactor 40 can be open or closed. In FIG. 1A, the CO.sub.2 from the storage tank is delivered to an open pond bioreactor via pathway 4a and/or delivered to a closed photobioreactor system via pathway 4b. In some embodiments, the one or more membranes are located in the lower light area of the reservoir. In some embodiments, a portion of the reservoir is blocked from a light source, and the one or more membranes are contained within the portion blocked from light. For example, the membrane system is placed near the middle of a closed photobioreactor, at the deeper levels of an open bioreactor, or behind a light shield, filter, or reflector in bioreactor 40 or in a separate enclosure through which the algae-containing media in bioreactor 40 is circulated. In some embodiments, the one or more membranes comprise a plurality of hollow fiber membranes through which the CO.sub.2-containing gas flows. In some embodiments, the one or more membranes are a non-porous polypropylene tubes. In some embodiments gas flows in one end of the hollow fiber membranes that are closed at the other end. In other embodiments, a hollow fiber membrane is not closed at the other end and after the gas stream passes through the membrane in contact with the liquid medium, it is returned back to the storage tank for further enrichment and then circulated through the hollow fiber membrane again. In some embodiments, the tubes have an outer diameter of about 100 μm to 500 μm, such as 120, 140, 160, 180, 200, 220, 240, 260, 280, 300, 320, 340, 360, 380, 400, 425, 450, 500 μm, or any value or range therebetween. In some embodiments, each membrane can have a lumen diameter of 50 μm to 150 μm, such as 54, 58, 62, 66, 70, 74, 78, 80, 85, 90, 100, 110, 120, 130, 140, 150 μm or any value or range therebetween. In some embodiments, each membrane can have an outer diameter of about 200 μm and a lumen diameter of 67 μm. In some embodiments, each membrane can have an apparent CO.sub.2 diffusivity of 10.sup.−4 m.sup.2/d to 10.sup.−6 m.sup.2/d, such as about 10.sup.−5 m.sup.2/d.

(23) The conditions of the liquid media and of the membrane system are such to facilitate diffusion of CO.sub.2 across the membrane at a rate substantially equivalent to the rate at which the culture consumes CO.sub.2. Such conditions include the pH of the liquid media, the specific surface area of the membrane, the surface area of the membranes contacting the liquid media, and the partial pressure of the CO.sub.2 on the one side of the membrane. For example, in some embodiments, the pH of the liquid media in bioreactor 40 is maintained between 7 to 10. For example, the pH of the liquid media is between 8.5 to 9.5 or between 8.8 to 9.2. In some embodiments, the pH of the liquid media is maintained at about a pH of 9. In other embodiments, the pH is greater than 10, such as 10.5, 10.7, 11 or any value or range therebetween, or less than 7, such as 6, 5, 4, 3.5, 3, 2.5, 2, 1.5, 1, 0.5, 0 or any value or range therebetween, to accommodate phototrophic microorganisms that favor those pH conditions. In some embodiments, the one or more membranes have a membrane specific surface area at or less than 25 m.sup.−1, 20 m.sup.−1, 18 m.sup.−1, 15 m.sup.−1, 12 m.sup.−1, 5 m.sup.−1, 1 m.sup.−1, 0.5 m.sup.−1 or any value or range therebetween. In some embodiments, the surface area of membranes interfacing with the fluid is between 0.005 to 0.025 m.sup.2 per L of algae-containing liquid. In some embodiments, the gaseous flow received by the membrane system comprises between 3% and 5% CO.sub.2, between 5% and 10% CO.sub.2, between 10% and 30% CO.sub.2, between 30% and 50% CO.sub.2, between 50% and 80% CO.sub.2, or between 80% and 100% CO.sub.2. In some embodiments, the CO.sub.2 transfer efficiency into the liquid media is at least 800/%, 85%, 900%, 95%, 98%, 99%, or 99.9%. Transfer efficiency, is the percentage of the CO.sub.2 that moves across the membrane wall and into the reactor fluid rather than escaping to a gas phase.

(24) In some embodiments, the concentration of dissolved inorganic carbon (DIC) in the liquid media is maintained at or below 10 mg DIC/L, such as at about 9.5, 9. 8.5, 8, 7.5, 7, 6.5, 6, 5.5, 5, 4.5, 4, 3.5, 3, 2.5, 2, 1.5, 1, or any value or range therebetween. In some embodiments, the concentration of DIC is maintained above 10 mg DIC/L, such as 11, 12, 13, 14, 15, 17.5, 20 or 25, or any value or range therebetween. In some embodiments, the concentration of DIC in the liquid media is maintained between 5 and 7 mg DIC/L. In some embodiments, the CO.sub.2 concentration within the membranes is monitored continuously with infrared gas analyzers.

(25) Referring now to FIG. 4, shown therein and designated by the reference numeral 200 is a second embodiment of an ACED system configured to concentrate CO.sub.2 from air and transfer the CO.sub.2 to an algae culture through a membrane system configured for diffusion-driven delivery of the CO.sub.2. Embodiment 200 is substantially similar to embodiment 100 described above, except that that system 200 further comprises a compressor 60 configured to compress CO.sub.2, extracted from the storage solution and/or released from the substrate during regeneration, to gas pressures at or greater than 1, 2, 3, 4 or 5 atm, or any value therebetween. Such a compressor can be used to increase the partial pressure of the CO.sub.2 for efficient delivery to bioreactor 40 through membrane system 50. In some embodiments, a gaseous flow from the headspace above the storage tank would flow into the compressor. In some embodiments, the gas produced during the regeneration process can flow into the compressor.

EXAMPLE

(26) By way of example, a study was conducted to demonstrate the effectiveness of CO.sub.2 taken up by an algae culture with a diffusion-delivery membrane system. FIG. 5 illustrates the experimental set-up. Membrane loops were placed near the middle of the photobioreactor so that they were not illuminated and did not accumulate significant biofilm. The systems were operated continuously to grow Synechocystis sp. PCC6883 with a modest external light intensity of 44 W/m.sup.2 (˜220 μE/m.sup.2-sec), CO.sub.2 pressure of about 1 atm, and different membrane specific surface areas <0.2 m.sup.−1. Long-term performance at a pH controlled to near 9 demonstrated that CO.sub.2 off-gas was zero, and at least 95% of the delivered CO.sub.2 was invested into organic products of photosynthesis, with ≤5% in DIC in the effluent liquid. Thus, membrane carbonation delivered CO.sub.2 with no waste and allowed good control at a suitable pH.

(27) The above specification and examples provide a complete description of the structure and use of an exemplary embodiment. Although certain embodiments have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the scope of this invention. As such, the illustrative embodiments of the present ACED systems and methods are not intended to be limited to the particular forms disclosed. Rather, they include all modifications and alternatives falling within the scope of the claims, and embodiments other than the one shown may include some or all of the features of the depicted embodiment. For example, components may be combined as a unitary structure and/or connections may be substituted. Further, where appropriate, aspects of any of the examples described above may be combined with aspects of any of the other examples described to form further examples having comparable or different properties and addressing the same or different problems. Similarly, it will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments.

(28) The claims are not to be interpreted as including means-plus- or step-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase(s) “means for” or “step for,” respectively.