WO2016060892A1

WO2016060892A1 - Systems and methods for cultivating algae

Info

Publication number: WO2016060892A1
Application number: PCT/US2015/054239
Authority: WO
Inventors: Daniel H. YEH; Ivy Lea Cormier DREXLER; Melanie Teresa PICKETT
Original assignee: University Of South Florida
Priority date: 2014-10-16
Filing date: 2015-10-06
Publication date: 2016-04-21

Abstract

In one embodiment, an algae cultivation system includes a passive membrane photobioreactor having an interior space in which algae can be cultivated and a porous membrane that separates growth media from the interior space, wherein water, carbon dioxide, and nutrients contained within the growth media can pass through the membrane and into the interior space but contaminants cannot.

Description

SYSTEMS AND METHODS FOR CULTIVATING ALGAE

Notice of Government-Sponsored Research

This invention was made with Government support under Grant Number 1236746, awarded by the National Science Foundation. The Government has certain rights in the invention.

Cross-Reference to Related Application

This application claims priority to co-pending U.S. Provisional Application Serial Number 62/064,556, filed October 16, 2014, which is hereby incorporated by reference herein in its entirety.

Background

Microalgae are increasingly recognized as a renewable source of biofuel, but also have many other commercially valuable applications, such as pharmaceuticals, animal/fish feed, fertilizers, etc. However, there remain many problems in the large- scale production of microalgae, which hinders its economic competitiveness with other biofuel crops. Microalgae are typically small in size (3-30 μιτι) and are cultivated in relatively dilute concentrations (generally less than 0.5 g dry biomass L-1 ), which makes harvesting and dewatering microalgal cultures particularly problematic and expensive. External inputs, such as nutrients, freshwater, and carbon dioxide, also add a substantial environmental and economic burden. Wastewater, on the other hand, is a free source of nutrients, water, and carbon dioxide and, if utilized in algae production, could improve the economics and environmental footprint of large-scale algae production. However, utilizing wastewater may increase the chances of introducing predators, grazers, and invasive species to a microalgal culture, which can lead to algal culture collapse. As a case in point, open raceway ponds are easily contaminated by environmental contaminants (e.g., bacteria, fungi, and rotifers). Although synthetic growth media can be sterilized before it is added to a closed algal culture, this can add- substantial cost to production.

From the above discussion, it can be appreciated that it would be desirable to have an alternative system and method with which algae can be cultivated.

Brief Description of the Drawings

The present disclosure may be better understood with reference to the following figures. Matching reference numerals designate corresponding parts throughout the figures, which are not necessarily drawn to scale.

Fig. 1 is a perspective view of an embodiment of a passive membrane photobioreactor.

Fig. 2 is a top view of a first embodiment of an algae cultivation system. Fig. 3 is a top view of a second embodiment of an algae cultivation system.

Fig. 4 is a top view of a third embodiment of an algae cultivation system.

Fig. 5 is a top view of a fourth embodiment of an algae cultivation system.

Fig. 6 is a side view of a passive membrane photobioreactor of the system of Fig.

5.

Fig. 7 is a side view of a fifth embodiment of an algae cultivation system.

Fig. 8 is a side view of a sixth embodiment of an algae cultivation system.

Fig. 9 is a side view of a seventh embodiment of an algae cultivation system. Fig. 10 is a top view of an eighth embodiment of an algae cultivation system. Fig. 1 1 is a partial side view of the system of Fig. 10.

Fig. 12 is a top view of a ninth embodiment of an algae cultivation system.

Detailed Description

As described above, it would be desirable to have an alternative system and method with which algae can be cultivated. Examples of such systems and methods are disclosed herein. In some embodiments, the systems and methods use passive membranes to take advantage of concentration gradients across a membrane to grow algae in a growth medium, such as wastewater. The membrane decouples the culture from the growth media, enabling the passive transport of constituents (i.e., nutrients and gases) from a larger nutrient pool while still maintaining a physical barrier for potential competitors/predators or contaminants, such as endemic wastewater species, airborne pathogens, or bacteria/protozoans/metazoans, contained in the growth media. Separating the culture from the growth media precludes the need to sterilize the media, which saves energy and cost.

In the following disclosure, various specific embodiments are described. It is to be understood that those embodiments are example implementations of the disclosed inventions and that alternative embodiments are possible. All such embodiments are intended to fall within the scope of this disclosure.

This disclosure addresses issues of cultivation, harvesting, and predator invasion in the production of algae. As described below, the disclosed systems can be utilized with various types of growth media. The nutrients and carbon dioxide in the growth media, such as wastewater, can be used as a feedstock for production, but the systems employ a membrane barrier to restrict passage of predators, grazers, and invasive species. Wastewater is often touted as a sustainable source of nutrients and carbon dioxide for algal cultivation, but wastewater treatment plants are often built in areas of high density. The disclosed systems and methods can take advantage of the existing footprint of a wastewater treatment plant or algal cultivation facility to grow algae instead of using arable or undeveloped land. For example, the disclosed systems can be installed on top of existing infrastructure, which not only saves space but further reduces capital costs. The systems and methods also preclude the need to sterilize growth media prior to feeding it to the algal culture, as the membrane barrier protects the culture from invasive bacteria, fungi, protozoa, or metazoa. Similarly, using the system in a raceway reactor further protects the culture from environmental contaminants to which algae in traditional suspended culture would otherwise be exposed.

The addition of a passive membrane prevents culture wash-out and retains the cells while still exposing them to fresh nutrients. The membrane encourages higher cell density, which eases harvesting and dewatering. In fact, it may be possible for the systems to passively dewater themselves through gas production, without intervention, further improving harvestability. Reducing the burden on downstream processing steps improves the economic competitiveness of the entire process.

In some embodiments, the systems comprise multiple passive membrane photobioreactors each being formed as a closed container adapted for growing micro or macro algae. Because the container is closed, it provides enhanced protection from predators and contaminants and better control over environmental conditions, such as light availability and temperature, than more conventional open systems, such as raceway ponds or lagoons. Although conventional photobioreactors tend to improve the productivity of algal cultures, they typically require more operation and maintenance and have higher energy costs and requirements than open systems. The disclosed passive membrane photobioreactors, however, may have lower energy requirements. For example, temperature regulation can be controlled by the growth medium below the reactor. In the case of wastewater cultivation, wastewater is a consistent temperature year-round and acts as a temperature sink in the summer and heat source in the winter. Nutrient and gas delivery occurs passively through the membrane, minimizing pumping requirements. The membrane restricts passage of large particles, reducing turbidity and improving sunlight penetration. Reactor depth may be larger due to the ability to provide light on multiple sides. Similarly, the passive protection from contamination reduces the need to add antibiotics, fungicides, or other external chemicals.

In some embodiments, the disclosed systems comprise the following basic design and operation principles:

1 . Membrane characteristics. Mennbrane characteristics (e.g., material, thickness, porosity, and permeability) can influence how the system functions and its selection is influenced by the overall goals of the system operation. Polymeric materials with a backing or cloth textile material in the range of approximately 0.01 to 0.2 μιτι (20 to 350 kDa) may be preferable because they have performed well in past experiments, are durable, and are effective in precluding entry of potential biological contaminants. However, other membrane types could be utilized for projects with goals different than optimizing biomass productivity or culture protection. For example, if predation was less of a concern than the capital cost of the passive membrane photobioreactor infrastructure, a membrane with a pore size of up to 1 μιτι or greater could be used to reduce costs. However, with large pore sizes, seed culture may escape and predatory organisms may invade until a biological coating layer (biofilm) forms on the membrane surface. The membrane characteristics may also influence constituent transport, which in turn affect overall productivity and/or algae metabolism. For example, if a project goal is to induce lipid production by restricting nitrogen, smaller membrane pores may be employed to retard the diffusion of nitrogen species across the membrane surface.

2. Culture depth. Submerging the passive membrane photobioreactors containers into a growth media reactor (e.g., a wastewater reactor) regulates the internal temperature of the algal culture to the relatively steady temperature range of recirculated media or continuously flowing wastewater. Photobioreactors often require external cooling mechanisms, which add energy and cost inputs. The passive temperature regulation is a major advantage of the disclosed passive membrane photobioreactors.

3. Available light. Light intensity can be controlled by a number of design parameters, such as geometry and transparency of the passive membrane photobioreactor container, depth of the passive membrane photobioreactor culture, and shading provided by the infrastructure. Example container geometries are discussed below. In some embodiments, the container material should maximize transparency and transparent surface area to enable light penetration for photosynthesis. The containers can, for example, have transparent sides and a transparent top. In some embodiments, the depth of the passive membrane photobioreactor culture should not exceed 1 m to prevent shading of the algae cake that may settle at the bottom of the passive membrane photobioreactor. Because the disclosed reactors can float on top of growth medium, algae can be cultivated in turbid water, such as activated sludge, yet still receive adequate light penetration.

4. Passive membrane photobioreactor configuration. While the passive membrane photobioreactor containers can be box-shaped, alternative designs are possible, including cylinders, rectangles, or other types of three-dimensional polygons. The passive membrane photobioreactor container can be designed to be best suited to optimize light, a project's specific goals, or to fit a particular growth media reactor, such as wastewater infrastructure or raceway ponds.

5. Growth media reactor configuration. The passive membrane photobioreactors can be installed in any open system, such as an algal raceway pond, lagoon, or wastewater reactor. A wastewater reactor can include clarifiers, activated sludge basins, lagoons, or any other wastewater basin with nutrients remaining in its fluid. The site chosen for passive membrane photobioreactor placement depends on the algal species cultivated and specific growth needs of that species (i.e., nitrogen preference, minimum carbon dioxide concentration). However, because passive membrane photobioreactor cultivation takes place in an independent reactor, the turbidity of the growth media (e.g., wastewater-activated sludge) is not a concern as long as the membrane characteristics do not enable passage of excessive turbidity and culture depth remains less than 1 m, light will still penetrate the passive membrane photobioreactors. The disclosed reactors can also be floated in natural or manmade waterways, such as stormwater ponds, catchment basins, or open water channels. The intent to deploy the disclosed reactor may be to provide nutrient removal capacity in an impaired waterway.

6. Harvest frequency. Harvest frequency will depend on the species being cultivated and the environmental conditions at the growth site, which will govern the growth rate of the algal culture. Site-specific goals of the culture (e.g., nutrient polishing or biomass production for commercial use) may govern timing and target cell density of the culture prior to harvesting.

7. Harvest procedure. In some cases, a removable cartridge (top or bottom) can be removed from the passive membrane photobioreactor container and the remaining water and suspended culture can be emptied into a collection basin. The suspended culture can be used to reseed the containers if contamination has not occurred. The remaining settled algae cake can be scraped into a separate collection bin to retain its high solids content. The harvest method can be achieved manually or mechanically. 8. Passive dewaterinq. Oxygen production via photosynthesis passively dewaters the algal culture when grown in a closed system as the oxygen displaces water within the passive membrane photobioreactors. This process can be exploited to increase the cell density of cultures prior to harvest (e.g., by removing water and concentrating cells).

The passive membrane photobioreactors and the systems in which they are used can take a variety of forms. Figs. 1 -12 illustrate some example embodiments of these passive membrane photobioreactors and systems. Beginning with Fig. 1 , illustrated is a passive membrane photobioreactor 10 that can be incorporated into an existing or future algae cultivation system, wastewater reactor, or other wastewater structure or waterway. As shown in Fig. 1 , the photobioreactor 10 comprises a generally rectangular container 12 that includes multiple generally planar wall panels. In the example of Fig. 1 , these panels include a top panel 14, a first end panel 16, a second end panel 18, a first side panel 20, and a second side panel 22. Each of these panels 14-22 can be generally perpendicular to each other to form an orthogonal rectangular box that defines an interior space 24. While an orthogonal rectangular box has been described and illustrated, it is noted that substantially any shape could be used, including three- dimensional polygons, cylinders, or hexagons. The particular shape that is used is not critical and may be influenced by various factors, such as the nature of the structure in which the photobioreactor 10 is to be used.

The size of the container 12 can be selected to suit the particular application in which it will be used. In some embodiments, however, the container 12 can have a length of approximately 1 to 10 m, a width of approximately 1 to 3 m, and a height of approximately 0.1 to 1 m, and the interior space 24 can have a volume of approximately 0.1 to 30 m³. As noted above, the container 12 can be transparent, or at least translucent, in which case the panels 14-22 can be made of a material that enables light to pass through the panels. In some embodiments, the panels 14-22 are made of a clear polymeric material, such as an acrylic material. Each of the panels 14-22 can be sealed along their common edges to prevent ingress or egress of fluids. In some embodiments, one or more of the panels 14-22 can be opened or removed from the container 12 to facilitate seeding of the container and harvesting of algae from the container.

As can be appreciated from the above discussion, the container 12 does not comprise a bottom panel similar to the other panels 14-22. Instead, the bottom of the container 12 is open but covered by a porous membrane 26 that enables water, carbon dioxide, and nutrients (e.g., nitrogen and phosphorus) to pass into the interior space 24 of the container 12 but prevents entry of contaminants and other unwanted components of the growth media into the interior space. The pores of the membrane can be less than approximately 1 μιτι in size. In some embodiments, the pores are in the range of approximately 0.01 to 0.2 μιτι (-20 to 350 kDa). By way of example, the membrane 26 can comprise an ultrafiltration membrane that forms part of a removable membrane cartridge that seals to the bottom edges of the end and side panels 16-22 of the container.

Passive membrane photobioreactors 10 of the type shown in Fig. 1 can be deployed in a growth media reactor to form an algae cultivation system 10. In particular, one or more photobioreactor containers 12 can be placed within an open-topped growth media reactor with their top panels 14 positioned near the surface of the growth media, such as wastewater (e.g., wastewater), and their end panels 16, 18, side panels 20, 22, and membrane 26 immersed in the media. The depth at which the containers 12 are submerged can be vary the volume of algae culture that is produced. Figs. 2 and 3 show such algae cultivation systems 30 and 31 . As indicated in these figures, the example systems 30, 31 include one or more passive membrane photobioreactors 10 that are immersed in the growth media 32 contained by an open-topped growth media reactor 34 including end walls 36 and 38, and side walls 40 and 42. Prior to immersion in the growth media 32, the photobioreactor containers 12 can be seeded with algal cells of the target algae species that is to be cultivated. Assuming correct conditions, the algae will then grow within the containers 12. As depicted in Figs. 2 and 3, the containers 12 can either float independently within the growth media 32 using floats 44 connected to the containers, and/or can be tethered to the side walls 40, 42 of the reactor 34 with tethers 46 to maintain the containers at a desired depth and position. The tethers 46 can extend between sides of the reactor 34 (i.e., transversely) or between the ends of the reactor (i.e., longitudinally).

During harvesting of the algae, the passive membrane photobioreactor containers 12 can be manually or automatically extracted from the reactor 34. Next, one of the container panels 14-22 (e.g., the top panel 14) can be removed to access the container's interior space 24 (Fig. 1 ). It is likely that algal culture, in the form of an algae cake, will settle at the bottom of the container 12. Residual water can be drained through the membrane 26 and any remaining water can be poured out of the container 12 and, if desired, retained for later seeding purposes. The algae culture can then be scraped from the membrane 26 and placed in an appropriate collection vessel. The outer surfaces of the container panels 14-22 can be cleaned and the interior space 24 can be reseeded. Once reseeding has been performed, the removed panel can be replaced and the container 12 can be returned to the reactor 34.

In some embodiments, algae harvesting can be at least partially automated. Fig. 4 illustrates an example of this. More particularly, Fig. 4 shows an algae cultivation system 50 that includes at least one passive membrane photobioreactor 52. The photobioreactor 52 is similar in many ways to the photobioreactor 10 shown in Fig 1 . Therefore, the photobioreactor 52 comprises a generally rectangular container 54 that includes multiple wall panels, which include a top wall (not numbered), end walls 56 and 58, and side walls 60 and 62 that together define an interior space 64 of the container. The container 54 further includes a porous membrane 66 that covers the bottom of the container.

Unlike the photobioreactor 10 of Fig. 1 , however, the photobioreactor 52 includes means for harvesting cultivated algae that include a recirculation system 68. As shown in Fig. 4, the recirculation system 68 comprises a dewatering system 70 that is in fluid communication with the interior space 64 via a container outlet 72 and a container inlet 74. The dewatering system 70 includes a pump mechanism that is used to draw algae from the container 54 through the outlet 72 and a dewatering mechanism (e.g., a gravity or filtration dewatering mechanism) that is used to dewater the algae so that concentrated algae sludge can be output from the system and collected. The dewatering system 70 then pumps the remaining algae and water back into the container 54 for further algae cultivation. Such harvesting can be performed on a continuous or batch basis.

As is further shown in Fig. 4, the photobioreactor container 54 can include multiple laterally extending baffles 76 provided within interior space 64 that force the algal culture to travel a serpentine path through the container from the inlet 74 to the outlet 72. This improves mixing, prevents internal biofouling, and potentially increases the concentration gradient between the growth media and algal culture as it prevents short circuiting in which the algae does not have enough time to grow.

Figs. 5 and 6 show an alternative algae cultivation system 80 that is similar to the system 50 of Fig. 4. The system 80 includes at least one passive membrane photobioreactor 82. As shown most clearly in Fig. 5, the photobioreactor 82 comprises a generally rectangular container 84 that includes multiple wall panels, which include a top wall 85 (Fig. 6), end walls 86 and 88, and side walls 90 and 92 that together define an interior space 94 of the container. The container 84 further includes a porous membrane 96 (Fig. 6) that covers the bottom of the container. The system 80 also includes a recirculation system 98 comprising a dewatering system 100 that is in fluid communication with the interior space 94 via a container outlet 102 and a container inlet 104.

As shown in Figs. 5 and 6, the photobioreactor container 54 further includes an internal scraping mechanism that comprises a continuous rotatable belt 106 that includes multiple scraping blades 108 that extend outward from the belt toward to the inner surface of at least the membrane 96. As depicted in Fig. 6, when the belt 106 is driven by an appropriate drive mechanism, such as a motor (not shown), its blades 108 can drive cultivated algae toward the container outlet 102 to help facilitate the harvesting process. This way, the algae is harvested from the container 54 at its most concentrated point. The algae culture is actively recirculated to improve mixing, reduce settling and internal biofouling, and improve the concentration gradient between the growth media and culture.

In other algae cultivation systems, the passive membrane photobioreactor containers can be mounted to an external conveyor belt that at least partially automates the harvesting process. Fig. 7 shows a first embodiment of such a system 1 10. In this embodiment, multiple passive membrane photobioreactors 1 12, which can have a configuration similar to that described above in relation to Fig. 1 , are mounted in a spaced manner to an outside surface of a continuous conveyor belt 1 14 that can be driven by an appropriate drive mechanism, such as a motor (not shown). In some embodiments, the tops of the photobioreactors 1 12 are mounted to the belt 1 14, in which case the bottoms of the photobioreactors each include a porous membrane. In other embodiments, however, the bottoms of the photobioreactors 1 12 can be mounted to the belt 1 14, which can be made of a porous membrane material so as to serve the same function as the individual membranes shown in Fig. 1 .

Irrespective of whether the photobioreactors 1 12 each include their own discrete membrane or the belt 1 14 forms this membrane for each photobioreactor, the belt can be rotated to sequentially immerse each photobioreactor 1 12 in growth media 1 16 contained in an open-topped growth media reactor 1 18 and later remove the photobioreactor from the growth media for algae harvesting. An advantage of this embodiment is that it reduces external attached growth on the photobioreactors 1 12 by preventing the photobioreactors from being stagnant. Furthermore, the photobioreactors 1 12 can be easily cleaned when they are out of the growth media 1 16.

Harvesting can be at least partially automated using a harvesting system 120 that removes and dewaters the cultivated algae before the photobioreactors 1 12 are re- immersed in the growth media 1 16. In addition, dewatering may naturally occur as water escapes the photobioreactors 1 12 via their membranes as the photobioreactors are removed from the growth media 1 16. Moreover, passive dewatering may occur while the photobioreactors 1 12 are submerged due to oxygen production during photosynthesis. The system 1 10 can be operated in a continuous or batch harvesting manner. Continuous harvesting may require the conveyor belt 1 14 to run at a constant speed (determined by the solids retention time of the culture), such that containers are continuously harvested by the harvesting system 120. In contrast, batch harvesting could occur on a set schedule based on time, culture density, or other parameters. Harvesting can be performed once for each rotation or at another predetermined interval, such once as every 10th cycle. The photobioreactors 1 12 can all have the same solids retention time and can be harvested after the same amount of time or can be staggered to promote metabolic changes in longer growth periods.

Fig. 8 shows a second embodiment of such a conveyor-based cultivation system 130. In this embodiment, multiple passive membrane photobioreactors 132 are mounted in a spaced manner to an inside surface of a continuous conveyor belt 134. In some embodiments, the tops of the photobioreactors 132 are mounted to the belt 134, in which case the bottoms of the photobioreactors each include a porous membrane. In other embodiments, the bottoms of the photobioreactors 132 can be mounted to the belt 134, which can be made of a porous membrane material so as to serve the same function as the individual membranes shown in Fig. 1 .

As in Fig. 7, the belt 134 can be rotated to sequentially immerse each photobioreactor 132 in growth media 136 contained in an open-topped growth media reactor 138 and later remove the photobioreactor from the growth media for algae harvesting. Harvesting can be at least be partially automated using a harvesting system 140 that removes and dewaters the cultivated algae before the photobioreactors 1 12 are re-immersed in the growth media. As with the embodiment of Fig. 7, the system 130 can be operated in a continuous or batch harvesting manner. Alternatively, harvesting can be performed continuously or intermittently.

Fig. 9 shows a further alternative embodiment of an algae cultivation system 150. In this embodiment, the passive membrane photobioreactors 152 are mounted to a rotatable wheel 154 having an axis of rotation that can, in some embodiments, be positioned near the surface of the growth media 156 of the growth media reactor 158. The wheel 154 can be rotated by an appropriate drive mechanism, such as a motor (not shown), to sequentially immerse the photobioreactors 152 and then lift them out of the growth media 156 for harvesting. The tops, bottoms, or both the tops and the bottoms of the photobioreactors 152 can comprise a porous membrane and the algal culture can settle as the photobioreactors are transported to a harvesting system 160. Similarly, the sides of the photobioreactor 152 can also comprise membrane material to increase the surface area for exchange. In some cases, the photobioreactors 152 can pivot on the frame of the wheel 154, which may enable easier harvesting. This embodiment has the advantage of reduced shading of the algae. Figs. 10 and 1 1 illustrate an algae cultivation system 170 in which the passive membrane photobioreactors 172 are supported by a transport mechanism 174 that transports the photobioreactors between two different growth media reactors 176 and 178, and alternately immerses the photobioreactors in the grow media 180 and 182 respectively contained in the reactors. The transport mechanism 174 can take a variety of forms. In some cases, the mechanism 174 includes conveyor belts and/or rails that support the photobioreactors 172. Harvesting of each photobioreactor 172 can take place upon its removal from each reactor 176, 178 using harvesting systems 184 and 186.

The reactors 176, 178 can contain the same type of growth media or different types of growth media. For example, the reactors 176, 178 can comprise different types of wastewater having differing dominant nutrient qualities or different synthetic media stationed in an order to induce a metabolic response. The system 170 enables timed delivery of varying types of nutrients, vitamins, gases, etc., and/or changes in environmental conditions (e.g., temperature, pH, light availability) that may enable the operator to change metabolic and biomass characteristics of the algal population. This may be especially beneficial in biofuel operations in which certain environmental conditions can trigger lipid production within an algal cell.

Fig. 12 shows an algae cultivation system 190 in which the growth media is driven through the photobioreactor. The system 190 includes a photobioreactor 192 that comprises a container 194 having multiple wall panels, including a top wall (not identified), a bottom wall (not identified), end walls 196 and 198, and side walls 200 and 202 that together define a closed interior space 204. Instead of including a porous membrane that covers the bottom of the container 194, the container includes one or more internal growth media tubes 206 that supply growth media, such as wastewater, to the container. Each tube is made of or comprises a porous membrane that, like the membranes of the other embodiments, enables water, carbon dioxide, and nutrients to pass from the growth media into water contained in the photobioreactor so that algae can be cultivated in the photobioreactor 192. The growth media can be pumped through the tubes 206 with inlet and outlet tubes 208 and 210 so that fresh media is continually or continuously supplied to the photobioreactor 192.

As with previous embodiments, the system 190 can also include a recirculation system 212 comprising a dewatering system 214 that is in fluid communication with the interior space 204 via a container outlet 216 and a container inlet 218.

Claims

CLAIMS Claimed are:

1 . An algae cultivation system comprising:

a passive membrane photobioreactor comprising an interior space in which algae can be cultivated and a porous membrane that separates growth media from the interior space, wherein water, carbon dioxide, and nutrients contained within the growth media can pass through the membrane and into the interior space but contaminants cannot.

2. The system of claim 1 , wherein the passive membrane photobioreactor comprises multiple transparent wall panels that define the interior space.

3. The system of claim 2, wherein the wall panels form a container having an open side.

4. The system of claim 3, wherein the porous membrane covers the open side of the container such that fluid can only enter the interior space if it passes through the membrane.

5. The system of claim 2, wherein the wall panels form a closed container.

6. The system of claim 5, wherein the porous membrane forms a growth media tube that passes through the container and wherein fluid can only enter the interior space if it passes through the tube.

7. The system of claim 1 , further comprising a recirculation system that circulates growth media through the passive membrane photobioreactor.

8. The system of claim 7, wherein the recirculation system comprises a dewatering system that dewaters algae cultivated in the passive membrane photobioreactor and removes the algae from the system.

9. The system of claim 7, wherein the passive membrane photobioreactor comprises internal baffles.

10. The system of claim 7, wherein passive membrane photobioreactor comprises an internal scraping mechanism that drives cultivated algae to the recirculation system.

1 1 . The system of claim 1 , further comprising a growth media reactor adapted to contain the growth media in which the passive membrane photobioreactor can be immersed.

12. The system of claim 1 1 , further comprising a conveyor belt to which the passive membrane photobioreactor is mounted, wherein the conveyor belt can be operated to alternately immerse the photobioreactor in the growth media for algae cultivation and remove it from the growth media for algae harvesting.

13. The system of claim 12, wherein the porous membrane is part of the conveyor belt.

14. The system of claim 12, further comprising a wheel to which the passive membrane photobioreactor is mounted, wherein the conveyor belt can be operated to alternately immerse the photobioreactor in the growth media for algae cultivation and remove it from the growth media for algae harvesting.

15. A passive membrane photobioreactor comprising:

multiple transparent wall panels that form a container that defines an interior space in which algae is to be cultivated and includes an open side; and

a porous membrane that covers the open side of the container that separates growth media from the interior space, wherein water, carbon dioxide, and nutrients contained within the growth media can pass through the membrane and into the interior space but contaminants cannot.

16. The photobioreactor of claim 15, wherein the porous membrane has pores that are less than approximately 1 μιτι in size.

17. The photobioreactor of claim 15, wherein the porous membrane has pores in the range of approximately 0.01 to 0.2 μιτι.

18. A passive membrane photobioreactor comprising:

multiple transparent wall panels that form a closed container that defines an interior space in which algae is to be cultivated; and

a growth media tube that passes through the container that comprises a porous membrane that separates growth media from the interior space, wherein water, carbon dioxide, and nutrients contained within the growth media can pass through the membrane and into the interior space but contaminants cannot.

19. The photobioreactor of claim 18, wherein the porous membrane has pores that are less than approximately 1 μιτι in size.

20. The photobioreactor of claim 18, wherein the porous membrane has pores in the range of approximately 0.01 to 0.2 μιτι.