WO2009129396A1

WO2009129396A1 - Photobioreactor systems and methods incorporating cultivation matrices

Info

Publication number: WO2009129396A1
Application number: PCT/US2009/040818
Authority: WO
Inventors: Toby Ahrens; Ben Fowler; Marcus Gay; Peter B. Heifetz; John J. Lewnard; Kevin Lockwood; Jason Prapas; Otto Pulz; Miles Walker; Christopher Wilson
Original assignee: GREENFUEL TECHNOLOGIES Corp; Greenfuel Tech Corp
Current assignee: GREENFUEL TECHNOLOGIES Corp; Greenfuel Tech Corp
Priority date: 2008-04-16
Filing date: 2009-04-16
Publication date: 2009-10-22
Anticipated expiration: 2010-10-16

Abstract

Certain embodiments and aspects of the present invention relate to a photobioreactor including covered photobioreactor units (100) enclosing at least one cultivation matrix (1012) upon which at least one species of phototrophic organism grows.

Description

PHOTOBIOREACTOR SYSTEMS AND METHODS INCORPORATING CULTIVATION MATRICES

Cross-Reference to Related Applications

[0001] This application claims priority to and incorporates by reference the entire disclosure of each of the following four provisional U.S. patent applications: U.S. Ser. Nos. 61/045,608 (filed April 17, 2008) and 61/048,287 (filed April 28, 2008), both titled "Photobioreactor Systems and Methods Incorporating Cultivation Matrices"; and U.S. Ser. Nos. 61/045,601 (filed April 16, 2008) and 61/048,302 (filed April 28, 2008), both titled "Cultivation Matrix for, Method for Production and Cultivation Method for Phototrophic Microoganisms."

Field of the Invention

[0002] The invention relates generally to photobioreactors and processes to operate and use photobioreactors for the treatment of gases, such as flue gases, and for the production of biomass.

Discussion of the Related Art

[0003] The power-generation industry is coming under increasing pressure to produce electricity from renewable energy sources. Many biofuels meet renewable-energy standards, but sources of conventional biofuels, such as biomass, biodiesel, and bioethanol, are not uniformly geographically distributed across the nation. Moreover, the cost of growing such sources on a commercial scale can make the resulting fuels economically unattractive.

[0004] At the same time, reductions in carbon dioxide emissions and other gas emissions from various sources are becoming increasingly necessary and/or desirable. Typically, however, capturing carbon dioxide from the flue gas of anthropogenic sources such as electric power plants is expensive.

[0005] Photosynthesis is the carbon recycling mechanism of the biosphere. In this process organisms performing photosynthesis, such as plants, synthesize carbohydrates and other cellular materials by CO₂ fixation. Microalgae are among the most efficient converters of CO₂ and solar energy to biomass. Often referred to herein simply as "algae," these are the fastest growing photo autotrophic organisms on earth and one of nature's simplest microorganisms. In fact, over 90% of CO₂ fed to algae can be absorbed, mostly through the production of cell mass. In addition, algae are capable of growing in saline waters that are unsuitable for agriculture.

[0006] CO₂ bio-regeneration through algae farming can be advantageous in using waste CO₂ to enhance production of useful, high-value products from algae. Thus, production of algal biomass using CO₂ from combustion gas sequesters the CO₂ from the environment and increases algae growth, enhancing yield of useful end products: dry algae has a heating value roughly equivalent to coal, and algal biomass can be turned into a high-quality liquid fuel similar to crude oil or diesel fuel ("biodiesel") through thermochemical conversion by known technologies. Algal biomass also can be used for gasification to produce highly flammable organic fuel gases suitable for use in gas-burning power plants.

[0007] Algal cultures also can be used for biological NO_x removal from combustion gases. Some algae species can remove NO_x at a wide range of NO_x concentrations and combustion gas flow rates. Nitrous oxide (NO), a major NO_x component, is dissolved in the aqueous phase, after which it is oxidized to NO₂ and assimilated by the algal cell. For example, NO_x removal using the algae Dunaliella can occur under both light and dark conditions, with an efficiency of NO_x removal of over 96% (under light conditions).

[0008] A major obstacle for feasible algal bio-regeneration and pollution abatement has been an efficient, yet cost-effective, growth system. Some current research has focused on growing algae in massive open ponds as big as 4 km . The ponds require low capital input; however, algae grown in open and uncontrolled environments result in low algal productivity that seldom

2 exceeds 20 g/m /day. The open pond technology makes growing and harvesting the algae expensive, because massive amounts of dilute algal waters require very large agitators, pumps and centrifuges. In addition, the high rates of evaporation from ponds in combination with their dilute algal biomass concentration result in very low algal cell: water ratios, especially when considered on a footprint area basis. Other deployed systems have utilized expensive, closed algal photobioreactors in which fiber optics provide light transmission. In these controlled environments, much higher algal productivity is achieved, but the algal growth rates generally are not high enough to offset the capital costs of the systems utilized.

Summary [0009] In one aspect, the invention relates to a photobioreactor system including an enclosure comprising a substantially transparent top portion and means for admitting a gas having a concentration of CO₂ elevated above ambient and, within the enclosure, at least one solid cultivation support. The support may include phototrophic organisms growing on at least one surface thereof. In various embodiments, the solid supports may each have a growing area of at least 1 m², 10 m², 13 m² or more; the system may have a total growing area (aggregated across all solid supports of) 100 m², 1000 m², or as much as 15,000 m² or more. The system may have a growth footprint area — i.e., the ground area below the at least one vertically aligned solid support — of at least 7.4 m , at least 73.5 m , or at least 1103 m . The solid supports may be in sheet form and arranged in parallel in a vertical orientation, and separated by a spacing of, for example, no more than 20 cm. In other embodiments the sheets may be separated by a spacing of no more than 15 cm, or not more than 10 cm.

[0010] In another embodiment, the ratio of the total growing area of the at least one solid support to the growth footprint area is at least 12.2 m². The total system growing volume may be at least 180 m³. In a vertical system, the growing area is vertical so that the system has a total growing volume equal to the aggregate vertical growing extent times average algae thickness. In one embodiment, the photobioreactor system also has a liquid (non-vertical) growing region within the enclosure, in which case the total growing volume is the sum of the vertical growing volume and the liquid growing volume. [0011] In various embodiments, the photobioreactor system includes a channel or basin under the at least one solid cultivation support for receiving liquid medium therefrom, and a means for recirculating at least a portion of the liquid medium back onto the at least one solid cultivation support. The ratio of the total amount of liquid in the system (the total system liquid volume) to the total growing area of 2.2 L/m² or less. The ratio of the total system liquid volume to the footprint area may be no greater than 26.5 L/m². The total system liquid volume per gram of biomass may be no greater than 0.144 L/g, no greater than 0.087 L/g, no greater than 0.072 L/g, or no greater than 0.054 L/g. The solid cultivation support may have a height and a width, where the width exceeds the height by at least a factor of 10 or a factor of 100. The phototrophic organisms may include eukaryotic algae or cyanobacteria. [0012] In one embodiment, the photobioreactor system may also include a liquid-medium delivery system configured to apply a liquid mist, spray or stream to at least a top portion of the at least one solid cultivation support. [0013] In another aspect, the invention relates to a photobioreactor system including an enclosure that comprises a substantially transparent top portion and means for admitting a gas having an elevated concentration of CO₂; a humidity level within the enclosure is at least 80%. Within the enclosure, at least one longitudinally extending photobioreactor unit includes (i) at least one solid cultivation support oriented substantially vertically, the support including phototrophic organisms growing on at least one surface thereof, and (ii) a channel or basin under the at least one solid cultivation support for receiving liquid medium therefrom. The system includes means for recirculating at least a portion of the liquid medium back onto the at least one solid cultivation support. The humidity level within the enclosure may, in some embodiments, be at least 95%. The system may include a plurality of solid cultivation supports in sheet form aligned in a longitudinal row.

[0014] In another aspect of the invention, a photobioreactor system includes at least one solid cultivation matrix positioned within a gas headspace that includes phototrophic organisms growing on at least one surface thereof. The cultivation matrix surface supports an average footprint areal productivity of biomass of at least 10 g/m -day. In various alternative embodiments, the average footprint areal productivity of biomass is at least 40 g/m -day, at least 80 g/m²-day, at least 100 g/m²-day, or between 10 and 170 g/m²-day.

[0015] In another aspect, a photobioreactor system in accordance with the invention includes an enclosure comprising a substantially transparent top portion and means for admitting a gas having an elevated concentration of CO₂. Within the enclosure, there is at least one longitudinally extending photobioreactor unit comprising (i) at least one solid cultivation support oriented substantially vertically, the support including phototrophic organisms growing on at least one surface thereof, and (ii) a channel or basin under the at least one solid cultivation support for receiving liquid effluent therefrom. The photobioreactor system also includes a means for sensing a temperature of the surface of the at least one solid cultivation support and an automatic liquid-medium delivery system configured to apply a nutrient-containing liquid medium to at least a portion of the surface at a target temperature when the sensed temperature deviates from the target temperature by a predetermined threshold.

[0016] In another aspect, the invention relates to a photobioreactor system including an enclosure comprising a substantially transparent top portion and means for admitting a gas having an elevated concentration of CO₂. Within the enclosure, there is at least one longitudinally extending photobioreactor unit comprising (i) at least one solid cultivation support oriented substantially vertically, the support including phototrophic organisms growing on at least one surface thereof, and (ii) a channel or basin under the at least one solid cultivation support for receiving liquid effluent therefrom. The system also may include a biomass harvester configured to move with respect to the cultivation support while dislodging phototrophic organisms therefrom.

[0017] In one embodiment, the system includes a means for automatically actuating the biomass harvester at predetermined intervals. The system may include at least one sensor that determines the density of the biomass on the at least one solid cultivation support. The biomass harvester may be activated at a target density of the biomass on the solid cultivation support(s) of at least 2 g/L without flocculation or at least 8 g/L without flocculation. The biomass harvester may include a plurality of nozzles for directing a stream of liquid onto the at least one surface of a solid cultivation support; the stream has sufficient velocity to dislodge and remove photrophic organisms therefrom. Alternatively, harvesting may be accomplished using an air knife rather than a liquid stream. The biomass harvester may include a trolley and upwardly projecting headers that direct a stream of liquid onto the surface(s) of a solid cultivation support; the stream is of sufficient velocity to dislodge and remove photrophic organisms therefrom. The trolley may include a pump that pulls liquid from the basin to be used in a stream of liquid used to dislodge and remove photrophic organisms.

Brief Description of the Drawings [0018] Other advantages, novel features, and uses of the invention will become more apparent from the following detailed description of non-limiting embodiments of the invention when considered in conjunction with the accompanying drawings, which are schematic and which are not intended to be drawn to scale. In the figures, each identical, or substantially similar component that is illustrated in various figures is typically represented by a single numeral or notation. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention.

[0019] In the drawings:

Fig. Ia is a perspective view of a photobioreactor unit according to one embodiment of the invention; Fig. Ib is a cross-sectional view of one photobioreactor section of a photobioreactor unit according to one embodiment of the invention;

Fig. 2 is a perspective view of a photobioreactor system according to one embodiment of the invention; Fig. 3 shows a block diagram of an overall gas treatment/biomass production system comprising a photobioreactor system according to one embodiment of the invention;

Fig. 4 is a cross-sectional view of a nutrient misting section of a photobioreactor unit, according to one embodiment of the invention;

Fig. 5 is a perspective view of an evaporative cooling zone of a photobioreactor unit according to one embodiment of the invention;

Fig. 6a is a perspective view of a first configuration of a photobioreactor unit zone for diverting liquid to a reflow channel;

Fig. 6b is a perspective view of a second configuration of the photobioreactor unit zone shown in Fig. 6a; Fig. 7 is a perspective view of two photobioreactor unit zones configured to divert liquid to a reflow channel, according to one embodiment of the invention;

Fig. 8a is a perspective view of one component of a bulkhead distribution unit according to one embodiment of the invention;

Fig. 8b is a cross-sectional view of the bulkhead distribution unit component shown in Fig. 8a;

Fig. 9 is a perspective view of a bulkhead distribution channel operatively connected with ten photobioreactor units according to one embodiment of the invention;

Fig. 10 is a block diagram of an overall gas treatment/biomass production system comprising a photobioreactor system according to an alternative embodiment of the invention; Fig. 11 is a perspective view of a photobioreactor system according to an alternative embodiment of the invention;

Fig. 12 shows a cross-sectional view of a photobioreactor unit adapted to float on a water body;

Fig. 13 is a block diagram of an overall gas treatment/biomass production system comprising a photobioreactor system which uses liquid associated with the system to quench flue gas; Fig. 14 is a block diagram of an overall gas treatment/biomass production system comprising a photobioreactor system which used liquid associated with the system to quench flue gas;

Fig. 15 is a block diagram of an overall gas treatment/biomass production system comprising a photobioreactor system which used liquid associated with the system to quench flue gas.

Fig. 16 is a cross-sectional view of a quench zone according to one embodiment of the invention;

Fig. 17 is a perspective view of the quench zone shown in Fig. 16; Fig. 18 is a perspective view of a heat exchange zone of a photobioreactor unit according to one embodiment of the invention;

Fig. 19 shows algae concentration versus time for one example of the use of a photobioreactor described herein; and

Fig. 20 shows carbon dioxide flux rates for embodiments employing different liquid spray rates;

Fig. 21 is a cross-sectional view of a photobioreactor unit according to one embodiment of the invention;

Fig. 22 is a cross-sectional view of a substrate sheet hanging mechanism according to one embodiment of the invention; Fig. 23 is a cross-sectional view of a photobioreactor unit according to one embodiment of the invention;

Figs. 24A-B are cross-sectional views of photobioreactor units according to two embodiments of the invention;

Fig. 25 illustrates a three-dimensional view of a photobioreactor unit according to one embodiment of the invention;

Fig. 26 is a side view of a photobioreactor unit according to one embodiment of the invention;

Fig. 27 is a top view of a photobioreactor unit according to one embodiment of the invention; Fig. 28 is a side view of a photobioreactor unit according to one embodiment of the invention;

Fig. 29 includes (a) a top view and (b) a side view of a photobioreactor unit according to one embodiment of the invention; Fig. 30 is a cross-sectional view of one photobioreactor section of a photobioreactor unit according to one embodiment of the invention;

Fig. 31 is a side view of a cultivation matrix according to one embodiment of the invention; and Fig. 32 is a side view of a photobioreactor unit according to one embodiment of the invention.

Detailed Description

[0020] Certain embodiments and aspects of the present invention relate to photobioreactor systems designed to contain a liquid medium comprising at least one species of phototrophic organism therein, and to methods of using the photobioreactor systems as part of a gas- treatment process and system able to at least partially remove certain undesirable pollutants from a gas stream.

[0021] Certain embodiments of the invention include one or more longitudinally oriented, elongated covered photobioreactor units arranged in parallel that extend across a land area or a body of water, such as a pond, to form at least a part of a photobioreactor system. In certain embodiments, each photobioreactor unit has a liquid channel (formed by a trench in some embodiments) and a gas headspace (enclosed by a light-transparent cover in some embodiments). Cθ₂-rich gas enters the photobioreactor unit and flows in the headspace above a liquid medium comprising at least one phototrophic organism such as algae. The algae uses the CO₂ from the gas and the light that passes through the cover to grow and produce biomass. Algae may be harvested from the liquid medium discharge and dewatered. The dewatered algae may go through additional processes and may be used as fuel and/or used to produce a fuel product (e.g., biodiesel). The liquid produced during the dewatering phase may be recycled back into the same photobioreactor unit and/or a different photobioreactor unit of the photobioreactor system (and/or another component of the photobioreactor system in some embodiments). In some cases, the photobioreactor units may be on the order of a few hundred feet or less, while in other cases, the photobioreactor units may extend half a mile to a mile or more.

[0022] A modular, sectional construction may be used to form at least some portion of at least some of the photobioreactor units in certain embodiments. For example, in certain embodiments, a photobioreactor unit may be made up of a plurality of individual photobioreactor sections interconnected in series. In certain such embodiments, the individual sections may comprise both a liquid flow channel and at least one cover. In other embodiments, a photobioreactor unit may comprise a single, uninterrupted liquid flow channel contained in a base (e.g., base 110 of Fig. Ia) , and the photobioreactor sections may be defined by the zones covered by one or a subset of a plurality of cover sections (e.g., cover sections 106 of Fig. Ia) over the base and channel. In this manner, the length of one or more photobioreactor units may be produced by selecting and interconnecting the appropriate number of photobioreactor sections, and thus custom manufacturing for specific applications may not be required. By employing a modular construction, in some cases, the length may be adjusted after installation if desired. Additionally, various types of photobioreactor sections may be used within a photobioreactor unit to create a plurality of operation zones with selected functionality, such as nutrient misting zones, cooling zones, liquid diversion zones, etc., and the number and positions of the various types of photobioreactor sections may be designed based on predicted operating conditions. Exchanging different types of photobioreactor sections after installation also may be possible when using a modular sectional construction. In some embodiments, a large number of photobioreactor units may be positioned near to one another (e.g., parallel to one another), and system scaling may be achieved by adding or subtracting photobioreactor units.

[0023] In certain embodiments, the disclosed photobioreactor systems, methods of using such systems, and/or gas treatment systems and methods provided herein can be used as part of an integrated method and/or system for treating waste gases produced by industrial processes, wherein phototrophic organisms used within the photobioreactor at least partially remove certain pollutant compounds contained within effluent gases (e.g., CO₂ and/or NO_x), and are subsequently harvested from the photobioreactor system, processed, and used as a fuel source for a combustion device (e.g., an electric power-plant generator, industrial furnace, and/or incinerator). Such uses can provide an efficient approach to recycling carbon contained within a combustion fuel (i.e., by converting CO₂ in a combustion gas to biomass fuel and/or biomass- derived fuel in a photobioreactor system), thereby reducing both CO₂ emissions and fossil-fuel requirements. [0024] In some embodiments, the liquid within a photobioreactor unit is sprayed into the gas headspace or otherwise exposed to CO₂-rich gas using one or more mass-transfer enhancement devices to increase the surface-to-volume ratio of the liquid. By providing surface area contact between the gas and the liquid medium via movement of the gas through a headspace rather than by exclusively sparging gas into a depth of liquid medium, certain embodiments of the photobioreactor system exhibit a low pressure drop when moving gas through the photobioreactor units. In some embodiments, the gas pressure drop along an entire photobioreactor unit may be below 0.5 p si.

[0025] In some embodiments, the flow of gas and liquid through the photobioreactor units may experience limited or essentially no backflow, and in this way exhibit the characteristics of a plug-flow system. With limited backflow, longitudinal zones may be defined in which different operating conditions such as, for example, algae density, liquid temperature, gas composition, gas temperature, media composition, media agitation/turbulence, gas/liquid mass/heat transfer, light exposure, media depth, etc. are generally known and controllable by changing operating parameters. For example, a single photobioreactor unit may include different zones within which one or more of the following operating parameters vary and/or are known and/or are controllable: nutrient concentrations; temperature; pH; liquid depth; surface- to-air ratio of the liquid; agitation levels; and others. In certain embodiments, these zones may be made up by or comprise one or more specially configured photobioreactor sections of the photobioreactor unit.

[0026] In some embodiments, advantages of a back-mixed bioreactor may be achieved while maintaining many of the characteristics of a plug-flow bioreactor. One or more reflow zones may be used to return algae-rich liquid from, for example, a longitudinal mid-area of the photobioreactor unit to the front end of the photobioreactor unit or to some other position upstream of the liquid removal position. By doing so, the addition of new inocula to the liquid medium at the front end of the photobioreactor unit may be reduced or eliminated and/or other desirable operating parameters may be maintained and/or established. [0027] Compared to raceway reactors, which can experience considerable thermal loss when ambient temperatures are below the reactor operating temperature, some embodiments of the invention limit thermal loss by covering a majority (or in some cases substantially all) of the liquid surfaces within the photobioreactor system. Compared to typical enclosed photobioreactors (e.g., certain tubular photobioreactors) which do not include a gas head space in contact with the liquid media over at least a substantial portion of its flow length, some of which use various methods of thermal management to remove heat from the reactors, certain embodiments disclosed herein are able to shed heat efficiently using controlled evaporative cooling.

[0028] According to certain embodiments, unlike systems that use gas pressure to support a cover, a self-supporting cover(s) (e.g., rigid individual interconnected cover section(s) or a continuous or sectioned cover formed of a flexible, non- self- supporting material that comprises ribs or other support elements) may be used to maintain a gas headspace regardless of the pressure of the gas flowing through a photobioreactor unit. The cover may be configured such that when gas is pulled through a photobioreactor unit by an induced-draft fan, thereby creating a negative pressure within the photobioreactor unit relative to atmospheric pressure, the cover maintains the gas headspace (i.e., does not collapse). In some embodiments, the cover is constructed and arranged to withstand external forces such as wind and snow.

[0029] Certain aspects of the invention are directed to photobioreactor designs and to methods and systems utilizing photobioreactors. A "photobioreactor," "photobioreactor unit" or "photobioreactor section" as used herein, refers to an apparatus containing, or configured to contain, a liquid medium comprising at least one species of phototrophic organism and having either a source of light capable of driving photosynthesis associated therewith, or having at least one surface at least a portion of which is partially transparent to light of a wavelength capable of driving photosynthesis (i.e., light of a wavelength between about 400-700 nm).

[0030] The term "photosynthetic organism," "phototrophic organism," or "biomass," as used herein, includes all organisms capable of photosynthetic growth, such as plant cells and microorganisms (including algae, cyanobacteria, photosynthetic bacteria and lemna) in unicellular or multi-cellular form that are capable of growth in a liquid phase or on a wetted surface (except that the term "biomass," when appearing in the titles of documents referred to herein or in such references that are incorporated by reference, may be used to more generically to refer to a wider variety of plant and/or animal-derived organic matter). These terms may also include organisms modified artificially or by gene manipulation. While certain photobioreactors disclosed in the context of the present invention are particularly suited for the cultivation of algae, cyanobacteria or photosynthetic bacteria, and while in the discussion below, the features and capabilities of certain embodiments that the inventions are discussed in the context of the utilization of algae as the photosynthetic organisms, it should be understood that, in other embodiments, other photosynthetic organisms may be utilized in place of or in addition to algae. In general, certain embodiments of the invention may be designed to support the growth of unicellular, motile or sessile, flagellated or non-flagellated phototrophic organisms that have volumetric yield rates which in certain embodiments may be greater than 0.2 g/L-day (dryweight), and have total lipid contents which in certain embodiments may be greater that 20% (by mass) and cell sizes which in certain embodiments may range from 1 to 50 micrometers. For an embodiment utilizing one or more species of algae, algae of various types, (for example Chlorella, Chlamydomonas, Chaetoceros, Dunaliella, Porphyridium, Haematococcus, the cyanobacterium Spirulina, etc.) may be cultivated, alone or in various combinations, in the photobioreactor. Of course one or more of these and/or other algae types may be used in certain embodiments, for example, one or more of Nannochloris sp., Tetraselmis chui (strain PLY 429), Dunaliella salina, Pleurochrysis carter ae, Tahitian Isochrysis sp., Rhodomonas salina, Pichochlorum oklahomensis, Pavlova lutheri, Phaeodactylum tricornutum, Nannochloris/Nannochloropsis, Skeletonema caustatum, Nannochloropsis oculata, Chlorella minutissima, Nannochloris sp., Botryodopsis arhiza, Scenedesmus dimorphus, Heterococcus mainxii, Chlorella protothecoides, Ankistrodesmus braunii, Heterococcus brevicellularis, Monodus subterraneus, Microspora sp., Ulothix sp., Nannochloropsis sp., Porphyridium sp., Chlorella sp., Neochloris oleoabundans, Chlamydomonas acidophila, Chlamydomonas reinhardtii, Spirulina platensis, Haematococcus lacustris, Aphanizomenon flos- aquae, Ankistrodesmus falcatus, Botryococcus sudeticus, Botryococcus braunii, Coscinodiscus sp., Coscinodiscus wailisii, Dunaliealla bardawil, Dunaliella tertiolecta, Chaetocerous muelleri, Chaetoceros gracilis, Amphora sp., Amphora cojfeaeformis, Tetraselmis suecica, Platymonas sp., Navicula lenzii, Chlamydomonas sp., Tetraselmis sp., Scenedesmus quadricuada, Selenastrum minutum, Chlorella sorokiniana, and Chlorella vulgaris. In some embodiments using one or more species of macroalgae, macroalgae of various types, (for example Chondrus, Porphyra, Palmaria, Laminaria, Ulva, etc.) may be cultivated, alone or in various combinations, in the photobioreactor.

[0031] The phrases "at least partially transparent to light" and "configured to transmit light," when used in the context of certain surfaces or components of a photobioreactor, refer to such surface or component being able to allow enough light energy to pass through, for at least some levels of incident light energy exposure, to drive photosynthesis within a phototrophic organism.

[0032] One embodiment of a photobioreactor unit 100 is illustrated in Figs. Ia and Ib. Liquid medium 101 flows along a trench (or, equivalently, channel) 102 within photobioreactor unit 100, and gas, such as flue gas from a power plant, flows through a gas headspace 104 formed between liquid medium 101 and a cover(s) 106 at least partially transparent to light. Cover(s) 106 may be constructed such that gas headspace 104 remains essentially constant when no gas pressure or a negative gas pressure is applied to the interior of photobioreactor unit 100.

[0033] As Cθ₂-rich gas flows over liquid medium 101, CO₂ dissolves into the liquid medium, and algae within the liquid medium use the CO₂ and sunlight (or other light source) to photosynthesize, grow and reproduce, thereby producing biomass. The liquid medium flows, in certain embodiments at a controlled rate, through photobioreactor unit 100, and the algae, in certain embodiments, is harvested at an outlet of photobioreactor unit 100 by removing the algae-rich liquid from the photobioreactor unit.

[0034] In some embodiments, photobioreactor unit 100 may be approximately 10 meters wide and the overall photobioreactor unit 100 may be a suitable length to process a desired amount of CO₂. In general, the photobioreactor unit length exceed the width, and the ratio of length to width may be greater than 100:1, and may exceed 1000:1. The CO₂ level in gas containing elevated concentrations of CO₂ (i.e., CO₂ concentrations which are higher than ambient air) may range from 1% - 100%, but typically is in the range of 4 - 20%. The operating pressure of the reactor may generally range from about 11 - 20 psia, preferably from 13 -16 psia. Flow rates of the gas may generally range from about 0.05 - 50 cm/sec, or other suitable flow rate. Liquid flow rates may generally range from about 1 - 100 cm/sec. Biomass concentrations generally may range from 0.01 - 10 g/L.

[0035] Several structural features of one embodiment of photobioreactor unit 100 will now be described, but it is important to note that the particular structural implementation of this embodiment are not intended to be limiting. [0036] Base 110 of photobioreactor unit 100 is formed of a compacted gravel base, and cover(s) 106 is supported by structural ribs 112. Structural ribs 112 are attached to supports 114 embedded in trench sidewalls 116 formed of the same material as the base (e.g., compacted gravel). A bottom liner 120 is laid over or formed within the base 110 to provide a liquid- impermeable surface. Liner 120 may be, for example a plastic sheet, e.g., a polyethylene sheet, or any other suitable liner.

[0037] Cover(s) 106 may be constructed from a wide variety of transparent or translucent materials that are suitable for use in constructing a bioreactor. Some examples include, but are not limited to, a variety of transparent or translucent polymeric materials, such as polyethylenes, polypropylenes, polyethylene terephthalates, polyacrylates, polyvinylchlorides, polystyrenes, polycarbonates, etc. Alternatively, cover(s) 106 may be formed from glass or resin- supported fiberglass. In certain embodiments, cover(s) 106, in certain embodiments in combination with support elements such as support elements 112/114, is sufficiently rigid to be self-supporting and to withstand typical expected forces experienced during operation without collapse or substantial deformation. Portions of cover(s) 106 may be non-transparent in certain embodiments, and such portions can be made out of similar materials as described above for the at least partially transparent portions of cover(s) 106, except that, when they are desired to be non-transparent, such materials should be opaque or coated with a light-blocking material.

[0038] Cover(s) 106 may include a material which is UV-stabilized and may, in certain embodiments, be between about 4-6 mils in thickness, depending on the material. The material, in certain embodiments in combination with support elements such as support elements 112/114, may be designed to support external loads such as snow, wind and/or negatives pressures applied by an induced-draft fan. Additionally, in some embodiments, cover(s) 106 may be able to withstand internal pressure, such as when a forced-draft fan is used to push gas through photobioreactor unit 100.

[0039] Each section 130 may include a separate cover 106 with each cover 106 being connected to adjacent covers when the sections 130 are interconnected. In some embodiments, each section has a support elements 112/114 and a single piece of polyethylene or other suitable material is used to span multiple sections 130.

[0040] Each photobioreactor unit 100 may be formed with multiple photobioreactor sections 130 defined, in the illustrated embodiment, by separate cover sections 106. In this manner, constructing the designed length of the photobioreactor unit 100 may be achieved simply by selecting and interconnecting the appropriate number of photobioreactor sections 130. In some embodiments, the length of photobioreactor unit 100 may be changed and the rate of gas and/or liquid flow may be changed to accommodate long-term changes in treatment needs. Additionally, retrofitting photobioreactor unit 100 such as by increasing or decreasing the length may be possible. [0041] While the photobioreactor unit shown in Figs. Ia and Ib includes a trench 102 to create a liquid flow channel, in some embodiments, no trench may be present and the channel for a liquid stream may be formed at or above grade. In certain embodiments, the base comprising the liquid flow channel may not be longitudinally continuous as illustrated, but may comprise a plurality of interconnected sections. For example, in certain embodiments, sections 130 may be defined by both separate a cover section and a separate base section in association with each other. The elevation of the photobioreactor unit may be substantially constant along the entire length of the channel or substantial portions thereof, and gravity flow of the liquid stream may be induced by adding liquid to a first end of the photobioreactor unit and allowing overflow (e.g., over a wall, weir, etc.) at the opposite end. In some embodiments, the photobioreactor unit may have a general, continuous downward pitch to promote liquid flow. In still other embodiments, abrupt elevation drops may be provided at the junctions of photobioreactor sections to create liquid flow and/or a cascading effect and/or to facilitate installation and operation over land areas with more substantial elevation changes.

[0042] Cover(s) 106 is shown as a semicircle or other curved surface in many of the embodiments disclosed herein, however, any suitable shape may be used, including a rectangular, triangular or trapezoidal shapes. [0043] Fig. 2 shows an embodiment of a large-scale photobioreactor system 200 in which the gas flows in the direction opposite to the liquid stream flow. In some implementations, however, the gas may flow in the same direction as the liquid stream. Ten parallel photobioreactor units 100 are shown in the embodiment of Fig. 2, but fewer (including a single photobioreactor unit) or more photobioreactor units may be used. While photobioreactor units 100 as illustrated comprise straight, linear segments, in alternative embodiments, one or more of the photobioreactor units may be arcuate, serpentine, or otherwise non-linear, if desired. A liquid inlet/gas outlet bulkhead 204 runs perpendicular to the photobioreactor units at a first end of photobioreactor system 200. At an opposite end of photobioreactor system 200, a liquid outlet/gas inlet bulkhead 206 also runs perpendicular to the photobioreactor units 100. An optional rainwater drainage and vehicle access channel 208 runs parallel to the outer side of the overall photobioreactor system; however, the drainage and vehicle access channel 208 may be positioned between parallel photobioreactor units, or may not be present at all. In some embodiments, smaller rainwater drainage channels which do not accommodate vehicles may be provided. [0044] The lengths of photobioreactor units 100 are selected to be sufficient, for a given desired liquid-medium circulation rate, to provide sufficient gas-liquid contact time to provide a desired level of mass transfer between the gas and the liquid medium. Optimal contact time depends upon a variety of factors, especially the algal growth rate and carbon and nitrogen uptake rate as well as feed gas composition and flow rate and liquid medium flow rate. Scalability of the photobioreactor system 200 as a whole may be achieved, for example, by simply by adding additional photobioreactor units to the system, such as by adding photobioreactor units in a parallel relationship to existing photobioreactor units.

[0045] As described above, each photobioreactor unit 100 may include various zones having different functionality. One or more photobioreactor sections may be configured as a misting zone 216 to controllably add nutrients/media to the system and facilitate gas-liquid mass transfer. The nutrients and/or the medium in which the nutrients are carried may be provided in certain embodiments at least in part by recycling algae-depleted medium from a dewatering system. More than one nutrient misting section 216 may be provided. By employing a modular section-based construction, channel and/or cover sections which include misters may be added or removed after construction if so desired. In other embodiments, nutrients may be added by methods other than misting such as by direct pumping into the liquid stream. Unrecycled nutrients and/or medium (i.e., fresh make-up) also, or exclusively, may be used to supply the liquid stream in some embodiments.

[0046] Of course in some embodiments, nutrients may be added using devices other than misters. For example, nutrients may flow from a pipe into the liquid medium stream, or nutrients may be showered from the top of the photobioreactor unit using a pipe with periodic openings.

[0047] Each photobioreactor unit 100 or certain of the photobioreactor units may in certain embodiments include a cooling zone 220 comprising, for example, cooling sections 222. As described below with reference to Fig. 5, cooling zone 220 may include portions in which the liquid stream is exposed to the atmosphere to provide for evaporative cooling. [0048] Harvesting algae, adjusting algal concentration, and introducing additional liquid medium can be facilitated via liquid medium inlet bulkhead 204 and liquid medium outlet bulkhead 206. Control of the concentration of algae can be important from the standpoint of maintaining a desirable level of algal growth and proliferation. Algae may be harvested periodically or continuously from an end(s) of the photobioreactor units, or, in some embodiments, from one or more locations located between the ends of the photobioreactor units. [0049] Various devices or mechanisms may in certain embodiments be included within photobioreactor units 100 to increase the interfacial surface area between the gas and the liquid medium to facilitate mass transfer. Sprayers which spray the liquid medium into the gas headspace may be used. In some embodiments, liquid medium may be directed onto or over sheets of plastic or other suitable material such that the liquid medium travels down and/or over the surfaces of the sheets and falls back into the liquid stream. Alternatively or additionally, sheets of material which include pockets may periodically be dipped into the liquid stream and pulled upwardly into the gas headspace to increase the available liquid surface area. In certain embodiments, floating objects and/or devices configured to be partially submerged in the liquid medium (e.g., a paddle wheel) may be used to facilitate enhancement of gas-liquid interfacial area and mass transfer. The objects may be transparent such that they also may act to allow penetration of light to greater depths within the media. In some embodiments, elements may be employed to produce surface ripples or even waves that travel laterally or longitudinally within the liquid medium to increase mass transfer between the gas and the liquid. [0050] At least one or each photobioreactor unit 100 may in certain embodiments include one or more diversion zones or sections 230, which divert portions of the liquid streams to at least one reflow unit such as a reflow channel 232. For example, at least one channel section or zone of a photobioreactor unit may allow liquid to flow perpendicularly to the photobioreactor unit to reach reflow channel 232 (see Figs. 6a-7). The liquid in the reflow channel may then flow toward to the liquid-medium inlet bulkhead 204 and may be added to the liquid inflow by a pump (e.g., an Archimedes screw pump). By recirculating some of the liquid medium comprising phototrophic organisms therein, the addition of new inocula to the liquid medium at the front end of the photobioreactor unit may be reduced or eliminated. In some embodiments, the recirculation rate may generally be in the range of 0.1 - 0.95, and preferably in the range of 0.5 - 0.7.

[0051] As will be apparent to those skilled in the art, particular configurations of the various photobioreactor units and components of the photobioreactor system will depend upon the use to which the photobioreactor is employed, the composition and quantity of the gas to be treated and other parameters specific to individual applications. Given the guidance provided herein and the knowledge and information available to those skilled in the arts of chemical engineering, biochemical engineering, and bioreactor design, one can readily select certain operating parameters and design configurations appropriate for a particular application, utilizing no more than a level of routine engineering and experimentation entailing no undue burden.

[0052] As discussed above in the description of Fig. 2, in certain embodiments, photobioreactor system 200 can comprise a plurality of identical or similar photobioreactor units 100 interconnected in parallel. Furthermore, in certain embodiments, at least one or each photobioreactor unit may comprise one photobioreactor section or a plurality of photobioreactor sections in series. Such scalability can provide flexibility to increase the capacity of the photobioreactor system and/or increase the degree of removal of particular components of the gas stream as a particular application or needs demand. In one such embodiment, a photobioreactor system is designed to separate algae species that are efficient in utilizing NO_x from species efficient in utilizing CO₂. For example, a nitrogen-efficient algae may be placed in a first photobioreactor unit or a first zone of a photobioreactor unit and carbon-efficient algae placed in a second photobioreactor unit or in a second zone of the same photobioreactor unit in series with the first zone. The flue gas enters the first photobioreactor unit/zone and is scrubbed of nitrogen (from NO_x), then flows through the second photobioreactor unit/zone and is scrubbed of carbon (from CO₂).

[0053] The term "fluidically interconnected," when used in the context of conduits, channels, chambers, or other structures provided herein that are able to contain and/or transport gas and/or liquid, refers to such conduits, channels, containers, or other structures being of unitary construction or connected together, either directly or indirectly, so as to provide a continuous coherent flow path from one conduit or channel, etc. to the other(s) to which they are fluidically interconnected. In this context, two conduits or channels, etc. can be "fluidically interconnected" if there is, or can be established, liquid and/or gas flow through and between the conduits and/or channels (i.e., two conduits/channels are "fluidically interconnected" even if there exists a valve between the two conduits/channels that can be closed, when desired, to impede fluid flow there between).

[0054] A channel or trench may comprise, in certain embodiments, fluid- impermeable wall(s) for partially or completely surrounding a fluid passing through the channel along its direction of flow. In other embodiments, wall(s) of a channel may only partially surround a fluid passing through the channel along its direction of flow and/or the wall(s) may have some degree of permeability with respect to a fluid flowing in the channel, so long as the wall(s) sufficiently surround the fluid and are fluid impermeable to a sufficient extent so as to be able to establish and maintain a bulk flow direction of fluid generally along a trajectory parallel to a longitudinal axis or curve defining the geometric center of the channel along its length.

[0055] The liquid medium contained within the photobioreactor system during operation typically comprises water or a saline solution (e.g., sea water or brackish water) containing sufficient nutrients to facilitate viability and growth of algae and/or other phototrophic organisms contained within the liquid medium. As discussed below, it is often advantageous to utilize a liquid medium comprising brackish water, sea water, or other non-portable water obtained from a locality in which the photobioreactor system will be operated and from which the algae contained therein was derived or is adapted to. Particular liquid medium compositions, nutrients, etc. required or suitable for use in maintaining a growing algae or other phototrophic organism culture are well known in the art. A wide variety of liquid media can be utilized in various forms for various embodiments of the present invention, as would be understood by those of ordinary skill in the art. Potentially appropriate liquid medium components and nutrients are, for example, discussed in detail in Rogers, LJ. and Gallon J.R., "Biochemistry of the Algae and Cyanobacteria," Clarendon Press Oxford, 1988; Burlew, John S. "Algal Culture: From Laboratory to Pilot Plant," Carnegie Institution of Washington Publication 600, Washington, D.C., 1961; and Round, F.E., The Biology of the Algae, St Martin's Press, New York, 1965. Each of these references is incorporated by reference.

[0056] Fig. 3 schematically shows one embodiment of a gas treatment/biomass production/photobioreactor system 300 that uses solar energy and photobioreactor system 200 comprising photobioreactor units 100 to produce biomass using a flue gas containing elevated concentrations of carbon dioxide (i.e., gas having a concentration of carbon dioxide greater than ambient air). Flue gas is sent from a CO₂ source 302 to a gas conditioner 306, such as a conventional quench zone, to reduce the gas temperature and possibly remove harmful species such as acid gases. In certain embodiments, a forced draft fan 308 may be used to facilitate this transfer of flue gas and/or push gas through photobioreactor units 100, but in some embodiments no forced draft fan is used. The gas is then sent through the photobioreactor units 100 so that the carbon dioxide (and potentially other gases) can interact with a liquid stream in the photobioreactor units to generate biomass. Photobioreactor system 200 may be constructed of one or more photobioreactor units 100 as described above. In the embodiment shown in Fig. 3, the gas is flowed countercurrently to the liquid stream, that is, the liquid stream flow from liquid inlet/gas outlet bulkhead 204 to liquid outlet/gas inlet 206. Make-up liquid medium (not shown) may be added during operation. In some embodiments, for example as described below with reference to Fig. 10, the flow of gas may be co-current with the liquid stream flow.

[0057] The photobioreactor units 100 may include different zones, e.g., zones 218, 219, 220, 221, along the lengths of the various photobioreactor units. In some embodiments, each photobioreactor unit may have similar zones, while in other embodiments, different zones and/or different zone locations may be provided in various of the photobioreactor units. For example, in a first zone 218, the bioreactor may include nutrient- addition capabilities such as nutrient misting facilities. A second zone 219 may receive a diverted portion of the liquid flow from the main photobioreactor units for return to an upstream zone. Third zone 220 may include cooling capabilities such as evaporative cooling. A fourth zone 221 may be designed and/or controlled to environmentally stress algae, for example, to increase lipids production. It should be noted that these particular zones are provided by way of example only, and as described further below, photobioreactor system 200 and/or individual photobioreactor units within photobioreactor system 200 may include fewer or more zones. The nature of the photobioreactor units 100 comprising growth of photosynthetic organisms fixed or immobilized on wetted surfaces as described herein is particularly amenable to rapid changes in the composition of growth media, e.g., for the purpose of increasing lipid production by inducing transient abiotic stress such as that due to depletion or limitation of a key nutrient such as nitrogen or phosphorus. Such changes are far more difficult or impossible to effect in pure liquid culture systems such as algae ponds where substantial separation of cells from bulk liquid must occur in order to enable a depletion of one or more nutrients by replacement or alteration of the medium composition.

[0058] Cθ₂-depleted gas exits photobioreactor units 100 through liquid inlet/gas outlet bulkhead 204 and may be vented to the atmosphere or passed to further treatment options. An induced-draft fan 312 may be used to pull gas through the bioreactor, or, as described above, a forced-draft fan 308 may be used upstream of the photobioreactor units 100 instead of or in addition to the induced-draft fan in some embodiments. An induced-draft fan allows the photobioreactor system and/or other portions of the overall system to be maintained at a negative pressure, thereby reducing the risk of unintentional venting of untreated gases to the atmosphere. Additionally, the use of an induced-draft fan (e.g., a blower) may simplify the integration of a photobioreactor system with existing power plants, reducing disruptions to power plant operations. A blower is considered fluidically connected to a photobioreactor unit even if it is not directly connected to the photobioreactor unit; that is, other pieces of equipment or other conduits may be connected between the photobioreactor unit and the blower.

[0059] In certain embodiments, a portion of the liquid stream may be diverted, as shown by arrow 318, from a downstream zone of the photobioreactor units 100 and returned to an upstream zone (or in some embodiments to liquid inlet/gas outlet bulkhead 204), which may provide some of the benefits of a "back-mixed" reactor system. In this regard, the amount of inoculum added to the liquid in the photobioreactor units may be reduced or eliminated. Additionally, overall average residence time for the liquid medium may be increased without extending the length of the photobioreactor units. The diverted liquid medium may be returned at a position and in a manner such that the returned liquid medium causes or increases turbulence in the liquid stream, which may enhance heating or cooling and/or photomodulation in certain photobioreactor unit sections.

[0060] As mentioned above, photobioreactor units 100 also may include a cooling zone(s) 220 such as an evaporative cooling zone. In some embodiments, while flowing through photobioreactor unit 100, the liquid stream temporarily exits the enclosed portion of the photobioreactor unit and is exposed to the atmosphere. Evaporation of some of the liquid cools the remaining liquid, which then reenters the enclosed portion of the photobioreactor unit 100. Each photobioreactor unit may be constructed and arranged such that the liquid stream does not significantly change direction or speed when exiting and reentering the enclosed portion of the photobioreactor unit. For example, as shown in Fig. 5, one or more photobioreactor sections of a photobioreactor unit may include walls that reduce the amount of cross-sectional area available for gas flow, but provide an area where the cover section(s) may be removed or indented, as shown, to allow exposure of the liquid stream to the atmosphere.

[0061] In some evaporative cooling zones, a portion of the liquid stream may be continuously exposed to the atmosphere; that is, for a relatively long zone of the photobioreactor unit, which may be made up of a large number of photobioreactor sections, the zone, or each section comprising such zone, may include an area (for example, on the lateral side of the trench) that provides an evaporative cooling area. Substantially continuous mixing of the exposed portion of the liquid stream with the unexposed portion of the liquid stream may provide adequate cooling for the reactor.

[0062] The photobioreactor sections and/or units may be heated and maintained at certain temperatures or temperature ranges suitable or optimal for productivity. These specific, desirable temperature ranges for operation will, of course, depend upon the characteristics of the phototrophic species used within the photobioreactor systems, the type of photobioreactor, etc. Typically, it is desirable to maintain the temperature of the liquid medium between about 5 ⁰C and about 45 ⁰C, more typically between about 15 ⁰C and about 37 ⁰C, and most typically between about 15 ⁰C and about 25 ⁰C. For example, desirable operating conditions for a photobioreactor utilizing Chlorella algae can involve a liquid-medium temperature controlled at about 30⁰C during the daytime and about 20⁰C during nighttime. In one embodiment, the temperature of the photobioreactor is maintained at about 20 ⁰C.

[0063] In certain embodiments, the temperature, velocity, residence time, depths and/or nutrient concentrations can be maintained at different levels/values in the various zones to control for different factors and/or provide particular functionality. For example, it is possible in certain embodiments to maintain one zone so as to maximize growth rates and to maintain conditions in another zone to maximum lipids production.

[0064] Algae-rich liquid exiting from photobioreactor system 200 may be sent to a dewatering system 322. Various conventional methods and/or systems of dewatering may be used to dewater the algae, including dissolved air floatation and/or tangential flow filtration, or any other suitable dewatering approach.

[0065] The dewatered algae may be sent for further processing as indicated at 324, for example, drying. Dried algal biomass can be used directly as a solid fuel for use in a combustion device or facility and/or could be converted into a fuel grade oil (e.g., biodiesel) and/or other fuel (e.g., ethanol, methane, hydrogen). The algae also may be used as food supplements for humans and animals. In certain embodiments, at least a portion of the biomass, either dried or before drying, can be utilized for the production of products comprising organic molecules, such as fuel-grade oil (e.g., biodiesel) and/or organic polymers. Methods of producing fuel grade oils and gases from algal biomass are well known in the art (see, e.g., Dote, Yutaka, "Recovery of liquid fuel from hydrocarbon rich micro algae by thermo chemical liquefaction," Fuel 73:12 (1994); Ben-Zion Ginzburg, "Liquid Fuel (Oil) From Halophilic Algae: A renewable Source of Non-Polluting Energy, Renewable Energy," 3:2/3 pp. 249-252, (1993); Benemann, John R. and Oswald, William J., "Final report to the DOE: System and Economic Analysis of Micro algae Ponds for Conversion of CO₂ to Biomass," DOE/PC/93204-T5, March 1996; and Sheehan, John, Dunahay, Terri, Benemann, John R. & Roessler, Paul, "A Look Back at the U.S. Department of Energy's Aquatic Species Program: Biodiesel from Algae," 1998, NERL/TP-580-24190, each of the foregoing references being incorporated by reference herein).

[0066] Algae-depleted medium resulting from dewatering operations may be disposed of or may be returned to photobioreactor system 200 (after optionally being mixed with fresh liquid medium), as shown by arrow 328, to return unused nutrients to the system. Such an approach may reduce the amount of fresh water and nutrients to be added to the system.

[0067] In some embodiments, other processes of the photobioreactor system may be integrated with the power plant or other CO₂ source. For example, the hot flue gas from the power plant may be used to at least partially dry the biomass produced by the photobioreactor system.

[0068] Algae, or other phototrophic organisms, may, in certain embodiments, be pre-adapted and/or pre-conditioned to specific environmental and operating conditions expected to be experienced in a full scale photobioreactor system of the invention during use. Methods and apparatus for adaptation and pre-conditioning algae are described in commonly-owned International Application Publication No. WO 2006/020177, which is hereby incorporated by reference in its entirety.

[0069] Although photobioreactor system 200 is described as being utilized with natural sunlight, in alternative embodiments, an artificial light source providing light at a wavelength able to drive photosynthesis may be utilized in supplement to or instead of natural sunlight. For example, a photobioreactor utilizing both sunlight and an artificial light source may be configured to utilize sunlight during the daylight hours and artificial light in the night hours, so as to increase the total amount of time during the day in which the photobioreactor system can convert CO₂ to biomass through photosynthesis.

[0070] Since different types of algae can require different light-exposure conditions for optimal growth and proliferation, in certain embodiments, especially those where sensitive algal species are employed, light modification apparatus or devices may be utilized in the construction of the photobioreactors according to the invention. Some algae species either grow much more slowly or die when exposed to ultraviolet light. If the specific algae species utilized in the photobioreactor is sensitive to ultraviolet light, then, for example, certain portions of cover(s) 106, or alternatively, the entire cover outer and/or inner surface, may be coated or covered with one or more light filters to reduce transmission of the undesired radiation. Such a light filter can readily be designed to permit entry into the photobioreactor system of wavelengths of the light spectrum that the algae need for growth while barring or reducing entry of the harmful portions of the light spectrum. Such optical filter technology is already commercially available for other purposes (e.g., for coatings on car and home windows). A suitable optical filter for this purpose could comprise a transparent polymer film optical filter such as SOLUS (manufactured by Corporate Energy, Conshohocken, PA). A wide variety of other optical filters and light blocking/filtering mechanisms suitable for use in the above context will be readily apparent to those of ordinary skill in the art. In certain embodiments, especially for photobioreactor systems utilized in hot climates, as part of a temperature-control mechanism, a light filter comprising an infrared filter could be utilized to reduce heat input into the photobioreactor system, thereby reducing the temperature rise in the liquid medium.

[0071] Referring now to Fig. 4, one embodiment of a nutrient/medium misting photobioreactor section or zone 400 is illustrated. A liquid inlet 402 may be formed of a conduit that also provides support for a mister 404. In some embodiments, liquid may flow into inlet 402 and all of the liquid may exit through mister 404. In some embodiments, liquid may flow through inlet 402 and some of the liquid may exit through mister 404 while the remaining liquid exits through an outlet 406 on the opposite side of section or zone 400 and continues to an adjacent photobioreactor unit. Mister 404 is shown as spraying liquid downwardly in Fig. 4, but in some embodiments the liquid may be aimed upwardly toward the inside of cover 106, such as directly upwardly. In this manner, mister 404 or other liquid injection device may help to clean the inside of cover 106 and the thin film of liquid formed on the inside surface of the cover can further enhance gas-liquid mass transfer.

[0072] Fig. 5 shows one embodiment of a cooling zone 220 for a photobioreactor unit 100. In this embodiment, cover(s) 106 forms three walls 502, 503, 504 which reduce the cross- sectional area of the gas headspace. Each wall 502, 503, 504 penetrates into liquid stream 101 such that photobioreactor unit 100 remains gas-tight. Walls 502, 503, 504 may not, however, in certain embodiments reach the base of photobioreactor unit 100, such that, therefore, in such embodiments, the liquid stream may readily flow into evaporative cooling area 508. In some embodiments, sprayers 510 or other devices which increase surface area exposure of the liquid stream to the atmosphere may be employed to enhance evaporative cooling.

[0073] While evaporative cooling area 508 is shown to be present only on one side of the photobioreactor unit in this embodiment, a second evaporative cooling area may additionally (or instead) be provided on the opposite side of the photobioreactor unit, or positioned at an intermediate location positioned between the two laterally opposed sides of photobioreactor unit 100. For embodiments in which cooling zone 220 comprises one or more interconnectable photobioreactor sections, as with photobioreactor sections that include nutrient misters for embodiments including such photobioreactor sections, the interchangeability of the photobioreactor sections may allow for the addition or subtraction of cooling areas after installation of the photobioreactor system.

[0074] One embodiment of a liquid flow diversion photobioreactor section or zone 230 is illustrated in Figs. 6a and 6b. As shown in Fig. 6a, a movable weir 240 may be deployed such that all liquid in the photobioreactor unit liquid stream 101 is directed through bypass conduits 242. In such a configuration, none of the liquid flowing through diversion photobioreactor section or zone 230 is diverted, and all of the liquid medium flowing through the section continues toward the liquid medium outlet. With the movable weir 240 lowered, as shown in Fig. 6b, a portion of the liquid medium is diverted into a transverse channel 244 which flows to a reflow channel such as reflow unit 232 illustrated in Fig. 2. In some cases, all of the liquid stream is diverted depending on the relative heights of bypass conduits 242, adjustable weir 240 and the liquid levels in trench 102 and transverse channel 244. In certain embodiments, the degree of diversion is controllable either or both of manually or through use of a computer operated process control system. [0075] A controller, e.g., a computer-implemented system, may be used to monitor and control the operation of the various components of the photobioreactor sections, units and systems disclosed herein, including valves, sensors, weirs, blowers, fans, dampers, pumps, etc. Certain embodiments may employ computer systems and methods described in commonly- owned International Publication No. WO2006/020177, particularly with reference to Fig. 7 A of that publication. In addition to automating operation of aspects of the photobioreactor system, use of a computer- implemented system may facilitate optimizing or improving the efficiency of the system by determining suitable values for various control parameters. In some embodiments, flow may be controlled to provided a desired level of turbulence and light/dark exposure intervals for improved growth, and described and determined according to methods also described in International Publication No . WO2006/020177.

[0076] Fig. 7 shows another embodiment of a diversion photobioreactor section or zone 230. In this embodiment, an adjustable weir 250 may be lowered to allow liquid medium to flow into transverse channel 244. When adjustable weir 250 is raised, the liquid medium flows through a bypass portion 254 of diversion zone 230 to continue along the photobioreactor unit.

[0077] One embodiment of a liquid and gas bulkhead zone 600 is shown in Figs. 8a and 8b. In certain embodiments, a series of sections 600 may be connected end to end and travel transversely to a plurality of parallel photobioreactor units, as shown in Fig. 9. Each bulkhead section 600 may include an automated weir 601 or other liquid control element for adjustably controlling the size and elevation of a liquid passageway 602. Each bulkhead section 600 also may include a flue gas damper 603 or other flue gas control element for controlling the size of a gas passageway 604. An embossing 606 or ridge for attachment to a photobioreactor unit may be provided on a side of bulkhead section 600. The sizes of liquid passageway 602 and gas passageway 604 may be fixed or adjustable. For example, in a system with a consistent liquid stream flow rate, the weirs for each of a plurality of photobioreactor units may be permanently set such that flow from the bulkheads is substantially equal for each photobioreactor unit. In other embodiments, each bulkhead section may include an adjustable weir so that the flow of liquid to each photobioreactor unit can be independently controlled. Similarly, gas passageways may be designed to equally distribute gas flow among all of the photobioreactor units, or, gas dampers may be configured and/or operated so that gas flow to each photobioreactor unit may be independently controlled. At least one cover 610 for the bulkhead section(s) may be transparent and otherwise similar to the covers for the photobioreactor units, or, in some embodiments, the cover may be opaque and/or made of a different material than the photobioreactor unit covers.

[0078] Ten bulkhead sections 600 are shown interconnected in Fig. 9 to form a bulkhead distribution unit 700. The open lateral inlet 701 to the gas head space of the bulkhead provides an inlet for flue gas that may be fluidically interconnected with a conduit(s) supplying feed gas from a CO₂ source and/or gas conditioner 306 and/or quench zone of the system (discussed below). Recirculated liquid 702 from a reflow channel 232 is shown being pumped into bulkhead distribution unit 700. The recirculated liquid 702 mixes with fresh liquid medium and/or liquid being recycled from dewatering operations, and the liquid is distributed to the various photobioreactor units 100 by gravity flow through liquid passageways 602. [0079] Although not shown, dampers, such as guillotine dampers, between one or more bulkhead sections may be used to limit gas and/or liquid flow to certain photobioreactor units. A guillotine damper and/or other flow control element may also be used within a single point entry to the bulkhead region so that all flow of gas and/or liquid may easily be stopped.

[0080] While many of the embodiments described herein employ the movement of liquid through a gas headspace to promote mass transfer between the gas and liquid, in certain embodiments, additionally or alternatively, gas may be sparged into the liquid. For example, while the bulk of gas distribution into the liquid medium present in a photobioreactor unit 100 may be through a gas passageway such as the one shown in Fig. Ia, a not-insignificant amount of gas may be sparged into the liquid medium in certain embodiments. The sparging, in addition to creating an additional gas-liquid interface, may create turbulence or additional turbulence in certain regions where such turbulence is desirable.

[0081] In an alternate embodiment of the invention, a photobioreactor system may include some or all of the elements of the photobioreactor system shown and described in Fig. 3, with the exception of the recycle for recirculating liquid from downstream in a photobioreactor unit to upstream in the photobioreactor unit. Fig. 10 shows one embodiment of such a system, which may include many of the same elements as the system described above with reference to Fig. 3. Additionally, Fig. 10 illustrates an embodiment in which gas flows co-currently with liquid flow through photobioreactor units 100. Thus, both liquid and gas flow from a liquid inlet/gas inlet bulkhead 340 to a liquid outlet/gas outlet bulkhead 342 in this embodiment.

[0082] A perspective view of one physical embodiment of the photobioreactor system 700 illustrated in Fig. 10 is shown in Fig. 11.

[0083] In many current photobioreactor systems, chosen, desirable strains of algae can be difficult to maintain in a photobioreactor that is not scrupulously sterilized and maintained in a condition that is sealed from the external environment. The reason for this is that the algal strains being used in such photobioreactors may not be well adapted or optimized for the conditions of use, whereas other, endemic algal strains in the atmosphere may be more suitably conditioned for the local environment, such that if they have the ability to contaminate the photobioreactor they will tend to predominate and eventually displace the desired algae species. Such phenomena may be mitigated and/or eliminated by using adaptation protocols and algal cultures described in International Publication No. WO2006/020177, and commonly owned U.S. Patent Application serial No. 11/632541, which is incorporated herein by reference. Use of such protocols and algae strains produced by such protocols may not only increase productivity and longevity of algal cultures in real photobioreactor systems, thereby reducing capital and operating costs, but also may reduce operating costs by reducing or eliminating the need to sterilize and environmentally isolate the photobioreactor system prior to and during operation, respectively.

[0084] Many power plants include ponds or other bodies of water to which waste heat is discharged. In some embodiments, especially in colder climates, a photobioreactor may be positioned on top of a wastewater pond to achieve one or more possible advantages. By floating or otherwise positioning a bioreactor on a body of water, the photobioreactor system may take advantage of the inherent flatness of the surface of a body of water over an expansive area. Further, by using an already existing pond, limited additional geographic area is required for the photobioreactor system. If the body of water accepts heated wastewater from the power plant (or other source) the photobioreactor system can be heated by the body of water to improve biomass production and/or prevent freezing in cold ambient conditions.

[0085] One embodiment of a photobioreactor unit 800 adapted for positioning on a body of water is shown in Fig. 12. Photobioreactor unit 800 is supported by two pontoon floats 802 that extend longitudinally along the length of the photobioreactor unit. Of course, other structures may be used to float or support one or more photobioreactor units on a body of water.

[0086] To accommodate rain water and/or melted snow runoff, a drain system (not shown) may be incorporated into any of the above described photobioreactor systems. In one embodiment of a drainage system, a drainage hole is provided periodically along a collection channel positioned between two photobioreactor units of the photobioreactor system. The drainage hole empties into a drainage conduit that transversely spans each of the photobioreactor units that are positioned side-by-side. The drainage conduit leads to a drainage trench to lead water away from the photobioreactor system. In some embodiments, the drainage trench may be wide enough to accommodate various vehicles (e.g., vehicle access channel 208 of Fig. 2 may comprise a drainage trench).

[0087] In certain embodiments, advantageously, hot flue gas received from a power plant may be cooled and/or scrubbed to remove undesirable components using a liquid otherwise employed in the photobioreactor system. For example, as illustrated in Fig. 13, in some embodiments of a gas treatment system 900, algae-free medium that results from dewatering operations may be sprayed in a quench zone 902 to cool/scrub hot flue gas before the gas enters photobioreactor system 200 comprising photobioreactor units 100. Liquid effluent from quench zone 902 may be disposed of, or in some embodiments, returned to photobioreactor units 100 (dashed line).

[0088] Using liquid medium to quench the flue gas heats the medium and may reduce the pH of the medium. One or both of these effects may help kill adventitious biological species, such as rotifers, cilitates, bacteria, and viruses that may impair the growth of the desired algae. If the quench effluent stream is returned to the system upstream of the dewatering step, it may improve the dewatering operation. For example, reducing the pH of the dewatering feed may improve the effectiveness of polycationic coagulants and alum-based flocculants. Additionally, thermally heating the algae-containing media may induce necrosis and autoflocculation, which simplifies the dewatering process and may reduce or eliminate the need for chemical additives.

[0089] In an alternative embodiment illustrated in Fig. 14, algae-rich medium harvested from the outlet of photobioreactor units 100 may be used in quench zone 902 to cool flue gas. The liquid effluent from quench zone 902 may then be sent to dewatering system 322 to enrich the algae. As with some other embodiments described herein, algae-free medium from dewatering system 322 may optionally be returned to photobioreactor units 100.

[0090] In a further embodiment of a photobioreactor system including quenching, illustrated in Fig. 15, enriched algae from dewatering system 322 may be used to cool hot flue gas in quench zone 902. Dewatered algae may be approximately 3% solids concentration after primary dewatering, and 10-20% solids after secondary dewatering. Using dewatered algae in quench zone 902 may help to stabilize the algae against decomposition, preheat the algae to aid in downstream processing, and allow some components to react with the acid gases, which may promote downstream processes such as fermentation.

[0091] In some embodiments, combustion gases are treated with the photobioreactor system to mitigate pollutants and to produce biomass, for example, in the form of harvested algae which can be used as a fuel for the combustion device and/or for the production of other products, such as products comprising organic molecules (e.g., fuel-grade oil and/or organic polymers). Further description of such an integrated system, which can be used in conjunction with embodiments of photobioreactor systems disclosed herein, may be found in PCT Publication No. WO2006/020177, and also in commonly owned U.S. Patent Application Publication Nos. US-2005-0064577 and US-2005-0239182, and PCT Application No.

US2005/025249, filed on July 18, 2005, each of which is hereby incorporated by reference in its entirety. [0092] One embodiment of a configuration for quench zone 902 is illustrated in Fig. 16. In this embodiment, spray elements 904 extend perpendicularly to a liquid supply conduit 906 and are configured spray liquid into a gas headspace 908. Liquid effluent is collected from the bottom of a trench 910 and either disposed of or recycled back into the photobioreactor system. A perspective view of one embodiment of quench zone 902 in Fig. 17 illustrates that spray elements 904 may be spray conduits 914 including longitudinal slits.

[0093] In some embodiments of the invention, waste heat (in the form of heated water) may be used to heat liquid media in a photobioreactor system. One embodiment of tubes 920 submerged in liquid medium 101 is shown in Fig. 18. Tubes 920 in Fig. 18 may continue longitudinally within the same photobioreactor section or unit, and/or may continue laterally to adjacent photobioreactor sections or units. In some embodiments, jets 922 may be used to increase the flow rate of liquid medium 101 past tubes 920 to increase the rate of heat transfer.

[0094] The interfacial surface area between the gas and the liquid medium may be increased by positioning, for example, in an essentially vertical orientation, a cultivation matrix within a photobioreactor unit and applying liquid medium (e.g., by dripping or spraying) onto the matrix. Such an arrangement can increase the productivity of photobioreactor units per land area (or water body area if the photobioreactor unit is positioned on a water body) covered by the photobioreactor units. Also, in some embodiments, the algae or other microorganisms on the cultivation matrix are exposed to reduced light levels in which the photon flux density supports improved light utilization.

[0095] In some embodiments, the cultivation matrix may comprise a textile, or other fiber- based material, in the form of a sheet, which may be made of knitted, woven, non-woven, or other suitable construction. Details of various cultivation matrices which may be used with embodiments disclosed herein, including treatments of the matrices to improve strength and wettability, are set forth in commonly-owned U.S. Provisional Application Serial Nos. 61/045,601 and 61/048,302.

[0096] As illustrated in Fig. 31, the cultivation matrices may be in the form of sheets 1012 having a growing area 1100 for algae or other microorganisms on the cultivation matrix. The sheet 1012 has a width 1102 and a height 1104. The total growing area 1100 of the sheet 1012 can range from 1 m² to 10,000 m². Generally, the total growing area 1100 is at least 1 m²; in some embodiments, the total growing area 1100 is at least 10 m², or at least 13 m². The aggregate growing area of system including multiple sheets (i.e., the sum of the growing areas 1100 of all sheets 1012 in the system) is generally at least 100 m , and may be at least 1000 m or in some cases 15,000 m or more. The system may have a growth footprint area of at least 7.4 m², at least 73.4 m², or at least 1103 m².

[0097] Systems incorporating cultivation matrices as described herein may be operated with a footprint areal productivity of between about 10 and 200 g/m²-day on an ash-free, dry- weight basis based on the footprint area of the photobioreactor unit(s). In some embodiments, footprint areal productivities ranging from 60 and 175 g/m -day, or from about 80 to about 150 g/m -day may be realized. "Areal Productivity" is defined herein as biomass produced per unit area of growing region per unit time (e.g., g/m²-day). Where B2 is biomass at time T2, and Bl is biomass density at time Tl, and A is the photobioreactor system's total growing area, areal productivity = (B2 - B 1) / (A x (T2 - Tl). The footprint areal productivity of biomass may be at least 10 g/m²-day, at least 40 g/m²-day, at least 80 g/m²-day, at least 100 g/m²-day, or at least 150 g/m²-day. Generally, the footprint areal productivity of biomass may be between 10 and 170 g/m²-day. [0098] One embodiment of a photobioreactor unit employing such an arrangement is illustrated in Fig. 21. In this embodiment, five cultivation matrices in the form of sheets 1012 are suspended with their widths aligned vertically from supports 1010 within photobioreactor unit 100. The growth footprint area is the area below the vertically aligned cultivation matrices. In one embodiment, the ratio of the total growing area of at least one solid support to the growth footprint area is at least 12.2 m² of growing area per m² of growth footprint area, for example, 1224 m² total growing area deployed over a 100 m² growth footprint area. Each m² of growing area corresponds to a growth volume, i.e., the growing area multiplied by the average thickness of the microorganism film on the growing area. In a system consisting solely of vertically deployed cultivation matrices 1012, the total system growth volume is the aggregate growth area times the average microorganism film thickness. However, as depicted in Fig. 32, the system total growing volume may include both a vertical growing volume 1110 and a liquid growing volume 1112. The liquid growing volume is generally the volume of liquid 1112 in which additional growth takes place.

[0099] Liquid medium may be applied to sheets 1012 via a stream, drip, mist or spray (as illustrated) 1014 provided by nozzles 1004. The application of liquid medium to sheets 1012 may be achieved by, for example, dripping, spraying, showering, or misting liquid medium onto sheets 1012 either continuously or at predetermined or controlled intervals.

[0100] In some embodiments, liquid medium may flow through a supply pipe inlet 1002 and some of the liquid medium may exit through nozzles 1004 while the remaining liquid medium exits through an outlet line 1006 on the opposite side of photobioreactor unit 100 and continues to an adjacent photobioreactor unit (not shown in Fig. 21). Nozzles 1004 are shown as spraying liquid medium downwardly in Fig. 21, but in some embodiments the liquid may be aimed upwardly toward the inside of cover 106, such as directly upwardly, or in any other suitable direction. The humidity within the photobioreactor unit 100 may be at least 80% or, in some embodiments, at least 95%. As illustrated in Fig. 24a and described in greater detail below, The photobioreactor may have a channel or basin to receive the liquid medium and a means 1024 for recirculating the liquid medium from the channel or basin back onto the cultivation matrices via nozzles 1004.

[0101] The total system liquid volume is the total volume of all the liquid in the photobioreactor system. In one embodiment, the ratio of the total system liquid volume to the total growing area is no greater than 2.2 L/m² of growing area, or 26.5 L/m² of growth footprint area. For example, the total system liquid volume may be 2650 L and the system may have a total growing area of 1224 m² from, for example, 40 sheets each having a width of 17 m and a height of 1.8 m in a photobioreactor having a 100 m² areal footprint. The total system liquid volume per gram of biomass may be no greater than 0.22 L/g. In preferred embodiments the total system liquid volume per gram of biomass may be no greater than 0.144 L/g, no greater than 0.087 L/g, no greater than 0.072 L/g, or no greater than 0.054 L/g. For example, the total system liquid volume may be 2650 L, corresponding to an area biomass density of 25 g/m² with a total growing area of 1225 m². By way of comparison, a typical 20 cm deep open-pond algae cultivation system having a similar 100 m² growth footprint area would have a total system liquid volume of 20,000 L. The same 100 m² pond can support at maximum a biomass density of 0.5 g/L, corresponding to an instantaneous system biomass of 10,000 g. In contrast, a photobioreactor in accordance with the present invention having a 100 m² areal footprint may support upwards of 25-40 g of biomass per m² of the matrix, corresponding to a total system biomass of approximately 30,000 to 50,000 g. Hence, the photobioreactor system of the present invention is about 10 times more efficient than a pond system on the basis of water use per footprint area, and 30 to 50 times more efficient than a pond system on the basis of water use per unit of algal biomass (when the water volume and biomass densities are both factored in). Moreover, due to the contained nature of the photobioreactor of the present invention little of the system water volume will be lost to evaporation vs. upwards of 0.5 to 1 cm per day loss expected in a pond system, which corresponds to 2.5 to 5% of the system volume (or 500 to 1000 L for the example 100 m² pond described above).

[0102] Cultivation matrices 1012 may be positioned at any suitable distance from one another, and in some embodiments, sheets 1012 may be spaced from one another by between about 5 and 40 centimeters. In particular, the spacing may range from about 10 to about 20 centimeters, e.g., no more than 20 cm, no more than 15 cm, or no more than 10 cm. A preferred spacing and number/total surface area of sheets 1012 will depend on the species and/or strain of algae or other microorganism being used and the degree of productivity, etc.

[0103] Any suitable method of positioning matrices 1012 within photobioreactor unit 100 may be used. In the embodiment of Fig. 21, sheets 1012 are hung from supports 1010 which run longitudinally along the length of photobioreactor unit 100. One embodiment of support 1010 is illustrated in Fig. 22. In this embodiment, support 1010 includes a hollow cylinder 1011 with a slot 1016, which extends along the bottom length of cylinder 1011. Each of the sheets 1012 is attached to one or more bearings 1040, and the assembly is inserted into cylinder 1011 such that bearing(s) 1040 are supported by cylinder 1011, and sheets 1012 hang through slot 1016. In some embodiments, cultivation matrix 1012 may be attached to bearing(s) 1040 after bearing(s) 1040 is/are inserted into cylinder 1011. It should be noted that this specific method of suspending substrate 1012 is not intended to be limiting, as any suitable method of suspending substrate 1012 may be used. In one embodiment, the solid cultivation support 1010 has a height and a width, and the width exceeds the height by at least a factor of 10, or alternatively by a factor of 100. [0104] In the embodiment illustrated in Fig. 21, sheets 1012 are oriented so that their longitudinal axes (extending along their lengths) run substantially parallel to the longitudinal axis extending along the length of photobioreactor unit 100. In other embodiments, such as that illustrated in Fig. 23, sheets 1012 may be oriented so their lengths are oriented substantially perpendicular to the length of photobioreactor unit 100, or in other angular orientations with respect to the longitudinal axis of the photobioreactor unit. In such embodiments, the direction of spray or other liquid medium application may be different from embodiments where substrates 1012 are positioned to be substantially parallel to the length of photobioreactor unit 100.

[0105] In some embodiments, liquid medium may not flow along or be continuously contained in the bottom portion (e.g., channel or basin) of photobioreactor unit 100, at least during certain periods of operation, e.g., during biomass harvesting. For example, the photobioreactor unit illustrated in Figs. 24a and 24b is constructed with a basin or channel 1020 that is sloped in the transverse direction toward a longitudinally-extending drain trench 1022. A pump tank 1024 may be included in the system to collect and store runoff liquid from sheets 1012 for removal from the system or back into the photobioreactor unit 100. The embodiment of Fig. 24a has drain trench 1022 positioned to one side of photobioreactor unit, while the embodiment illustrated in Fig. 24b has a drain trench 1023 positioned toward the transverse center of photobioreactor unit 100. As illustrated, a pitched roof 1006 is used as a cover for photobioreactor unit 100 in the embodiments of Fig. 24a and 24b as opposed to the semi- hemispherical cover of the embodiment illustrated in Fig. 21. Fig. 25 is a perspective view of the embodiment of Fig. 24a.

[0106] Fig. 26 illustrates a side view of a photobioreactor unit 100 according to another embodiment. In this embodiment, base 1020 slopes in the longitudinal direction toward a drain trench 1022, which extends transversely to the length of photobioreactor unit 100 and any other photobioreactor units (not shown) positioned adjacent and essentially parallel thereto in the overall photobioreactor system. Nozzles 1004 are positioned at regular intervals along the length of photobioreactor unit 100, although in some embodiments, nozzles 1004 may be positioned at irregular intervals.

[0107] One embodiment of a liquid delivery system constructed and arranged to apply liquid medium to cultivation matrices is illustrated in plan in Fig. 27. In this embodiment, nozzles 1004 are positioned in a two-dimensional array such that the range of each nozzle (shown as dashed circles 1005) overlaps with adjacent nozzles. The range of each liquid delivery location is not necessarily fixed as the range may be adjusted by modifying flow rates and/or changing the type or nozzle or flow distributor present at the liquid delivery location.

[0108] Fig. 28 is a side view one embodiment of a photobioreactor unit 100 including cultivation matrices 1012 and a liquid delivery system 1032. In Fig. 28, nozzles 1004 are positioned above cultivation matrices 1012 and the cultivation matrix supports such that liquid medium falls from the nozzles on to the cultivation matrices. In some embodiments, nozzles 1004 are integrated within the supports. Nozzles 1004 may be directly integrated with a header pipe in some embodiments. In other cases, as illustrated in Fig. 28, nozzles 1004 may be attached to the ends of conduits 1030 branching from a main header pipe 1032. Any number of headers may be used to feed any number of nozzles in the system. [0109] Cooling liquid may be introduced into the photobioreactor units using the liquid medium delivery systems described herein, and/or cooling liquid may be introduced using a separate cooling liquid delivery system. In some embodiments, an external heat exchanger may be used to cool the cooling liquid. The flow of cooling liquid into the photobioreactor unit may be pulsed or continuous, and may use a subset of the nozzles or other liquid delivery devices present in the liquid medium delivery system.

[0110] In some embodiments, temperature sensors 1106 (see Fig. 21) that are configured to monitor the internal temperature of the photobioreactor units and/or cultivation matrices may be incorporated into the system to facilitate maintenance of a target temperature or cooling so that a maximum temperature is not exceeded. For example, pump tank 1024 may include a control system responsive to the temperatures sensors 1106 to automatically control delivery of cooling liquid to the cultivation matrices when the measured temperature exceeds a predetermined threshold. In certain embodiments, the cooling spray may be spatially controlled so that spray is emitted from only nozzles positioned to direct liquid onto portions of the matrices exceeding the set point temperature but not from other nozzles. Temperature sensors 1106 may include one or more of thermocouples, fluid bulb thermometers, IR thermometers, bimetallic strips, change-of- state-thermometers, and/or any other suitable temperature sensor.

[0111] In some embodiments, biomass may be removed from the cultivation matrices on a daily or other periodic basis, while in other embodiments, continuous harvesting of biomass from a photobioreactor unit may be conducted. In some instances, control and operation of the biomass harvesting system may be automatic. In certain such embodiments, one or more sensors 1108 (see Fig. 30) that determine the density of biomass on the matrices may be utilized by a control system to control operation of the harvesting system. For example, the sensors 1108 may be optical density sensors deployed as illustrated. Alternatively, sensors 1108 may be conductivity sensors positioned against the growing areas or weight sensors mechanically coupled to the support bearings 1040 (see Fig. 22). The biomass harvester may be activated when, for example, the density of the biomass on the matrices is at least 2 g/L without flocculation, or at least 8 g/L without flocculation.

[0112] In an exemplary embodiment, the removal of 5-7 grams of algae material per square meter of matrix on a daily basis may enhance system performance. A pressurized, high- velocity water or liquid-medium jet stream is directed to impinge upon the cultivation matrices to dislodge and remove biomass. In some embodiments, a user employs a hand-held harvest wand to direct the water jet stream. The harvest wand may, for example, include a plurality of nozzles and deliver water at pressures between about 40 and 90 psi. A range of harvest levels may be achieved by varying the water pressure. In some cases, an increase in pressure and/or an providing a pressure pulse and variation in the pulse frequency may be used to increase the amount of biomass removed. In an alternative embodiment, the harvester may be an air knife that removes the biomass from the cultivation matrices.

[0113] In certain embodiments, especially in large systems, an automated harvesting system may be used. Automated harvesting can be advantageous in embodiments, for example, in which manual harvesting would require system shutdown and/or result in contamination of the algae by the entry of a human operator in the photobioreactor unit. Automated harvesting may increase the speed or efficiency with which harvesting can be accomplished.

[0114] In some harvesting system embodiments, the cultivation matrices are movable, as illustrated in Fig. 29A (top view) and Fig. 29B (side view). In this embodiment, each cultivation matrix 1012 passes over rollers 1036 which provide tension to the cultivation matrix. Rollers also may, in some cases, provide vertical support along the length of the growth structure. Harvesting may be implemented at one or more stationary locations, for example with a harvest wand 1038 or other water streams, while the cultivation matrices move past the harvesting location(s). [0115] In some embodiments, the cultivation matrices remain stationary within the photobioreactor unit, and the harvesting system is movable. One example of a mobile harvesting system is illustrated in Fig. 30. In this embodiment, a mobile harvesting system 1039 moves along the length of the photobioreactor unit 100 and removes biomass from cultivation matrices 1012. The harvesting system includes a trolley 1040 and upwardly projecting headers 1042, from which liquid jet streams 1044 are directed onto the cultivation matrices 1012. The trolley may, in some cases, comprise rollers, wheels, tractor treads, etc. 1046 to facilitate movement. In some embodiments, the trolley may include a pump that pulls liquid from the media collection basin below the sheets. Pure water or dilute harvest streams (e.g., between about 0.01 - 0.05 % solids) may be used in some cases for harvesting the biomass. The harvesting system may include a conventional controller 1110 and a hydraulic, pneumatic, or belt drive system for moving the trolley 1040 back and forth along the cultivation matrices 1012. The controller 110 may be responsive to sensors 1108, causing ejection of liquid jet streams 1044 and movement of the trolley 1040 when harvesting is appropriate based on sensor measurements. Alternatively, the controller 1110 may actuate the harvesting system 1039 at predetermined intervals.

[0116] A variety of algal species may be used in the systems disclosed herein. In certain such embodiments, the culture comprises at least one species of algae, such as, for example, one or more species of Chlorella, Scenedesmus, Chlamydomonas, Dunaliella, Chaetoceros, and/or P orphyridum, such as, for example, at least one of Dunaliella tertiolecta, Porphyridium sp., Dunaliella parva, Dunaliella salina, Chlorella pyrenoidosa, Chaetoceros muelleri, and/or Chlamydomonas reinhardtii. [0117] Some embodiments may require matrix materials that meet minimum tensile strength requirements, especially for embodiments such as that illustrated in Figs. 24a-30 in which the cultivation matrices are positioned with their lengths running along a substantial portion of the photobioreactor unit. In such designs, especially where the cultivation matrices are formed from non-woven textile webs or fabric, the width direction of the matrix sheet supporting the weight of the media and biomass against the force of gravity will be aligned in the cross- direction of the material (i.e., essentially perpendicular to the machine direction during manufacture). This is typically the direction along which conventional non- woven fabric/web materials are weakest. The tensile strength of the cultivation matrix material should be large enough to support the wet weight of the material, and may be anisotropic. For example, the tensile strength of the material in the direction along the length of the support may be different from the tensile strength of the material in the direction from the support to the ground. In such cases, tensile strength of the material in the direction from the support to the ground should be large enough to support the wet weight of the substrate material.

[0118] Algal biofilms grow efficiently, in some cases, only within a particular range of temperatures. The appropriate temperature range may vary depending on the species. In some embodiments, operating temperatures of 20 to 30⁰C promote efficient growth. In some cases, temperatures higher than 40 ⁰C may lead to death of the species. To facilitate temperature control, the liquid applied to the cultivation matrices by a liquid distribution system of the invention may be cooler than the temperature of the matrix/portion of the matrix to which it is applied by at least 10 ⁰C. As discussed above, the liquid may be applied in response to sensed temperature within the system. [0119] In an exemplary embodiment, a cultivation matrix sheet may hold about 500 ml (500 g) of water per square meter. Each square meter of such matrix may hold sufficient macro nutrients in the medium held in the sheet to synthesize 1 to 1.5 g of biomass. Thus, a sheet spacing within a photobioreactor unit of about 6 sheets per meter may be used to produce an areal productivity of about 80-100g/m -day. Based upon an exemplary sheet dimensions (e.g., 2 meters wide along the vertical direction when hanging), between about 5 and 7 grams of algae per square meter of sheet per day may be produced in some cases. To meet nutrient uptake needs, the medium may need to be refreshed as often as every 2 hours to support productivities of 5 to 7 grams of algae per square meter of sheet per day. For an embodiment where the liquid is supplied to the sheets continuously rather than at intervals by the liquid distribution system, to achieve the same effect as above, a continuous liquid supply of about 4.16 ml/minute-m of sheet would be appropriate.

[0120] In certain embodiments, liquid in the cultivation matrices remains entrapped in the pore space of the sheet material. For embodiments in which a gas supply to the photobioreactor comprises flue gas, due to the operating conditions of the photobioreactor, the moisture content of the flue gas entering the system and the reservoir of bulk media within the photobioreactor unit (e.g., within the channel/basin thereof), it can be assumed that, in some cases, there is 100% humidity in the photobioreactor unit. Thus, in such embodiments, the matrix may require only a single administration of water flow to support hydration of the biofilm on the matrix, with no additional flow necessary during the growth period between harvestings.

Examples

Example 1

[0121] In this example, a laboratory test of an embodiment of a photobioreactor of the present disclosure is compared to a model of the same. Algae species Nannochloris sp. is grown in a 20cm depth of Media 1, which is sea water comprising 0.75 g/L NaNO₃ and 0.0565 g/L NaH₂PO₄^H₂O. The growth rates for the algae as a function of time, concentration, and light intensity, measured as photon flux, were estimated from data derived from laboratory tests with well-stirred open tanks fed with gas containing 5 mol% CO₂ and the balance O₂ and N₂ in a 1:5 molar ratio. The test results are shown in Fig. 19 for insolation rates of 2000, 1000, and 750 μE/m²-sec, and the productivity is tabulated in Table 1. It should be noted that the data derived from the laboratory tests with well-stirred open tanks may substantially overestimate the reactor productivity and CO₂ recovery that may be attained during actual operation of photobioreactors of the present disclosure. As shown in Figure 8, the productivity is not a function of concentration in this operating range. Independently, the growth rate can be estimated using the model developed by Wu and Merchuk ("A Model Integrating Fluid Dynamics in the Photosynthesis and Photoinhibition Processes," Chemical Engineering Science 56: 3527-3538) under steady-state conditions, using a finite-element simulation approach ("Simulation of Algae Growth in a Bench Scale Bubble Column," Biotechnology and Bioengineering 80(2): 156-168; and "Simulation of Algae Growth in a Bench Scale Internal Loop Airlift Reactor," Chemical Engineering Science 59(14): 2899-2912). The parameter μmax was averaged 0.077 /hr in duplicate tests, and parameter kx is taken as 0.22 m²/g per Oswald (The Engineering Aspect of Microalgae. In: Laskin, I., and Lechevalier, H. A., Editors. CRC Handbook of Microbiology. Cleveland CRC Press, pp 519-552, 1977.) The model productivities matched the measured productivities well, as shown in Table 1.

[0122] The bioreactor gas/liquid exchange is measured in a flowing rectangular conduit with 5 mol% CO₂ flowing above a media containing base so that CO₂ uptake can be measured by carbonate analysis in the liquid phase. The results are shown in Fig. 20, expressed as CO₂ flux (mmol/m²-sec) vs. pH of the media. Recycled media from dewatering is used to enhance the CO₂ gas-liquid exchange. Fig. 20 also show the enhanced gas-liquid mass transfer rates that can be achieved by spraying the recycled media into the headspace of the reactor for two different spray rates, normalized to the reactor area. The test results illustrate the increase in CO₂ transfer rates which can be obtained by properly re-injecting the dewatering fluid into the reactor. These higher CO₂ transfer rates can reduce the bioreactor area requirements in situations where the algal productivity is limited by gas mass transfer. Alternatively these higher CO₂ transfer rates can be used to increase the total biomass production rates from a bioreactor of fixed size.

[0123] A covered bioreactor is modeled using the algal growth model discussed above and the mass transfer rates from the gas-liquid tests. The bioreactor has a depth of 20 cm and a liquid velocity of 20 cm/sec to ensure a high level of turbulence. The bioreactor is sufficiently long that the flow is essentially plug flow; i.e., the Peclet number is high. The liquid phase comprises Media 1 maintained at pH 7.8 with an initial algae recycle rate to maintain the algae concentration in the feed end at 0.1 g cell dry weight/liter. The flue gas contains 5 mol% CO₂, and flows through channels with a gas freeboard height of 2 m. The bioreactor is covered with polyethylene plastic film, with a measured visible light transmission of 95%. The media recycled from the dewatering system is split with 80% returned to the bioreactor to enhance the CO₂ mass transfer rate, and 20% sent to the open areas of the bioreactor to generate a spray that enhances liquid cooling. The ambient dry-bulb temperature is assumed to be 3O⁰C, with a wet- bulb temperature of 25 ⁰C. The reactor productivity, CO₂ conversion, power requirements for the flue gas handling and water consumption are listed in Table 2 for three levels of solar insolence.

Example 2

[0124] This example illustrates the advantage of embodiments of photobioreactors disclosed herein compared to a conventional raceway pond. The reactor productivity, CO₂ conversion, power requirements for the flue gas handling, and water consumption are listed in Table 3 for the highest level of solar insolence using the same operating conditions of Example 1, based on published values for CO₂ conversion and evaporation rates. Flue gas is sparged into a 2-meter deep well in the raceway via a blower that compresses flue gas to 8 psig. The results show that the hybrid bioreactor achieves comparable growth rates, while attaining greater CO₂ conversion and using substantially less power. The raceway pond power consumption is significantly higher due to its lower CO₂ capture efficiency, requiring higher flue gas flows per unit of algae produced, and its higher pressure drop. Water consumption for both reactors is comparable because both use evaporative cooling to maintain reactor temperature.

Example 3

[0125] This example illustrates the advantage of the hybrid open/closed bioreactor at a lower ambient temperature, 5 ⁰C, compared to a raceway pond. The system of Examples 1 and 2 is operated at identical conditions, with the exception that none of the recycled media of Example 1 is directed towards the cooling zone, and low-level heat from the power plant condenser cooling loop is used to maintain the bioreactor temperature. Table 3 lists the productivity and heat duty for maintaining 25 ⁰C in the two reactors. The results show that this reactor has significant advantages over an open raceway pond.

I Table 3 Comparison of Bioreactor Performance at 5°C Ambient |

Example 4

[0126] This example illustrates options for integrating the dewatering operation with the bioreactor. Nannochloris sp. is grown in Media 1 at bioreactor temperatures ranging from 17- 27 ⁰C, with insolation ranging up to about 2200 μE/m²-sec. The biomass concentration ranges from 0.2 to 10 g/L. The algae is dewatered using the techniques known in the art as dissolved air flotation. The feed to the dewatering system is mixed with aluminum sulfate to attain a concentration of 50-300 ppm in the media, and contacted with bubbles generated by dissolving air into the filtrate that is recycled to the dewatering unit at a 10% rate. The algal biomass creates a floe that is 4-5 wt% solids. Essentially algae-free filtrate is recycled to the reactor, allowing unreacted nutrients to be returned to the system. Recycling this stream reduces total water and nutrient requirements. Optionally, a portion or all of the dewatering feed stream can be contacted with flue gas in the quench zone prior to dewatering. For flue gases containing acid gases such as SO₂, NO_x, and HCl, absorption of the acid gases reduces pH from approximately 7-9 range to a more preferred range of 6.5-7.5. In this pH range, the quantity of aluminum sulfate required to dewater the algae is reduced. Example 5

[0127] This example illustrates the use of tangential flow filtration for dewatering the algae. The algae of Example 5 is run in a system using tangential flow filtration instead of dissolved air floatation. The filtration process uses a sterile-grade membrane and operates at low trans- membrane pressure and low shear rates to increase the algae concentration by a factor of 10- 200. Cellular debris and bacterial contaminants are concentrated with the algae-rich stream. The sterilized permeate stream is recycled to the reactor, conserving water and nutrients while reducing risk due to recycle of deleterious species such as bacteria and cell lysates.

Example 6 [0128] This example illustrates the use of different operating conditions upstream and downstream of the algae recycle point(s) to affect changes in the algae growth rates and algae composition. The bioreactor of Example 1 is operated with the algae recycle zone located 2/3 down the length of the reactor channel. Recycled media is used to add nitrate such that the concentration in the feed end is 0.85 g/L and the concentration in the recycle stream is 0.3 g/L. In the zone downstream of the algae recycle stream split, the recycled media contains nutrients such as phosphate, but no nitrate. The algae in the first zone experience growth rates of 1.4 g/m -hr, and lipid content is approximately 14 wt%. The algae in the second, nitrate-poor region demonstrate lower growth rates, but have lipids content that exceeds 14 wt%.

[0129] While several embodiments of the invention have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and structures for performing the functions and/or obtaining the results or advantages described herein, and each of such variations, modifications and improvements is deemed to be within the scope of the present invention. More generally, those skilled in the art would readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that actual parameters, dimensions, materials, and configurations will depend upon specific applications for which the teachings of the present invention are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described. The present invention is directed to each individual feature, system, material and/or method described herein. In addition, any combination of two or more such features, systems, materials and/or methods, provided that such features, systems, materials and/or methods are not mutually inconsistent, is included within the scope of the present invention. In the claims (as well as in the specification above), all transitional phrases or phrases of inclusion, such as "comprising," "including," "carrying," "having," "containing," "composed of," "made of," "formed of," "involving" and the like shall be interpreted to be open-ended, i.e., to mean "including but not limited to" and, therefore, encompassing the items listed thereafter and equivalents thereof as well as additional items. Only the transitional phrases or phrases of inclusion "consisting of and "consisting essentially of are to be interpreted as closed or semi-closed phrases, respectively. The indefinite articles "a" and "an," as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean "at least one."

[0130] As used herein in the specification and in the claims, the phrase "at least one," in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements that the phrase "at least one" refers to, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, "at least one of A and B" (or, equivalently, "at least one of A or B," or, equivalently "at least one of A and/or B") can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc. In cases where the present specification and a document incorporated by reference and/or referred to herein include conflicting disclosure, and/or inconsistent use of terminology, and/or the incorporated/referenced documents use or define terms differently than they are used or defined in the present specification, the present specification shall control. [0131] What is claimed is:

Claims

1. A photobioreactor system comprising: an enclosure comprising a substantially transparent top portion and means for admitting a gas having a concentration of CO₂ elevated above ambient; and within the enclosure, at least one solid cultivation support, the support comprising phototrophic organisms growing on at least one surface thereof, the at least one surface having a growing area of at least 1 m².

2. The system of claim 1 wherein the growing area is at least 10 m².

3. The system of claim 1 wherein the growing area is at least 13 m².

4. The system of claim 1 wherein the system has a total growing area of the at least 100 m².

5. The system of claim 4 wherein the system has a growth footprint area of at least 7.4 m².

6. The system of claim 1 wherein the system has a total growing area of at least 1000 m².

7. The system of claim 6 wherein the system has a growth footprint area of at least 73.5 m .

8. The system of claim 1 wherein the system has a total growing area of at least 15,000 m².

9. The system of claim 8 wherein the system has a growth footprint area of at least 1103 m².

10. The system of claim 1 wherein the solid supports are in sheet form, arranged in parallel in a vertical orientation, and separated by a spacing of no more than 20 cm.

11. The system of claim 1 wherein the solid supports are in sheet form, arranged in parallel in a vertical orientation, and separated by a spacing of no more than 15 cm.

12. The system of claim 1 wherein the solid supports are in sheet form, arranged in parallel in a vertical orientation, and separated by a spacing of no more than 10 cm.

13. The system of claim 1 wherein the at least one solid cultivation support has a growth footprint area, the ratio of the total growing area of the at least one solid support to the growth footprint area is at least 12.2.

14. The system of claim 1 wherein the total growing area corresponds to a vertical growing volume, the system having a vertical growing volume of at least 180 m³.

15. The system of claim 14 wherein the total growing volume is equal to the vertical growing volume.

16. The system of claim 14 further comprising a liquid growing region within the enclosure and having a liquid growing volume, the system having a total growing volume equal to the sum of the vertical growing volume and the liquid growing volume.

17. The system of claim 1 further comprising a channel or basin under the at least one solid cultivation support for receiving liquid medium therefrom, and means for recirculating at least a portion of the liquid medium back onto the at least one solid cultivation support.

18. The system of claim 17 wherein the system has a total system liquid volume, a ratio of the total system liquid volume to the total growing area being no greater than 2.2 Um².

19. The system of claim 17 wherein the system has a total system liquid volume, a ratio of the total system liquid volume to a growth footprint area of at least one solid cultivation support being no greater than 26.5 L/m².

20. The system of claim 17 wherein the total system liquid volume per gram of biomass is no greater than 0.144 L/g.

21. The system of claim 17 wherein the total system liquid volume per gram of biomass is no greater than 0.087 L/g.

22. The system of claim 17 wherein the total system liquid volume per gram of biomass is no greater than 0.072 L/g.

23. The system of claim 17 wherein the total system liquid volume per gram of biomass is no greater than 0.054 L/g.

24. The system of claim 1 wherein the solid cultivation support has a height and a width, the width exceeding the height by at least a factor of 10.

25. The system of claim 1 wherein the width exceeds the height by at least a factor of 100.

26. The system of claim 1 wherein the phototrophic organisms comprise eukaryotic algae or cyanobacteria.

27. The system of claim 1 further comprising a liquid- medium delivery system configured to apply a liquid mist, spray or stream to at least a top portion of the at least one solid cultivation support.

28. A photobioreactor system comprising: an enclosure comprising a substantially transparent top portion and means for admitting a gas having an elevated concentration of CO₂, wherein a humidity level within the enclosure is at least 80%; within the enclosure, at least one longitudinally extending photobioreactor unit comprising (i) at least one solid cultivation support oriented substantially vertically, the support comprising phototrophic organisms growing on at least one surface thereof, and (ii) a channel or basin under the at least one solid cultivation support for receiving liquid medium therefrom; and means for recirculating at least a portion of the liquid medium back onto the at least one solid cultivation support.

29. The system of claim 28 wherein a humidity level within the enclosure is at least 95%.

30. The system of claim 28 comprising a plurality solid cultivation supports in sheet form aligned in a longitudinal row.

31. A photobioreactor system comprising: at least one solid cultivation matrix positioned within a gas headspace that comprises phototrophic organisms growing on at least one surface thereof, wherein the cultivation matrix surface supports an average footprint areal productivity of biomass of at least 10 g/m²-day.

32. The photobioreactor system of claim 31, wherein the average footprint areal productivity of biomass is at least 40 g/m²-day.

33. The photobioreactor system of claim 31, wherein the average footprint areal productivity of biomass is at least 80 g/m²-day.

34. The photobioreactor system of claim 31 wherein the cultivation matrix surface supports an average footprint areal productivity of biomass of at least 100 g/m -day .

35. The photobioreactor system of claim 31 wherein the cultivation matrix surface supports an average footprint areal productivity of biomass of at least 150 g/m -day.

36. The photobioreactor system of claim 31 wherein the cultivation matrix surface supports an average footprint areal productivity of biomass of between 10 and 170 g/m²-day.

37. A photobioreactor system comprising: an enclosure comprising a substantially transparent top portion and means for admitting a gas having an elevated concentration of CO₂; within the enclosure, at least one longitudinally extending photobioreactor unit comprising (i) at least one solid cultivation support oriented substantially vertically, the support comprising phototrophic organisms growing on at least one surface thereof, and (ii) a channel or basin under the at least one solid cultivation support for receiving liquid effluent therefrom; means for sensing a temperature of the surface of the at least one solid cultivation support; and an automatic liquid-medium delivery system configured to apply a nutrient-containing liquid medium to at least a portion of the surface at a target temperature when the sensed temperature deviates from the target temperature by a predetermined threshold.

38. A photobioreactor system comprising: an enclosure comprising a substantially transparent top portion and means for admitting a gas having an elevated concentration of CO₂; within the enclosure, at least one longitudinally extending photobioreactor unit comprising (i) at least one solid cultivation support oriented substantially vertically, the support comprising phototrophic organisms growing on at least one surface thereof, and (ii) a channel or basin under the at least one solid cultivation support for receiving liquid effluent therefrom; and a biomass harvester configured to move with respect to the cultivation support while dislodging phototrophic organisms therefrom.

39. The system of claim 38 further comprising means for automatically actuating the biomass harvester at predetermined intervals.

40. The system of claim 39 further comprising at least one sensor for determining a density of the biomass on the at least one solid cultivation support, the biomass harvester being activated at a target biomass density.

41. The system of claim 40 wherein the target biomass density is at least 2 g/L without flocculation.

42. The system of claim 40 wherein the target biomass density is at least 8 g/L without flocculation.

43. The system of claim 38 wherein the biomass harvester comprises a plurality of nozzles for directing a stream of liquid onto the at least solid cultivation support surface, the stream being of sufficient velocity to dislodge and remove photrophic organisms therefrom.

44. The system of claim 38 wherein the biomass harvester comprises an air knife.

45. The system of claim 38 wherein the biomass harvester comprises a trolley and headers for directing a stream of liquid onto the at least one solid cultivation support surface, the stream being of sufficient velocity to dislodge and remove photrophic organisms therefrom.

46. The system of claim 45 wherein the trolley comprises a pump that pulls liquid from the basin to be used in a stream of liquid used to dislodge and remove photrophic organisms therefrom.