Enclosed outdoor photobioreactors: light regime,photosynthetic efficiency,scale-up,and future prospects

Enclosed outdoor photobioreactors need to be developed and designed for large-scale production of phototrophic microorganisms. Both light regime and photosynthetic efficiency were analyzed in characteristic examples of state-of-the-art pilot-scale photobioreactors. In this study it is shown that productivity of photobioreactors is determined by the light regime inside the bioreactors. In addition to light regime, oxygen accumulation and shear stress limit productivity in certain designs. In short light-path systems, high efficiencies, 10% to 20% based on photosynthetic active radiation (PAR 400 to 700 nm), can be reached at high biomass concentrations (>5 kg [dry weight] m(-3)). It is demonstrated, however, that these and other photobioreactor designs are poorly scalable (maximal unit size 0.1 to 10 m(3)), and/or not applicable for cultivation of monocultures. This is why a new photobioreactor design is proposed in which light capture is physically separated from photoautotrophic cultivation. This system can possibly be scaled to larger unit sizes, 10 to >100 m(3), and the reactor liquid as a whole is mixed and aerated. It is deduced that high photosynthetic efficiencies, 15% on a PAR-basis, can be achieved. Future designs from optical engineers should be used to collect, concentrate, and transport sunlight, followed by redistribution in a large-scale photobioreactor.     

Photoautotrophic Production of Lipids by Some <Emphasis Type="Italic">Chlorella</Emphasis> Strains

The microalgae Chlorella protothecoides UTEX 25, Chlorella sp. TISTR 8991, and Chlorella sp. TISTR 8990 were compared for use in the production of biomass and lipids under photoautotrophic conditions. Chlorella sp. TISTR 8990 was shown to be potentially suitable for lipid production at 30°C in a culture medium that contained only inorganic salts. For Chlorella sp. TISTR 8990 in optimal conditions in a stirred tank photobioreactor, the lipid productivity was 2.3 mg L⁻¹ h⁻¹ and after 14 days the biomass contained more than 30% lipids by dry weight. To attain this, the nitrogen was provided as KNO₃ at an initial concentration of 2.05 g L⁻¹ and chelated ferric iron was added at a concentration of 1.2 × 10⁻⁵ mol L⁻¹ on the ninth day. Under the same conditions in culture tubes (36 mm outer diameter), the biomass productivity was 2.8-fold greater than in the photobioreactor (0.125 m in diameter), but the lipid productivity was only 1.2-fold higher. Thus, the average low-light level in the photobioreactor actually increased the biomass specific lipid production compared to the culture tubes. A light-limited growth model closely agreed with the experimental profiles of biomass production, nitrogen consumption, and lipid production in the photobioreactor.     

Short-term evaluation of the photosynthetic activity of an alkaliphilic microalgae consortium in a novel tubular closed photobioreactor

A novel lab-scale tubular closed photobioreactor was developed and used for the assessment of the photosynthetic activity of an alkaliphilic microalgae mixed consortium under non-substrate limitation (i.e., bicarbonate excess), controlled irradiance, and mixing conditions. Two prominent haloalkaliphilic strains were identified as members of the consortium: Halospirulina sp. and Picochlorum sp. The photobioreactor (vol?=?0.5 L) consists of two interconnected U-shaped borosilicate glass tubes (internal diameter 2 cm) reaching a surface/volume ratio of 200 m² m^?3. This configuration specifically addressed the issue of the homogeneous light distribution among the microalgae suspended cells cultured by using fixed equidistant cool white light LEDs nearby the surface of the glass tubes. A soft homogeneous pneumatic mixing (i.e., airlift) was implemented in the culture fostering Reynolds numbers around 3000. The photosynthetic activity of the microalgae consortium was evaluated during different short-term kinetic assays by fitting the dynamics of the dissolved oxygen concentration to an oxygenic kinetic model. The photobioreactor operated in a closed loop allowed to control the produced oxygen by the extraction of the cumulated gas in the headspace. The use of this novel photobioreactor allowed the photosynthetic activity of microalgae suspended cells to be assessed, where the dissolved oxygen concentration and irradiance were the main parameters affecting the oxygenic rates under alkaline pH.     

Microaerobic hydrogen production by photosynthetic bacteria in a double-phase photobioreactor

The rate of hydrogen production by the marine nonsulfur photosynthetic bacterium, Rhodovulum sp., increased with increasing light intensity. A light intensity of 1800 W/m(2) hydrogen production rate was achieved at the rate of 9.4 micromol/mg dry weight/h. The hydrogen production of this strain was enhanced by the addition of a small amount of oxygen (12 micromol O(2)/reactor). Intracellular ATP content was most efficiently accumulated under microaerobic, dark conditions. Hydrogen production rate by Rhodovulum sp. was investigated using a double-phase photobioreactor consisting of light and dark compartments. This rate was compared with data obtained using a conventional photobioreactor. Rhodovulum sp. produced hydrogen at a rate of 0.38+/-0.03 micromol/mg dry weight/h under microaerobic conditions using the double-phase photobioreactor. The hydrogen production rate was four times greater under microaerobic conditions, as compared with anaerobic conditions using either type of photobioreactor. Hydrogen production using a double-phase photobioreactor was demonstrated continuously at the same rate for 150 h.     

A parallel bubble column system for the cultivation of phototrophic microorganisms

An incubator with up to 16 parallel bubble columns was equipped with artificial light sources assuring a light supply with a homogenous light spectrum directly above the bioreactors. Cylindrical light reflecting tubes were positioned around every single bubble column to avoid light scattering effects and to redirect the light from the top onto the cylindrical outer glass surface of each bubble column. The light reflecting tubes were equipped with light intensity filters to control the total light intensity for every single photo-bioreactor. Parallel cultivations of the unicellular obligate phototrophic cyanobacterium, Synechococcus PCC7942, were studied under different constant light intensities ranging from 20 to 102 microE m(-2)s(-1) at a constant humidified air flow rate supplemented with CO(2).     

A photobioreactor with pH control: demonstration by growth of the marine diatom Skeletonema costatum

A photobioreactor with pH control for cultivation of algae isdescribed. The magnetically stirred culture flask is connectedto separate reservoirs for medium and for acid and base (dilutedHCl and NaOH, respectively). A pH electrode is inserted intothe culture flask and coupled to a pH controller, which activatesacid and base titration at set points of pH 8.1 and 7.8, respectively.Illumination is provided by light tubes with a diel light :dark cycle. The use of the photobioreactor in batch mode isillustrated by showing pH curves in different growth phasesof the marine diatom Skeletonema costatum. The photobioreactorcan also be run as a semicontinuous or continuous reactor withslight modifications.     

Laboratory scale photobioreactor

Summary A photobioreactor using three concentric glass cylinders, with a light source mounted on the axis within them is described. The space between two innermost cylinders is used as a waterjacket, while the culture of phototrophic microorganisms is in the chamber between the outer cylinder and the middle one. This chamber is also equipped with a stirrer. Rhodobacter capsulatus has been grown in the device at biomass concentration up to 550 mg/l without light limitation.     

Technique for the Determination of the Rate of Ethylene Production by Pseudomonas solanacearum

A tube culture system was designed for measurement of ethylene evolved by the phytopathogenic bacterium, Pseudomonas solanacearum. The system consisted of 10 glass tubes joined together in series and coated on the inside surface with a dextrose-peptone-casamino acids agar medium. The system provided a large surface for bacterial growth in relation to the volume of air. The system was seeded with a bacterial suspension (7 × 10⁸ cells/ml) drawn through all the tubes by vacuum applied at one end and was then placed in a water bath at 30 C. Air was pumped through the system at 3 ml/min; the outlet was connected directly to the inlet port of a gas sampling loop and ethylene in the sample was determined by gas chromatography.     

Design of static mixers for inclined tubular photobioreactors

Static mixers, which improve gas-liquid mass transfer inside tubular photobioreactors and move the cells between the upper and lower parts of the tubes were designed. Each static mixer was equipped with an opening (v-cut slit) at the top (for gas dispersion) and an orifice at the lower part (for liquid circulation). When the static mixers were installed in the riser tube of an inclined tubular photobioreactor, vertical movement of the liquid was induced so that cells were moved between the surface and bottom part of the photobioreactor. The mass transfer rates in the tubular photobioreactor without static mixers decreased sharply when they were scaled up by increasing the tube diameters. However, by installation of static mixers, the mass transfer rates in 12.5-cm diameter tubes were almost as high as those of 3.8-cm tubes without static mixers. The effectiveness of the static mixers in improving the mass transfer characteristics of the tubular photobioreactors was higher in large than small diameter tubes. The ratio of the diameters of the static mixers to the diameter of the tube, the areas of the v-cut slits and the orifice were the important design parameters that affected the mass transfer characteristics. The gas hold up and and k_La were higher when the areas of the slits (v-cut) and the orifice were reduced.     

Effect of oxygen concentration on the growth of Nannochloropsis sp. at low light intensity

In large-scale microalgal production in tubular photobioreactors, the build-up of O(2) along the tubes is one of the major bottlenecks to obtain high productivities. Oxygen inhibits the growth, since it competes with carbon dioxide for the Rubisco enzyme involved in the CO(2) fixation to generate biomass. The effect of oxygen on growth of Nannochloropsis sp. was experimentally determined in a fully controlled flat-panel photobioreactor operated in turbidostat mode using an incident photon flux density of 100?μmol photons m(-2) s(-1) and with only the oxygen concentration as variable parameter. The dissolved oxygen concentration was varied from 20 to 250% air saturation. Results showed that there was no clear effect of oxygen concentration on specific growth rate (mean of 0.48?±?0.40?day(-1)) upon increasing the oxygen concentration from 20% to 75% air saturation. Upon further increasing the oxygen concentration, however, a linear decrease in specific growth rate was observed, ranging from 0.48?±?0.40?day(-1) at a dissolved oxygen concentration of 75% air saturation to 0.18?±?0.01?day(-1) at 250% air saturation. In vitro data on isolated Rubisco were used to predict the quantum yield at different oxygen concentrations in the medium. The predicted decrease in quantum yield matches well with the observed decrease that was measured in vivo. These results indicate that the effect of oxygen on growth of Nannochloropsis sp. at low light intensity is only due to competitive inhibition of the Rubisco enzyme. At these sub-saturating light conditions, the presence of high concentrations of oxygen in the medium induced slightly higher carotenoid content, but the increased levels of this protective antioxidant did not diminish the growth-inhibiting effects of oxygen on the Rubisco.     

Stabilization and scale-up of photosynthesis-driven ω-hydroxylation of nonanoic acid methyl ester by two-liquid phase whole-cell biocatalysis

Photoautotrophic organisms are promising hosts for biocatalytic oxyfunctionalizations because they supply reduction equivalents as well as O₂ via photosynthetic water oxidation. Thus far, research on photosynthesis-driven bioprocesses mainly focuses on strain development and the proof of principle in small-scale biocatalytic reaction setups. This study investigates the long-term applicability of the previously developed cyanobacterial strain Synechocystis sp. PCC 6803_BGT harboring the alkane monooxygenase system AlkBGT catalyzing terminal alkyl group oxyfunctionalization. For the regiospecific ω-hydroxylation of nonanoic acid methyl ester (NAME), this biocatalyst showed light intensity-independent hydroxylation activity and substantial hydrolysis of NAME to nonanoic acid. Substrate mass transfer limitation, substrate hydrolysis, as well as reactant toxicity were overcome via in situ substrate supply by means of a two-liquid phase system. The application of diisononyl phthalate as organic carrier solvent enabled 1.7-fold increased initial specific activities (5.6 ± 0.1 U/g_CDW) and 7.6-fold increased specific yields on biomass (3.8 ± 0.1 mmol_H-NAME/g_CDW) as compared with single aqueous phase biotransformations. Finally, the whole-cell biotransformation system was successfully scaled from glass tubes to a stirred-tank photobioreactor. This is the first study reporting the application of the two-liquid phase concept for efficient phototrophic whole-cell biocatalysis.     

A two-plane tubular photobioreactor for outdoor culture of Spirulina

A photobioreactor in the form of a 245-m-long loop made of plexiglass tubes having an inner diameter of 2.6 cm was designed and constructed for outdoor culture of Spirulina. The loop was arranged in two planes, with 15 8-m-long tubes in each plane. In the upper plane, the tubes were placed in the vacant space between the ones of the lower plane. The culture recycle was performed either with two airlifts, one per plane, or with two peristaltic pumps. The power required for water recycle in the tubular photobioreactor, with a Reynolds number of 4000, was 3.93 x 10(-2) W m(-2). The photobioreactor contained 145 L of culture and covered an overall area of 7.8 m(2). The photobioreactor operation was computer controlled. Viscosity measurements performed on Spirulina cultures having different biomass concentrations showed non-Newtonian behavior displaying decreasing viscosity with an increasing shear rate. The performance of the two-plane photobioreactor was tested under the climatic conditions of central Italy (latitude 43.8 degrees N, longitude 11.3 degrees E). A biomass concentration of 3.5 g L(-1) was found to be adequate for outdoor culture of Spirulina. With a biomass concentration of 6.3 g L(-1), the biomass output rate significantly decreased. The net biomass output rate reached a mean value of 27.8 g m(-2) d(-1) in July; this corresponded to a net photosynthetic efficiency of 6.6% (based on visible irradiance). (c) 1993 John Wiley & Sons, Inc.     

Photobioreactor design: Mixing, carbon utilization, and oxygen accumulation

Photobioreactor design and operation are discussed in terms of mixing, carbon utilization, and the accumulation of photosynthetically produced oxygen. The open raceway pond is the primary type of reactor considered; however small diameter (1-5 cm) horizontal glass tubular reactors are compared to ponds in several respects. These are representative of the diversity in photobioreactor design: low capital cost, open systems and high capital cost, closed systems. Two 100-m(2) raceways were operated to provide input data and to validate analytical results. With a planktonic Chlorella sp., no significant difference in productivity was noted between one pond mixed at 30 cm/s and another mixed from 1 to 30 cm/s. Thus, power consumption or CO(2) outgassing limits maximal mixing velocities. Mixing power inputs measured in 100-m(2) ponds agreed fairly well with those calculated by the use of Manning's equation. A typically configured tubular reactor flowing full (1 cm diameter, 30 cm/s) consumes 10 times as much energy as a typical pond (20 cm deep flowing at 20 cm/s). Tubular reactors that flow only partially full would be limited by large hydraulic head losses to very short sections (as little as 2 m length at 30 cm/s flow) or very low flow velocities. Open ponds have greater CO(2) storage capacity than tubular reactors because of their greater culture volume per square meter (100-300 L/m(2) vs. 8-40 L/m(2) for 1-5-cm tubes). However, after recarbonation, open ponds tend to desorb CO(2) to the atmosphere. Thus ponds must be operated at higher pH and lower alkalinity than would be possible with tubular reactors if cost of carbon is a constraint. The mass transfer coefficient, K(L), for CO(2) release through the surface of a 100-m(2) pond was determined to be 0.10 m/h. Oxygen buildup would be a serious problem with any enclosed reactor, especially small-diameter tubes. At maximal rates of photosynthesis, a 1-cm tubular reactor would accumulate 8-10 mg O(2)/L/min. This may result in concentrations of oxygen reaching 100 mg/L, even with very frequent gas exchange. In an open pond, dissolved oxygen rises much more slowly as a consequence of the much greater volume per unit surface area and the outgassing of oxygen to the atmosphere. The maximum concentration of dissolved oxygen is typically 25-40 mg/L. The major advantage of enclosed reactors lies in the potential for aseptic operation, a product value which justifies the expense. For most products of algal mass cultivation, open ponds are the only feasible photobioreactor design capable of meeting the economic and operating requirements of such systems, provided desirable species can be maintained.     

Photosynthetic performance of phototrophic biofilms in extreme acidic environments

Photosynthesis versus irradiance curves and their associated photosynthetic parameters from different phototrophic biofilms isolated from an extreme acidic environment (Río Tinto, SW, Spain) were studied in order to relate them to their species composition and the physicochemical characteristics of their respective sampling locations. The results indicated that the biofilms are low light acclimated showing a photoinhibition model; only floating communities of filamentous algae showed a light saturation model. Thus, all the biofilms analysed showed photoinhibition over 60 μmol photon m(-2) s(-1) except in the case of Zygnemopsis sp. sample, which showed a light-saturated photosynthesis model under irradiations higher that 200 μmol photon m(-2) s(-1). The highest values of compensation light intensity (I(c)) were showed also by Zygnemosis sp. biofilm (c. 40 μmol photon m(-2) s(-1)), followed by Euglena mutabilis and Chlorella sp. samples (c. 20 μmol photon m(-2) s(-1)). The diatom sample showed the lowest I(c) values (c. 5 μmol photon m(-2) s(-1)). As far as we know this is the first attempt to determine the photosynthetic activity of low pH and heavy metal tolerant phototrophic biofilms, which may give light in the understanding of the ecological importance of these biofilms for the maintenance of the primary production of these extreme and unique ecosystems.     

Reduction of CO2 by a high-density culture of Chlorella sp. in a semicontinuous photobioreactor

The microalga incorporated photobioreactor is a highly efficient biological system for converting CO2 into biomass. Using microalgal photobioreactor as CO2 mitigation system is a practical approach for elimination of waste gas from the CO2 emission. In this study, the marine microalga Chlorella sp. was cultured in a photobioreactor to assess biomass, lipid productivity and CO2 reduction. We also determined the effects of cell density and CO2 concentration on the growth of Chlorella sp. During an 8-day interval cultures in the semicontinuous cultivation, the specific growth rate and biomass of Chlorella sp. cultures in the conditions aerated 2-15% CO2 were 0.58-0.66 d(-1) and 0.76-0.87 gL(-1), respectively. At CO2 concentrations of 2%, 5%, 10% and 15%, the rate of CO2 reduction was 0.261, 0.316, 0.466 and 0.573 gh(-1), and efficiency of CO2 removal was 58%, 27%, 20% and 16%, respectively. The efficiency of CO2 removal was similar in the single photobioreactor and in the six-parallel photobioreactor. However, CO2 reduction, production of biomass, and production of lipid were six times greater in the six-parallel photobioreactor than those in the single photobioreactor. In conclusion, inhibition of microalgal growth cultured in the system with high CO2 (10-15%) aeration could be overcome via a high-density culture of microalgal inoculum that was adapted to 2% CO2. Moreover, biological reduction of CO2 in the established system could be parallely increased using the photobioreactor consisting of multiple units.     

Development of laboratory-scale photobioreactor for water purification by use of a biofilter composed of the aerial microalga Trentepohlia aurea (Chlorophyta)

Biofilms formed by the green alga Trentepohlia aurea could be a useful tool in the removal of nitrate and phosphate from water. When a prepared biofilter was dampened with medium and incubated under low light intensity (10 μmol photons m⁻² s⁻¹) between 5 and 50 μmol photons m⁻² s⁻¹, the efficiency of removal of inorganic compounds from water was higher without the decomposition of chlorophylls in the cells. Algal cells immobilized on a glass fiber filter could be kept for 12 weeks under dark conditions at 4°C in the refrigerator. We tried to construct a laboratory-scale photobioreactor for the removal of inorganic nitrogen and phosphate from water by the biofilm. In this study, the synthetic wastewater was prepared by diluting 18-fold Bold’s basal medium with deionized water. The photobioreactor could efficiently remove nitrate and phosphate from the synthetic wastewater under continuous illumination. The removal ability of nitrate and phosphate per sheet of the biofilter in the photobioreactor exhibited about an 8- and 16-fold increase, respectively, in 3 days, compared with the bath experimental results. This study showed that the cycling of wastewater in the reactor by the pump led to a significant improvement in the efficiency of the inorganic ion uptake from water.     

Bioconversion of synthesis gas to hydrogen using a light-dependent photosynthetic bacterium,Rhodospirillum rubrum

Biological hydrogen production from synthesis gas was carried out in batch culture. The phototrophic anaerobic bacterium, Rhodospirillum rubrum was used to oxidize CO and water to CO₂ and hydrogen. The bacteria were grown under anaerobic conditions in liquid medium; also acetate was used as carbon source in presence of synthesis gas. Biological hydrogen production was catalysed by R. rubrum via the water–gas shift reaction. A light-dependent cell growth modelled with a desired rate of hydrogen production and CO uptake was determined. The effect of light intensity on microbial cell growth was also studied at 500, 1,000 and 1,500 m.cd. A complete conversion of CO to hydrogen and maximum light efficiency were obtained with an acetate concentration of 1 g/l and light intensity of 500 m.cd. Utilization of the carbon monoxide from the gas phase was often considered as a mass transfer limited process, which needed to diffuse through the gas–liquid interface and then further diffuse into liquid medium prior to reaction. The results from this study showed that maximum cell propagation and hydrogen production were achieved with a limited light intensity of 1,000 m.cd. It was also found that high-light intensity may interfere with cell metabolism. In low-light intensity and substrate concentration, no inhibition was observed, however at extreme conditions, non-competitive inhibition was identified. The adverse effect of high-light intensity was shown at 5,000 m.cd, where the CO conversion drastically dropped to as low as 21%. Maximum CO conversion of 98% and maximum yield of 86% with an acetate concentration of 1.5 g/l and a light intensity of 1,000 m.cd were achieved.     

Simulated cell trajectories in a stratified gas–liquid flow tubular photobioreactor

The fluid dynamic environment within a photobioreactor is critical for performance as it controls mass transfer of photosynthetic gases (CO₂ and O₂) and the mixing environment of the algal culture. At a cellular level, light fluctuation will occur when cells move between the “light”, well-illuminated volume of the culture near the light source and the “dark”, self-shaded zone of the culture. Controlled light/dark frequency may increase the light to biomass yield and prevent photoinhibition. Knowledge of cell trajectories within the reactor is therefore important to optimize culture performance. This study examines the cell trajectories and light/dark frequencies in a stratified gas–liquid flow tubular photobioreactor. Commercially available computational fluid dynamics software, ANSYS Fluent, was used to investigate cell trajectories within the half-full solar receivers at different liquid velocities and reactor tube diameters. In the standard configuration 96-mm solar receiver tube, the light/dark cycle frequencies ranged from 0.104 to 0.612?Hz over the liquid velocity range of 0.1 to 1?m s^?1. In comparison, the smaller diameter 48- and 24-mm tubes exhibit higher light/dark frequencies, 0.219 to 1.30?Hz and 0.486 to 2.67?Hz, respectively.     

Design and construction of a photobioreactor for hydrogen production,including status in the field

Several species of microalgae and phototrophic bacteria are able to produce hydrogen under certain conditions. A range of different photobioreactor systems have been used by different research groups for lab-scale hydrogen production experiments, and some few attempts have been made to upscale the hydrogen production process. Even though a photobioreactor system for hydrogen production does require special construction properties (e.g., hydrogen tight, mixing by other means than bubbling with air), only very few attempts have been made to design photobioreactors specifically for the purpose of hydrogen production. We have constructed a flat panel photobioreactor system that can be used in two modes: either for the cultivation of phototrophic microorganisms (upright and bubbling) or for the production of hydrogen or other anaerobic products (mixing by “rocking motion”). Special emphasis has been taken to avoid any hydrogen leakages, both by means of constructional and material choices. The flat plate photobioreactor system is controlled by a custom-built control system that can log and control temperature, pH, and optical density and additionally log the amount of produced gas and dissolved oxygen concentration. This paper summarizes the status in the field of photobioreactors for hydrogen production and describes in detail the design and construction of a purpose-built flat panel photobioreactor system, optimized for hydrogen production in terms of structural functionality, durability, performance, and selection of materials. The motivations for the choices made during the design process and advantages/disadvantages of previous designs are discussed.     

Microalgal biomass production and on-site bioremediation of carbon dioxide, nitrogen oxide and sulfur dioxide from flue gas using Chlorella sp. cultures

The growth and on-site bioremediation potential of an isolated thermal- and CO?-tolerant mutant strain, Chlorella sp. MTF-7, were investigated. The Chlorella sp. MTF-7 cultures were directly aerated with the flue gas generated from coke oven of a steel plant. The biomass concentration, growth rate and lipid content of Chlorella sp. MTF-7 cultured in an outdoor 50-L photobioreactor for 6 days was 2.87 g L?1 (with an initial culture biomass concentration of 0.75 g L?1), 0.52 g L?1 d?1 and 25.2%, respectively. By the operation with intermittent flue gas aeration in a double-set photobioreactor system, average efficiency of CO? removal from the flue gas could reach to 60%, and NO and SO? removal efficiency was maintained at approximately 70% and 50%, respectively. Our results demonstrate that flue gas from coke oven could be directly introduced into Chlorella sp. MTF-7 cultures to potentially produce algal biomass and efficiently capture CO?, NO and SO? from flue gas.