J. M. Fernández Sevilla

Airlift-driven external-loop tubular photobioreactors for outdoor production of microalgae: assessment of design and performance

A methodology is presented for designing photobioreactors with tubular loop solar receivers in which the fluid is circulated by an airlift device. The design method effectively combines the relevant aspects of external irradiance-dependent cell growth, oxygen accumulation in the solar loop, oxygen removal in the airlift device, and hydrodynamics of the airlift system that determine the flow velocity through the solar receiver. The design approach developed was used to model and build a outdoor photobioreactor. A compact degasser in the airlift section eliminated dead zones and dark zones, while achieving complete separation of gas and liquid. The measured gas–liquid hydrodynamics, mass transfer, and culture productivity were consistent with the model predictions. The reactor was tested with continuous culture of the microalga Phaeodactylum tricornutum at various liquid velocities through the tubular solar receiver. A biomass productivity of (or ) was obtained at a dilution rate of . Solar receiver linear liquid velocities of 0.50 and gave similar biomass productivities, but the culture collapsed at lower velocities. An adverse effect of high dissolved oxygen concentration on productivity was observed. Oxygen accumulation could be reduced by increasing the liquid velocity and this enhanced the biomass yield.

A model for light distribution and average solar irradiance inside outdoor tubular photobioreactors for the microalgal mass culture

A mathematical model to estimate the solar irradiance profile and average light intensity inside a tubular photobioreactor under outdoor conditions is proposed, requiring only geographic, geometric, and solar position parameters. First, the length of the path into the culture traveled by any direct or disperse ray of light was calculated as the function of three variables: day of year, solar hour, and geographic latitude. Then, the phenomenon of light attenuation by biomass was studied considering Lambert-Beers law (only considering absorption) and the monodimensional model of Cornet et al. (1900) (considering absorption and scattering phenomena). Due to the existence of differential wavelength absorption, none of the literature models are useful for explaining light attenuation by the biomass. Therefore, an empirical hyperbolic expression is proposed. The equations to calculate light path length were substituted in the proposed hyperbolic expression, reproducing light intensity data obtained in the center of the loop tubes. The proposed model was also likely to estimate the irradiance accurately at any point inside the culture. Calculation of the local intensity was thus extended to the full culture volume in order to obtain the average irradiance, showing how the higher biomass productivities in a Phaeodactylum tricornutum UTEX 640 outdoor chemostat culture could be maintained by delaying light limitation. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 55: 701-714, 1997.

Modeling of biomass productivity in tubular photobioreactors for microalgal cultures: effects of dilution rate, tube diameter, and solar irradiance

A macromodel is developed for estimating the year-long biomass productivity of outdoor cultures of microalga in tubular photobioreactors. The model evaluates the solar irradiance on the culture surface as a function of day of the year and the geographic location. In a second step, the geometry of the system is taken into account in estimating the average irradiance to which the cells are exposed. Finally, the growth rate is estimated as a function of irradiance, taking into account photoinhibition and photolimitation. The model interconnects solar irradiance (an environmental variable), tube diameter (a design variable), and dilution rate (an operating variable). Continuous cultures in two different tubular photobioreactors were analyzed using the macromodel. The biomass productivity ranged from 0.50 to 2.04 g L-1 d-1, and from 1.08 to 2. 76 g L-1 d-1, for the larger and the smaller tube diameter photobioreactors, respectively. The quantum yield ranged from 1.1 to 2.2 g E-1; the higher the incident solar radiation, the lower the quantum yield. Simultaneous photolimitation and photoinhibition of outdoor cultures was observed. The model reproduced the experimental results with less than 20% error. If photoinhibition was neglected, and a growth model that considered only photolimitation was used to fit the data, the error increased to 45%, thus reflecting the inadequacy of previous outdoor growth models that disregard photoinhibition. Copyright 1998 John Wiley & Sons, Inc.

Production of lovastatin by Aspergillus terreus: effects of the C:N ratio and the principal nutrients on growth and metabolite production

Production of lovastatin and microbial biomass by Aspergillus terreus ATCC 20542 were influenced by the type of the carbon source (lactose, glycerol, and fructose) and the nitrogen source (yeast extract, corn steep liquor, and soybean meal) used and the C:N mass ratio in the medium. Use of a slowly metabolized carbon source (lactose) in combination with either soybean meal or yeast extract under N-limited conditions gave the highest titers and specific productivity ( ∼0.1 mg g −1 h −1 ) of lovastatin. The maximum value of the lovastatin yield coefficient on biomass was ∼30 mg g −1 using the lactose/soybean meal and lactose/yeast extract media. The optimal initial C:N mass ratio for attaining high productivity of lovastatin was ∼40. The behavior of the fermentation was not affected by the method of inoculation (fungal spores or hyphae) used, but the use of spores gave a more consistent inoculum in the different runs.

Mixotrophic growth of Phaeodactylum tricornutum on glycerol: growth rate and fatty acid profile

Mixotrophic growth of the eicosapentaenoic acid (EPA)producing diatom Phaeodactylum tricornutum UTEX640 was carried out in 1-L batch cultures under anexternal irradiance of 165 μmol photons m-2s-1 by supplementing the inorganic culture mediumwith glycerol. The effect on the growth and the fattyacid profile was studied for different initialglycerol concentrations (0–0.1 M). The optimalglycerol concentration was 0.1 M.A lag phase was observed at high glycerolconcentrations. The present study also shows thatsuccessive additions of glycerol at 0.1M concentrationand using ammonium chloride as a nitrogen sourceremarkably increased the maximum biomass concentration(16.2 g L-1) and maximum biomass productivity(61.5 mg L-1 h-1). These values wererespectively 9 and 8-fold higher than in thephotoautotrophically grown control. The level ofsaponifiable lipids in mixotrophically cultured cellswas significantly higher than in photoautotrophicallycultured cells and increased with the glycerolconcentration in the medium. The concentration ofstorage lipids, saturated and monounsaturated fattyacids, were enhanced but the EPA content did notchange significantly. The EPA content was around 2.2%of biomass dry weight. The maximum EPA yield was33.5 mg L-1 d-1 and was obtained in aculture containing 0.1 M glycerol, supplementedperiodically by ammonium chloride. This productivitywas 10-fold higher than the EPA productivity obtainedunder mixotrophic conditions.Mixotrophic growth of the eicosapentaenoic acid (EPA)producing diatom Phaeodactylum tricornutum UTEX640 was carried out in 1-L batch cultures under anexternal irradiance of 165 μmol photons m-2s-1 by supplementing the inorganic culture mediumwith glycerol. The effect on the growth and the fattyacid profile was studied for different initialglycerol concentrations (0–0.1 M). The optimalglycerol concentration was 0.1 M.A lag phase was observed at high glycerolconcentrations. The present study also shows thatsuccessive additions of glycerol at 0.1M concentrationand using ammonium chloride as a nitrogen sourceremarkably increased the maximum biomass concentration(16.2 g L-1) and maximum biomass productivity(61.5 mg L-1 h-1). These values wererespectively 9 and 8-fold higher than in thephotoautotrophically grown control. The level ofsaponifiable lipids in mixotrophically cultured cellswas significantly higher than in photoautotrophicallycultured cells and increased with the glycerolconcentration in the medium. The concentration ofstorage lipids, saturated and monounsaturated fattyacids, were enhanced but the EPA content did notchange significantly. The EPA content was around 2.2%of biomass dry weight. The maximum EPA yield was33.5 mg L-1 d-1 and was obtained in aculture containing 0.1 M glycerol, supplementedperiodically by ammonium chloride. This productivitywas 10-fold higher than the EPA productivity obtainedunder mixotrophic conditions.

A study on simultaneous photolimitation and photoinhibition in dense microalgal cultures taking into account incident and averaged irradiances

Abstract From chemostat cultures of the marine microalga Isochrysis galbana (CCAP 927/15) simultaneous photolimitation and photoinhibition was observed. The extent of each phenomenon depends on the light gradient inside the culture and therefore on the incident irradiance. Variations in biomass concentration and average irradiance inside the culture with dilution rate at three incident irradiances, I o , were studied (from I o = 820 to 3270 μE m −2 s −1 ). At I o above 1630 μE m −2 s −1 a photoinhibition effect was observed, although the specific growth rate remained a hyperbolic function of average irradiance regardless of incident irradiance. To calculate average irradiance, a three-dimensional irradiance distribution model for cylindrical geometry is proposed, improving the estimation of the irradiance field inside culture with regard to other methods used up to now since the variations in illumination along the vertical axis are considered. Lastly, a new approach to model simultaneous photolimitation and photoinhibition is proposed by considering that specific growth rate is related to average irradiance and that parameters representing the cell adaptability to light are a function of the maximum irradiation at which cells are exposed, that is, the incident irradiance.

Conversion of CO2 into biomass by microalgae: how realistic a contribution may it be to significant CO2 removal?

Microalgae have been proposed as a CO2 removal option to contribute to climate change avoidance and problems coming from the use of fossil fuels. However, even though microalgae can be used to fix CO2 from air or flue gases, they do not permit long-term CO2 storage because they are easily decomposed. On the other hand, microalgae can contribute to an enhancement in human sustainability by producing biofuels as an alternative to fossil fuels in addition to the production of other useful chemicals and commodities. Moreover, microalgae can contribute to enhancing the sustainability of waste treatment processes, reducing the energy consumed, and improving the recycling of nutrients contained within them. This paper reviews the potential contribution of these processes and the existing knowledge in these areas.

Utilization of the cyanobacteria Anabaena sp. ATCC 33047 in CO2 removal processes.

In this paper the utilization of the cyanobacteria Anabaena sp. in carbon dioxide removal processes is evaluated. For this, continuous cultures of this strain were performed at different dilution rates; alternatives for the recovery of the organic matter produced being also studied. A maximum CO(2) fixation rate of 1.45 g CO(2) L(-1) day(-1) was measured experimentally, but it can be increased up to 3.0 g CO(2) L(-1) day(-1) outdoors. The CO(2) is mainly transformed into exopolysaccharides, biomass representing one third of the total organic matter produced. Organic matter can be recovered by sedimentation with efficiencies higher than 90%, the velocity of sedimentation being 2.10(-4) s(-1). The major compounds were carbohydrates and proteins with productivities of 0.70 and 0.12 g L(-1) day(-1), respectively. The behaviour of the cultures of Anabaena sp. has been modelized, also the characteristics parameters requested to design separation units being reported. Finally, to valorizate the organic matter as biofertilizers and biofuels is proposed.

Photobioreactors for the production of microalgae.

Microalgae are produced today for human and animal markets, as food-feed and source of active compounds. Microalgae can be also used in wastewater treatment and they has been proposed as biofuels source to reduce global warming problem. Whatever the final application of microalgae its production is based on the same principles as light availability, enough mass and heat transfer and adequate control of culture parameters. In this paper these principals are revised. Moreover, the production must be carried out at adequate scale using photobioreactors. Design of photobioreactor is determined by the final use of biomass and quality required. Different designs today used are revised, including last designs proposed, identifying his characteristics parameters and applications. In addition, the obligation of adequate control strategies is discussed. Finally, the bottlenecks for the scale-up of the different technologies and thus of microalgae production are summarized.

Outdoor continuous culture of Porphyridium cruentum in a tubular photobioreactor: quantitative analysis of the daily cyclic variation of culture parameters

The present work reports on the daily cyclic variation of oxygen generation rates, carbon consumption rates, photosynthetic activities, growth rates and biochemical composition of the biomass in a pilot plant continuous outdoor culture of the microalgae Porphyridium cruentum. A linear relationship between the external irradiance and the average irradiance inside the culture was found. In addition, the oxygen generation and carbon consumption rates were found to be a function of the average irradiance inside the culture. A reduction in photosynthetic activity of the cells at noon and recovery in the afternoon was also observed. Therefore, the cells showed a short-term response of parameters such as oxygen generation rate as well as carbon consumption rate with external and average irradiance; a model of photosynthesis rate considering photoinhibition is proposed. This model is a useful tool for the operation and scaleup of tubular photobioreactors, and can be used for determining CO2 requirements of the system. The higher the photosynthesis rates, the lower the carbon losses, ranging from 25% at noon to 100% during the night. The growth rate showed a linear relationship with the daily mean average irradiance inside the culture with a long-term response. Likewise, a linear relationship among the oxygen generation rate and the growth rate was obtained. With respect to the biochemical composition of the biomass, the cells showed a long-term response of metabolic routes to mean daily culture conditions. During the illuminated period, energy was stored as carbohydrates and synthesis of proteins was low. During the night, the stored carbohydrates were consumed. The fatty acid dry weight (DW) content decreased during the daylight period, whereas the fatty acid profile, as total fatty acids, was a function of growth rate. A short-term variation of exopolysaccharides synthesis with solar irradiance was also observed, i.e. the higher the external irradiance the higher the excretion of exopolysaccharides as a protection against adverse culture conditions.