A new photobioreactor for continuous microalgal production in hatcheries based on external-loop airlift and swirling flow

This study deals with the scale of a new photobioreactor for continuous microalgal production in hatcheries. The combination of the state-of-art with the constraints inherent to hatcheries has turned the design into a closed, artificially illuminated and external-loop airlift configuration based on a succession of elementary modules, each one being composed of two transparent vertical interconnected columns. The liquid circulation is ensured pneumatically (air injections) with respect to a swirling motion (tangential inlets). A single module of the whole photobioreactor was built-up to scale its geometry (diameter and length) and to optimize its design (air sparger, tangential inlets). The volumetric productivities were predicted by modeling radiative transfer and growth of Isochrysis affinis galbana (clone Tahiti). The hydrodynamics of the liquid phase was modeled in terms of global flow behavior (circulation and mixing times, Péclet number) and of swirling motion decay along the column (Particle Image Velocimetry). The aeration performances were determined by overall volumetric mass transfer measurements. Continuous cultures of Isochrysis affinis galbana (clone Tahiti) were run in two geometrical configurations, generating either an axial or a swirling flow. Lastly, the definitive options of design are presented as well as a 120-L prototype, currently implemented in a French mollusk hatchery and commercialized.     

Factors controlling eicosapentaenoic acid production in semicontinuous cultures of marine microalgae

Three marine microalgal species with a high content of eicosapentaenoic acid (EPA), Phaeodactylum tricornutum, Isochrysis galbana and Porphyridium cruentum, were cultured semicontinuously in order to study the effect of renewal rate on EPA productivity. The percentage of EPA in total fatty acids increased with increasing renewal rates in nitrogen limited cultures, but while for Phaeodactylum tricornutum and Isochrysis galbana a plateau around 20–25% of total fatty acids was reached with renewal rates that were not nitrogen-limiting, in Porphyridium cruentum EPA percentage increased continuously with increasing renewal rate even for those cultures that were nitrogen sufficient. Maximal EPA productivities of4.6 mg L^-1 day^-1 for Isochrysis galbana and 5.2 mg L^-1 day^-1 for Phaeodactylum tricornutum were achieved with renewal rates of 20% and 30% respectively. On the other hand for Porphyridium cruentum maximal EPA productivity, 5.3 mg L^-1 day^-1, was obtained with the maximal renewal rate tested. Results indicate that different culture strategies should be adopted for the production of a particular polyunsaturated fatty acid depending on the microalgal species being used. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Production of Dunaliella salina biomass rich in 9-cis-beta-carotene and lutein in a closed tubular photobioreactor

Performance of Dunaliella salina cultures outdoors in a closed tubular photobioreactor has been assessed. Optimization of conditions involved verification of the effect of several determining factors on the yield of both biomass and carotenoids. Maximal biomass productivity (over 2g (dry weight) m(-2) d(-1) or 80 gm(-3) d(-1)) was achieved at 38 cm s(-1), flow rate; 2 x 10(9) cells l(-1), initial population density; 25 degrees C, temperature; semi-continuous regime, keeping a cell density interval between 2 x 10(9) and over 4 x 10(9) cells l(-1). Coverage of the tubular loop with a sunshade screen to avoid light-induced damage of cells was essential to maintain growth performance. The cellular beta-carotene level increased significantly during the light period, as also did that of lutein. The rise in the beta-carotene level could be accounted by the 9-cis-isomer, with all-trans-beta-carotene remaining steady during the light period. By sunset, the ratio between 9-cis- and all-trans-isomers of beta-carotene amounted to 1.5, with over 60% of total beta-carotene corresponding to the 9-cis-isomer. Removal of sunshade enhanced carotenoid accumulation by cells to reach up to 10% of dry biomass. Cultivation of Dunaliella in closed tubular photobioreactor, thus represents a suitable approach for the production of a high-quality microalgal biomass enriched in the valuable 9-cis-isomer of beta-carotene and lutein.     

The technology of microalgal culturing

This review outlines the current status and recent developments in the technology of microalgal culturing in enclosed photobioreactors. Light distribution and mixing are the primary variables that affect productivities of photoautotrophic cultures and have strong impacts on photobioreactor designs. Process monitoring and control, physiological engineering, and heterotrophic microalgae are additional aspects of microalgal culturing, which have gained considerable attention in recent years.     

Analyzing and modeling of photobioreactors by combining first principles of physiology and hydrodynamics

Mixing in photobioreactors is known to enhance biomass productivity considerably, and flow dynamics play a significant role in the reactor's performance, as they determine the mixing and the cells' movement. In this work we focus on analyzing the effects of mixing and flow dynamics on the photobioreactor performance. Based on hydrodynamic findings from the CARPT(Computer Automated Radioactive Particle Tracking) technique, a possible mechanism for the interaction between the mixing and the physiology of photosynthesis is presented, and the effects of flow dynamics on light availability and light intensity fluctuation are discussed and quantitatively characterized. Furthermore, a dynamic modeling approach is developed for photobioreactor performance evaluation, which integrates first principles of photosynthesis, hydrodynamics, and irradiance distribution within the reactor. The results demonstrate the reliability and the possible applicability of this approach to commercially interesting microalgae/cyanobacteria culture systems.     

Optimization of algal photobioreactors using flashing lights

It has been reported that flashing light enhances microalgal biomass productivity and overall photosynthetic efficiency. The algal growth kinetics and oxygen production rates under flashing light with various flashing frequencies (5 Hz-37 kHz) were compared with those under equivalent continuous light in photobioreactors. A positive flashing light effect was observed with flashing frequencies over 1 kHz. The oxygen production rate under conditions of flashing light was slightly higher than that under continuous light. The cells under the high frequency flashing light were also observed to be healthier than those under continuous light, particularly at higher cell concentrations. When 37 kHz flashing light was applied to an LED-based photobioreactor, the cell concentration was higher than that obtained under continuous light by about 20%. Flashing light may be a reasonable solution to overcome mutual shading, particularly in high-density algal cultures.     

Hydrodynamic characteristics and microalgae cultivation in a novel flat‐plate photobioreactor

Flat‐plate photobioreactors (FPPBRs) are widely reported for cultivation of microalgae. In this work, a novel FPPBR mounted with inclined baffles was developed, which can make the fluid produce a “spirality” flow. The flow field and cell trajectory in the photobioreactor were investigated by using computational fluid dynamics. In addition, the cell trajectory was analyzed using a Fast Fourier transformation. The influence of height of the baffles, the angle α between the inclined baffle and fluid inlet flow direction (z), and the fluid inlet velocity on the frequency of flashing light effect and pressure drop were examined to optimize the structure parameters of the inclined baffles and operating conditions of the photobioreactor. The results showed that with inclined baffles built‐in, significant swirl flow could be generated in the FPPBR. In this way, the flashing light effect for microalgal cell could also be achieved and the photosynthesis efficiency of microalgae could be promoted. In outdoor cultivation of freshwater Chlorella sp., the maximum biomass productivity of Chlorella sp. cultivated in the photobioreactor with inclined baffles was 29.94% higher than that of the photobioreactor without inclined baffles. © 2012 American Institute of Chemical Engineers Biotechnol. Prog., 2013     

Photosynthetic efficiency of Chlorella sorokiniana in a turbulently mixed short light‐path photobioreactor

To be able to study the effect of mixing as well as any other parameter on productivity of algal cultures, we designed a lab‐scale photobioreactor in which a short light path (SLP) of (12 mm) is combined with controlled mixing and aeration. Mixing is provided by rotating an inner tube in the cylindrical cultivation vessel creating Taylor vortex flow and as such mixing can be uncoupled from aeration. Gas exchange is monitored on‐line to gain insight in growth and productivity. The maximal productivity, hence photosynthetic efficiency, of Chlorella sorokiniana cultures at high light intensities (1,500 μmol m^?1 s^?1) was investigated in this Taylor vortex flow SLP photobioreactor. We performed duplicate batch experiments at three different mixing rates: 70, 110, and 140 rpm, all in the turbulent Taylor vortex flow regime. For the mixing rate of 140 rpm, we calculated a quantum requirement for oxygen evolution of 21.2 mol PAR photons per mol O₂ and a yield of biomass on light energy of 0.8 g biomass per mol PAR photons. The maximal photosynthetic efficiency was found at relatively low biomass densities (2.3 g L^?1) at which light was just attenuated before reaching the rear of the culture. When increasing the mixing rate twofold, we only found a small increase in productivity. On the basis of these results, we conclude that the maximal productivity and photosynthetic efficiency for C. sorokiniana can be found at that biomass concentration where no significant dark zone can develop and that the influence of mixing‐induced light/dark fluctuations is marginal. © 2010 American Institute of Chemical Engineers Biotechnol. Prog., 2010     

A theoretical consideration on oxygen production rate in microalgal cultures

Because algal cells are so efficient at absorbing incoming light energy, providing more light energy to photobioreactors would simply decrease energy conversion efficiency. Furthermore, the algal biomass productivity in photobioreactor is always proportional to the total photosynthetic rate. In order to optimize the productivity of algal photobioreactors (PBRs), the oxygen production rate should be estimated. Based on a simple model of light penetration depth and algal photosynthesis, the oxygen production rate in high-density microalgal cultures could be calculated. The estimated values and profiles of oxygen production rate by this model were found to be in accordance with the experimental data. Optimal parameters for PBR operations were also calculated using the model.     

Study of hydrodynamic characteristics in tubular photobioreactors

In this work, the hydrodynamic characteristics in tubular photobioreactors with a series of helical static mixers built-in were numerically investigated using computational fluid dynamics (CFD). The influences of height and screw pitch of the helical static mixer and fluid inlet velocity on the cell trajectories, swirl numbers and energy consumption were examined. In order to verify the actual results for cultivation of microalgae, cultivation experiments of freshwater Chlorella sp. were carried out in photobioreactor with and without helical static mixer built-in at the same time. It was shown that with built-in helical static mixer, the mixing of fluid could be intensified, and the light/dark cycle could also be achieved which is of benefit for the growth of microalgae. The biomass productivity of Chlorella sp. in tubular photobioreactor with helical static mixer built-in was 37.26 % higher than that in the photobioreactor without helical static mixer.     

State and parameter estimation of microalgal photobioreactor cultures based on local irradiance measurement

Local photosynthetic photon flux fluence rate (PPFFR) determined by a submersible 4pi quantum micro-sensor was used in developing a versatile on-line state estimator for stirred-tank microalgal photobioreactor cultures. A marine micro-alga Dunaliella salina was used as a model organism in this study. On-line state estimation was realized using the extended Kalman filter (EKF), based on a state model of the photobioreactor and on-line local PPFFR measurement. The dynamic state model for the photobioreactor was derived based on mass-balance equations of the relevant states. The measurement equation was established based on an empirical correlation between the microalgal biomass concentration and the local PPFFR measured at a fixed point inside the photobioreactor. An internal model approach was used to estimate the specific growth rate without the need of state-based kinetic expression. The estimator was proven to be capable of estimating biomass concentration and specific growth rate, as well as phosphate and dissolved oxygen concentrations in a photobioreactor illuminated with either fixed or time-varying incident radiation. The quantum sensor was shown to be robust and able to quickly respond to dynamic changes in local PPFFR. In addition, the quantum sensor outputs were not affected by bubble aeration or agitation within the typical operating range. The strong filtering capacity of EKF gives the state estimator superior performance compared to direct calculation from the empirical biomass/local PPFFR correlation. This state estimation system makes use of inexpensive and reliable sensor hardware to report key process dynamics of microalgal photobioreactor cultures on-line, enabling improved operation of such a process.     

Nitrogen starvation strategies and photobioreactor design for enhancing lipid content and lipid production of a newly isolated microalga Chlorella vulgaris ESP-31: implications for biofuels

Microalgae are recognized for serving as a sustainable source for biodiesel production. This study investigated the effect of nitrogen starvation strategies and photobioreactor design on the performance of lipid production and of CO(2) fixation of an indigenous microalga Chlorella vulgaris ESP-31. Comparison of single-stage and two-stage nitrogen starvation strategies shows that single-stage cultivation on basal medium with low initial nitrogen source concentration (i.e., 0.313 g/L KNO(3)) was the most effective approach to enhance microalgal lipid production, attaining a lipid productivity of 78 mg/L/d and a lipid content of 55.9%. The lipid productivity of C. vulgaris ESP-31 was further upgraded to 132.4 mg/L/d when it was grown in a vertical tubular photobioreactor with a high surface to volume ratio of 109.3 m(2)/m(3) . The high lipid productivity was also accompanied by fixation of 6.36 g CO(2) during the 10-day photoautotrophic growth with a CO(2) fixation rate of 430 mg/L/d. Analysis of fatty acid composition of the microalgal lipid indicates that over 65% of fatty acids in the microalgal lipid are saturated [i.e., palmitic acid (C16:0) and stearic acid (C18:0)] and monounsaturated [i.e., oleic acid (C18:1)]. This lipid quality is suitable for biodiesel production.     

Optimization of microalgal photobioreactor system using model predictive control with experimental validation

To maximize biomass and lipid concentrations, various optimization methods were investigated in microalgal photobioreactor systems under mixotrophic conditions. Lipid concentration was estimated using unscented Kalman filter (UKF) with other measurable sources and subsequently used as lipid data for performing model predictive control (MPC). In addition, the maximized biomass and lipid trajectory obtained by open-loop optimization were used as target trajectory for tracking by MPC. Simulation studies and experimental validation were performed and significant improvements in biomass and lipid productivity were achieved in the case where MPC was applied. However, occurence of a lag phase was observed while manipulating the feed flow rates, which is induced by large amount of inputs. This is an important phenomenon that can lead to model–plant mismatch and requires further study for the optimization of microalgal photobioreactors.     

A novel photobioreactor with transparent rectangular chambers for cultivation of microalgae

An open tank photobioreactor containing transparent rectangular chambers (TRCs) was developed to improve the photosynthetic efficiency of microalgal cultivation. The TRCs, made of transparent acrylic, conducted light deep into the photobioreactor, especially at high cell concentrations. The average irradiance, I_av, was calculated by Lambert–Beer's law, and was used to determine the light conditions in the cultivation system. The photobioreactor provided large areas of illumination that improved the effective utilization of light energy for microalgae growth and created a good artificial environment for a high rate of cell growth, even at low I_av. The biomass concentration of Chlorella sp. reached 3.745 g L⁻¹ on the 13th day, with biomass productivity of 0.340 g L⁻¹ d⁻¹. The total biomass obtained was 56% more than that of similar culture systems without TRCs.     

Productivity and photosynthetic efficiency ofSpirulina platensis as affected by light intensity,algal density and rate of mixing in a flat plate photobioreactor

The effect of the rate of mixing on productivity of algal mass in relation to photon flux density and algal concentration was quantitatively evaluated in cultures ofSpirulina platensis grown in a newly designed flat-plate photobioreactor. Special emphasis was placed on elucidating the principles underlying efficient utilization of high photon flux density for maximal productivity of algal-mass. Whereas the rate of mixing exerted little influence on productivity and photosynthetic efficiency in cultures of relatively low algal density, its effect became ever more significant as algal concentration was increased. Maximal mixing-enhanced cell concentrations and productivity of biomass were obtained at the highest light intensity used. At each level of incident light intensity, maximum productivity and photosynthetic efficiency could be achieved only when algal concentration and mixing rates were optimized. The higher the intensity of the light source, the higher became the optimal culture density, highest algal concentrations and productivity of biomass being obtained at the highest light intensity used. The rate of mixing required careful optimization: when too low, maximal productivity resulting from the most efficient utilization of light could not be obtained. Too high a rate of mixing resulted in cell damage and reduced output rate.Author for correspondence     

Cultivation, photobioreactor design and harvesting of microalgae for biodiesel production: A critical review

Microalgae have the ability to mitigate CO₂ emission and produce oil with a high productivity, thereby having the potential for applications in producing the third-generation of biofuels. The key technologies for producing microalgal biofuels include identification of preferable culture conditions for high oil productivity, development of effective and economical microalgae cultivation systems, as well as separation and harvesting of microalgal biomass and oil. This review presents recent advances in microalgal cultivation, photobioreactor design, and harvesting technologies with a focus on microalgal oil (mainly triglycerides) production. The effects of different microalgal metabolisms (i.e., phototrophic, heterotrophic, mixotrophic, and photoheterotrophic growth), cultivation systems (emphasizing the effect of light sources), and biomass harvesting methods (chemical/physical methods) on microalgal biomass and oil production are compared and critically discussed. This review aims to provide useful information to help future development of efficient and commercially viable technology for microalgae-based biodiesel production.     

Optimisation of cultivation parameters in photobioreactors for microalgae cultivation using the A-stat technique

Light availability inside the reactor is often the bottleneck in microalgal cultivation and for this reason much attention is being given to light limited growth kinetics of microalgae, aiming at the increase of productivity in photobioreactors. Steady-state culture characteristics are commonly used for productivity optimisation and for cell physiology studies in continuous cultures, and are normally achieved using chemostat cultivations. In the present study, we investigated the applicability of a new and dynamic cultivation method called acceleration-stat (A-stat) to microalgae cultivations where light is the limiting substrate. In the A-stat, the dilution rate is increased at a constant rate. This acceleration rate should be a compromise between a short cultivation time, in order to make it a fast process, and the metabolic adaptation rate of the microorganism to changes in the environment. Simulations of the A-stat were done with different acceleration rates to have an indication of the best rate to use. An A-stat was performed in a pilot plant bubble column (65 l) with Dunaliella tertiolecta as a model organism, and results showed that a pseudo steady state was maintained throughout the experiment. From this work, it was concluded that the A-stat can be used as a fast and accurate tool to determine kinetic parameters and to optimise any specific type of photobioreactor.     

Effect of liquid circulation velocity and cell density on the growth ofParietochloris incisa in flat plate photobioreactors

For more accurately describing the durations of the light and the dark phases of microalgal cells over the whole light-dark cycle, and probing into the relationship between the liquid circulation time or velocity, the aeration rate and cell density, a series of experiments was carried out in 10 cm light-path flat plate photobioreactors. The results indicated that the liquid flow in the flat plate photobioreactor could be described by liquid dynamic equations, and a high biomass output, higher content and productivity of arachidonic acid, 70.10 gm⁻²d⁻¹, 9.62% and 510.3 mg/L, respectively, were obtained under the optimal culture conditions.