Flat-panel Airlift Photobioreactors Research Articles

Improving microalgae growth modeling of outdoor cultivation with light history data using machine learning models: A comparative study

Accurate prediction of microalgae growth is crucial for understanding the impacts of light dynamics and optimizing production. Although various mathematical models have been proposed, only a few of them have been validated in outdoor cultivation. This study aims to investigate the use of machine learning algorithms in microalgae growth modeling. Outdoor cultivation data of Phaeodactylum tricornutum in flat-panel airlift photobioreactors for 50 days were used to compare the performance of Long Short-Term Memory (LSTM) and Support Vector Regression (SVR) with traditional models, namely Monod and Haldane. The results indicate that the machine learning models outperform the traditional models due to their ability to utilize light history as input. Moreover, the LSTM model shows an excellent ability to describe the light acclimation effect. Last, two potential applications of these models are demonstrated: 1) use as a biomass soft sensor and 2) development of an optimal harvest strategy for outdoor cultivation.

Influence of light conditions on the production of chrysolaminarin using Phaeodactylum tricornutum in artificially illuminated photobioreactors.

The light conditions are of utmost importance in any microalgae production process especially involving artificial illumination. This also applies to a chrysolaminarin (soluble 1,3-β-glucan) production process using the diatom Phaeodactylum tricornutum. Here we examine the influence of the amount of light per gram biomass (specific light availability) and the influence of two different biomass densities (at the same amount of light per gram biomass) on the accumulation of the storage product chrysolaminarin during nitrogen depletion in artificially illuminated flat-panel airlift photobioreactors. Besides chrysolaminarin, other compounds (fucoxanthin, fatty acids used for energy storage [C16 fatty acids], and eicosapentaenoic acid) are regarded as well. Our results show that the time course of C-allocation between chrysolaminarin and fatty acids, serving as storage compounds, is influenced by specific light availability and cell concentration. Furthermore, our findings demonstrate that with increasing specific light availability, the maximal chrysolaminarin content increases. However, this effect is limited. Beyond a certain specific light availability (here: 5 µmolphotons gDW -1 s-1 ) the maximal chrysolaminarin content no longer increases, but the rate of increase becomes faster. Furthermore, the conversion of light to chrysolaminarin is best at the beginning of nitrogen depletion. Additionally, our results show that a high biomass concentration has a negative effect on the maximal chrysolaminarin content, most likely due to the occurring self-shading effects.

Influence of specific light availability and solvent on process economics — The production of fucoxanthin and eicosapentaenoic acid from P. tricornutum using flat-panel airlift photobioreactors with artificial light

This study presents an economic assessment of an up- and downstream process (USP and DSP) using P. tricornutum in flat-panel airlift photobioreactors (FPA-PBRs) with artificial illumination. Depending on the solvent, it is possible to obtain separately a crystalline fucoxanthin and eicosapentaenoic acid-rich (EPA) oil or a fucoxanthin- and EPA-rich oleoresin. The objective was to investigate the potential of optimizing light supply during USP and changing the extraction solvent during DSP to reduce fixed capital investment (FCI) and production costs. Comprehensive experimental data on the cultivation of P. tricornutum and the extraction and separation of the products were used to perform techno-economic assessments (TEA) and profitability analyses of different scenarios. In the first part, six scenarios were developed to determine the economic performance with a biomass concentration (c0) of either 3.0 or 6.0 g l−1 and a specific light availability (Ispec) of 2.0, 5.0 or 8.0 μmolphotons g−1 s−1 during the annual production of 10.0 t fucoxanthin ((component of biomass) until harvest. In the second part, the scenario with the economically most favourable results served as the basis for TEA depending on the solvent. It was demonstrated that production costs of dry weight biomass (CX) could be reduced from 644.31 to 184.30 € kgDW−1 by increasing Ispec and volumetric biomass productivity (QX) with the lowest biomass yield on light energy (YX,ph). A sensitivity analysis was performed to evaluate the possibilities of reducing costs and increasing profitability. Increasing QX proved to be the most effective, resulting in a decrease in CX of up to 20.1 %. Subsidising the FCI was most effective in reducing FCI, resulting in a decrease of up to 49.6 %. By using ethanol instead of a nonpolar solvent, the specific production costs of fucoxanthin (CFx) could be decreased by 60.0 % from around 39,825 € kgFx−1 to 16,111 € kgFx−1.

Comparing three different Phaeodactylum tricornutum strains for the production of chrysolaminarin in flat panel airlift photobioreactors

In recent years, various applications for algae-based ß-1,3-glucans have been postulated, including animal feed and human nutrition. Chrysolaminarin is a ß-1,3–1,6-glucan produced by diatoms such as Phaeodactylum tricornutum for energy storage. It is accumulated under nutrient-depleted cultivation conditions. In this study, the production of chrysolaminarin in artificially illuminated scalable flat panel airlift photobioreactors (FPA) was investigated by using P. tricornutum in a two-stage production process. In the growth stage primarily biomass is produced, and the subsequent nitrogen-depleted stage induces the accumulation of chrysolaminarin. Three P. tricornutum strains (SAG 1090-1a, SAG 1090-1b, SAG 1090–6) were cultured at laboratory scale in 6 L-FPA reactors under controlled light conditions to characterize the process and identify a production strain. The chrysolaminarin content of the algae strains was analysed and additionally their contents of eicosapentaenoic acid and fucoxanthin, both of which could be involved in a possible co-production. Strain SAG 1090-1b exhibited the highest biomass productivity and chrysolaminarin content (317 ± 9 mg gDW−1) after nitrogen depletion, and thus stood out as the most suitable for chrysolaminarin production in a two-stage process. A co-production of the three compounds is possible. However, during nitrogen depletion there occurred trade-offs between the compounds. As chrysolaminarin was produced, the amount of fucoxanthin in the culture stagnated or even decreased depending on the selected strain.

Economic evaluation of up- and downstream scenarios for the co-production of fucoxanthin and eicosapentaenoic acid with P. tricornutum using flat-panel airlift photobioreactors with artificial light

This study presents an economic assessment of an up- and downstream process for the co-production of fucoxanthin and eicosapentaenoic acid using the diatom P. tricornutum in commercial flat-panel airlift photobioreactors with artificial illumination. For the first time, comprehensive experimental data on the cultivation of P. tricornutum and the extraction and separation of both target compounds were used to evaluate production costs in three scenarios, from pilot to industrial scale. Biomass-specific production costs of 882 to 228 € kg−1 were determined, depending on the scale of the production plant. This results in minimum production costs of 32,042 € kg−1 for purified fucoxanthin (>90%w/w) and 7343 € kg−1 for highly concentrated eicosapentaenoic acid (EPA). Operating costs - in particular energy requirements for the artificial illumination and cooling of the reactors - were found to be the main overall cost drivers. Furthermore, the results show that a sizable cost reduction by upscaling is only possible to a limited extent. The assessment revealed that the overall profitability of the entire process is much more dependent on fucoxanthin than on EPA, and thus the prevailing market price of fucoxanthin is crucial for an investment decision.

Online estimation of biomass, lipid and nitrate dynamic profile using innovative light evolution kinetic model in flat panel airlift photobioreactor for Botryococcus braunii under varying light conditions

Light availability inside the photobioreactor is generally measured by the Lambert – Beer law. However, Lambert - Beer law assumptions are not valid at high cell density cultural conditions, because light scattering and differential absorption have significant impact on light transfer in photobioreactor. In this study Lambert – Beer law was modified by applying differential absorbance in flat panel photobioreactor. As light intensity fluctuate with location and age in the cultivation process, aconsolidated model based on Lambert – Beer law including temporal light evolution and microorganism growth is presented. The model was calibrated under specific light intensity of 133 μmol m−2 s−1 and validated using the experimental data of microalgae growth under varying light conditions of 174, 261 and 348 μmol m−2 s−1. Maximum biomass concentration, maximum lipid content and lipid productivity were 2.52 g dcw L−1, 19.76% and 114.92 mg L−1d−1 at 261 μmol m−2 s−1 light intensity respectively.

Temperature Control of Flat-panel Airlift Photobioreactors for Microalgae Prduction – A Numerical Investigation

Within the blue biotechnology, the cultivation of microalgae has an important role. Aimed is the production of valuable bio products, including biofuels. Microalgae can be cultivated in open raceway ponds or in different types of photobioreactors (PBRs). Besides their higher investment costs, PBRs are gaining more importance due to the possibilities they offer for controlling the production parameters like, light, pH, Nutrients, CO2 supply, etc. This study presents the influence of temperature control on the operating cost of a culture in a flat-panel airlift photobioreactor, based on a simulation model. The data used are those of a coastal range in Cyprus, at Zygi, with mild climate, requiring heating in Winter and cooling in the Summer. Microalgae grow optimally between 20°C and 24°C, but choosing the right set temperatures for Winter and Summer plays an important role in the economy of the system. The most energy saving option seems to be that of a stepwise set-temperature control, with a temperature varying in steps between 19 and 24°C that are considered to be economic acceptable minimum and maximum values. For the estimation of the yearly fuel consumption of the PBR a new term, the Burner ON Ratio was introduced.

Culture of Spirogyra sp. in a flat-panel airlift photobioreactor.

Spirogyra is a green filamentous freshwater algae on which recent studies reveal several promising properties and potential application possibilities in biotechnology. However, little is known about cultivation of Spirogyra and even less about large-scale cultivations in closed growth systems. Therefore, the aim of the present study was to elaborate the growth kinetics of Spirogyra sp. in a commercially available and scalable photobioreactor. For this purpose, Spirogyra sp. was grown indoors in distinct flat-panel airlift photobioreactors equipped with culture-flow directing installations. Hereby, special attention was laid on light administration and specific light availability and it was found that Spirogyra sp., in combination with the photobioreactor in question, required high photon-flux densities (100µmolm-2s-1gDW-1) for maximum proliferation which is in accordance with its abundance in epipelagial waters in nature. Applying photon-flux densities of up to 1400µmolm-2s-1, a maximum volumetric productivity and final biomass concentration of 1.15gDWL-1day-1 and 14.28gDWL-1 were achieved, respectively, the highest to be reported for the alga. To the knowledge of the authors, this is the first report on the growth of Spirogyra in a flat-panel photobioreactor.

Empirical operating strategy to reduce the light-specific energy consumption of a flat-panel airlift photobioreactor with intrinsic static mixers cultivating Thermosynechococcus elongatus BP-1

During photoautotrophic cultivations using optimized synthetic media, light is the limiting “substrate” of high-density cultures, especially throughout outdoor cultivations prohibiting adjustment of photon-flux density (PFD). Convective mass transfer (turbulence) is the method of choice to cope with that limiting effect. Then again, turbulence comes at costs through the energy required for its generation. In this context and based on laboratory data generated with Thermosynechococcus elongatus BP-1, an empiric operating strategy for flat-panel airlift photobioreactors with intrinsic static mixers is suggested. Repeated cultivations were performed from sub- to supra-saturating PFD (180 to 780μmolm−2s−1) at aeration rates ranging from 0.11 to 0.83vvm assessing the cultures' productivities along the courses of biomass concentrations. The results indicate that, owing to the culture-flow directing mixers, there is a strong interrelation between the effects of PFD, biomass concentration and aeration rate (thus energy input) applied. Hereby, the positive impact of directed turbulence on the cultures' performance with respect to productivity and yield was greatest at high biomass concentration (>5gDWL−1) and PFDs with effects becoming less dominant while the latter two were reduced. Based on the quantitative findings and utilizing the easily monitored parameters PFD and biomass concentration, a computational model may be defined automatically controlling the easily adjustable parameter aeration rate. Without a negative influence on cultures' performance, this will allow for both, increase of culture performance at times of intense light and high biomass concentration as well as reduction of operational expenditures (OPEX) at times of dim light or low biomass concentration. Compared to the standard, continuous aeration regime, this may lead to a reduction of energy required for the generation of turbulence of 37%, especially when considering outdoor cultivations in temperate climate zones.

Effects of pH on cell growth, lipid production and CO2 addition of microalgae Chlorella sorokiniana

Microalgae have emerged as one of the most promising alternative energy feedstocks. Some advantages include the simple cellular structure, short production cycle, high lipid content, and fast growth. However, high production costs due to high CO2 usage and low lipid productivity have been some of the major challenges impeding the commercial production of algal biodiesel. Here, cell growth and lipid content of Chlorella sorokiniana DOE1412 were first evaluated at different pH in flask cultivation. Culture pH was manipulated by CO2 addition. The optimal pH for DOE1412 is approximately 6.0 when only accounting for cell growth and lipid production and not considering the CO2 efficiency. A flat panel airlift photobioreactor (PBR) was used for scale-up cultivation at five different pH levels (6.5, 7, 7.5, 8 and 8.5). Data of pH values and CO2 addition was collected by a data logger. Biomass productivity increased with decreasing pH. By taking into account not only the cell growth and lipid production but also CO2 addition, the lowest value of CO2 addition was achieved at pH8 (2.01g CO2/g biomass). The fatty acid profiles and biodiesel properties, such as iodine value (IV), saponification value (SV), cetane number (CN), degree of unsaturation (DU), long-chain saturated factor (LCSF), and cold filter plugging point (CFPP), were determined as a function of pH. CN of biodiesel produced at pH6.5, 7 and 7.5 satisfied the US standard ASTM D6751; among them, the pH6.5 products met the European standard EN 14214. Finally, protein content in microalgal biomass increased with increasing pH, while C/N ratio in cells decreased.

Hydrodynamic and mass transfer characterization of flat-panel airlift photobioreactors for the cultivation of a photosynthetic microbial consortium

The design and build of industrial photobioreactors are critical steps for the production of photoautotrophic microorganisms. Flat-panel airlift photobioreactors (PBRs) of 50L and 150L were built; hydrodynamics (gas hold-up (ε), mixing time (tm) and Reynolds number (Re)) and mass transfer properties were characterized in the two flat-panel airlift PBRs (50 and 150L). The geometric ratio (illuminated area: operation volume (A/V) ratio) selected for building the 150L PBR was adequate to carry out biomass production of a microbial consortium (MC). A high CO2 utilization efficiency in the PBRs was obtained. Finally, the experimental results of kLa were correlated mathematically to describe the influences of the considered factors (the overall power input and density) on the studied superficial gas velocity range. The predicted kLa values yielded acceptable results compared to the experimental results.

A novel flat plate air-lift photobioreactor with inclined reflective broth circulation guide for improved biomass and lipid productivity by Desmodesmus subspicatus LC172266

The aim of this study was to construct a novel photobioreactor with improved mixing and light supply efficiencies. A 4-L flat panel airlift photobioreactor with inclined broth circulation guide installed in the downcomer column was constructed. The circulation time (θc) and mixing time (θm) were determined, using the solid flow follower technique and dye method, respectively. The circulation fluid was either tap water or 0.2% potato starch solution. The effects of nature and angles of inclination of the circulation guides and the liquid heights above the circulation guides on the mixing efficiencies of the photobioreactor were studied. A decrease in the angle of inclination and increase in liquid heights resulted in decreased θc and θm in tap water or 0.2% potato starch solution. At 45° angle of inclination and liquid height of 0.02 m, the shortest θc (2.67 s), θm (10.0 s) in tap water, and θc (3.3 s) in 0.2% starch solution were achieved. The photobioreactor was evaluated by cultivation of high lipid-producing Desmodesmus subspicatus LC172266 isolated from Maiduguri, Nigeria. The installation of broth circulation guides (transparent, opaque, or reflective guides) improved the productivity of D. subspicatus, though significant (P < 0.05) only in the case of reflective guide. The lipid productivities (g L−1 day−1) of D. subspicatus in the photobioreactor equipped with inclined circulation guides were 0.052, 0.0215, 0.020, and 0.016 for reflective, opaque, transparent, and control (none) circulation guides, respectively. Under a mixotrophic condition using a reflective circulation guide and glucose as the carbon source, the lipid productivity was 0.217 g L−1 day−1 while the value obtained with glycerol was 0.122 g L−1 day−1. The lipid productivity by D. subspicatus in the photobioreactor with a reflective broth guide was 94% (0.217 g L−1 day−1), higher than the value obtained in batch (flask) cultivation (0.112 g L−1 day−1) with glucose concentration of 10.0 g L−1 and 46% (0.122 g L−1 day−1) higher than the value obtained in batch flask cultivation (0.086 g L−1 day−1) with glycerol concentration of 5.0 g L−1 under a mixotrophic condition.

Continuous production of diatom Entomoneis sp. in mechanically stirred tank and flat-panel airlift photobioreactors

ABSTRACTContinuous production of diatom Entomonies sp. was performed in mechanically stirred tank and flat-panel airlift photobioreactors (FPAP). The maximum specific growth rate of diatom from the batch experiment was 0.98 d−1. A series of dilution rate and macronutrient concentration adjustments were performed in a stirred tank photobioreactor and found that the dilution rate ranged from 0.7 to 0.8 d−1 and modified F/2 growth media containing nitrate at 3.09 mg N/L, phosphate at 2.24 mg P/L, and silicate at 11.91 mg Si/L yielded the maximum cell number density. Finally, the continuous cultivation of Entomonies sp. was conducted in FPAP using the optimal conditions determined earlier, resulting in the maximum cell number density of 19.69 × 104 cells/mL, which was approximately 47 and 73% increase from the result using the stirred tank photobioreactor fed with modified and standard F/2 growth media, respectively.

Repeated fed-batch cultivation of Thermosynechococcus elongatus BP-1 in flat-panel airlift photobioreactors with static mixers for improved light utilization: Influence of nitrate, carbon supply and photobioreactor design

Microalgae mass cultivation is limited by light availability due to effects of absorption and reflection. These effects can only partially be coped with by increasing the photon-flux density. Further inhibitive effects, especially during outdoor cultivations, are described by photoinhibition and photosaturation. Therefore, moving the cells towards the light source and away from it in a distinct mode by convective mass transfer is the method of choice to cope with these inhibitive effects. Hereby, the utilization of culture-flow directing installations, e.g. static mixers, within photobioreactors can be of great benefit. The thermophilic model organism Thermosynechococcus elongatus BP-1 was cultivated in flat-panel airlift photobioreactors with and without static mixers in order to show their positive influence on growth kinetics. Optimal nitrate and carbon concentrations were 2000mgL−1 NO3−, 0.04gL−1 Na2CO3 combined with 6.3% v/v CO2. It was shown that photobioreactors with static mixers increase volumetric productivity by a factor of 3.4 and final biomass concentration by a factor of 2.0. A maximum productivity of 2.9gDWL−1d−1 was demonstrated, to the knowledge of the authors the highest to be reported for the cyanobacterium T. elongatus BP-1.

Manufacture of a large-sized flat panel airlift photobioreactor (FPA PBR) case with characteristic shapes using a thermoforming process

The objective of this paper is to manufacture a large-sized Flat panel airlift photobioreactor (FPA PBR) case with characteristic shape using thermoforming process. Two distinguished characteristic shapes, including a blade and a stiffener, are designed to improve mixing characteristics and light utilization of the FPA PBR case. Structural analyses are carried out to obtain a proper design of the large-sized FPA PBR case. The design of the thermoforming mould is created using the proper design of the FPA PBR case. Proper conditions of the thermoforming process are estimated via a thermoforming analysis. The desired FPA PBR case with characteristic shapes is fabricated via thermoforming and bonding processes. Through durability and culture experiments, the applicability and the efficiency of the fabricated FPA PBR case have been investigated. From the results of the experiments, it has been shown that the designed FPA PBR case can significantly improve the productivity of the microalgae and the efficiency of the FPA PBR case.

Microalgae cultivation in urban wastewater: Nutrient removal and biomass production for biodiesel and methane

The freshwater microalgae species Chlorella kessleri and Chlorella vulgaris, and the marine microalgae species Nannochloropsis oculata were cultivated in urban wastewater. The freshwater species demonstrated the possibility of growing in urban wastewater reaching high biomass production and nutrient removal when cultured in batch mode using a flat-panel airlift photobioreactor. Both microalgae species reached high biomass dry weights, 2.70±0.08g/L and 2.91±0.02g/L respectively, accompanied by nitrogen concentration reduction around 96% and 95%, and a phosphorous concentration reduction around 99% and 98% respectively. N. oculata was able to uptake nutrients from wastewater to grow but with less efficiency, indicating the need of microalgae acclimation or process optimisation to achieve high nutrient removals. During C. kessleri and C. vulgaris cultivation, the nitrogen consumption led to a progressive N-starvation process which increased the microalgae potential for biofuels production; both species produced 346±3mLCH4/gVS and 415±2mLCH4/gVS during anaerobic digestion, and 7.4±0.2gBiodiesel/100gVS and 11.3±0.1gBiodiesel/100gVS respectively.

Astaxanthin Induction in Microalga H. pluvialis With Flat Panel Airlift Photobioreactors Under Indoor and Outdoor Conditions

Astaxanthin was induced from Haematococcus pluvialis (NIES-144) under indoor and outdoor conditions using 17-, 50-, and 90-L flat-panel airlift photobioreactors (FP-APBRs). Preliminary experiments in 1.5-L bubble column photobioreactors (BC-PBRs) revealed that sterilized clean water with 3% CO2 aeration (1.47 cm3 s−1 CO2 loading) could best encourage astaxanthin accumulation at 18.21 g m−3 (3.63% by weight). Operating 17-L FP-APBRs with these bubble column parameters under indoor conditions could further enhance astaxanthin to 26.63 g m−3 (5.34% by weight). This was potentially due to the inherited up-lift force from the reactor that helped avoid cell precipitation by allowing the cells to be circulated within the reactor. In addition, the various sizes of FP-APBRs exhibited similar performance, implying a potential scale-up opportunity. However, similar operation under outdoor condition exhibited slightly poorer performance due to the light inhibition effect. The best outdoor performance was obtained with the FP-APBR covered with one layer of shading net, where 20.11 g m−3 (4.47% by weight) of astaxanthin was resulted.

Experimental characterization and numerical simulation of the hydrodynamics in an airlift photobioreactor for microalgae cultures

The development of photobioreactors optimized to increase the biomass production from phototrophic microalgae cultures assumes a major concern for the economic viability of different processes (biofuel production, carbon dioxide mitigation, pigments …). The main aim of this study is to establish and validate a CFD hydrodynamic model for a flat panel airlift photobioreactor configuration. In this regard, experimental water flow rates induced in the riser of the reactor and the liquid circulation were compared to the CFD solution obtained thanks to the Fluent® software. The numerical simulation results were relevant. Subsequently, cultures of Scenedesmus obliquus (CCAP 276/3A) were achieved to determine an optimal injected air flow rate for our configuration of photobioreactor. This air flow rate was 1.5L·min−1. Measurement of the mass transfer coefficient (kLa) of carbon dioxide in the culture medium was 3.10−4s−1 and the mixing time for this optimal operating condition is 312s.

Producing synthetic natural gas from microalgae via supercritical water gasification: A techno-economic sensitivity analysis

A techno-economic sensitivity analysis of the production of synthetic natural gas (SNG) via catalytic supercritical water gasification (SCWG) of microalgae produced in raceway ponds (RP), tubular-, or flat-panel-airlift photobioreactors (FPA-PBR) has been perfomed. The aim of combining microalgae production with SCWG is to close material flows with respect to water and nutrients, the so called SunCHem process. The sensitivity analysis is based on an annual production of 86,500 t of microalgae biomass yielding 1.14 PJ of methane per year. The sensitivity analysis showed that with an annual algae productivity of 38.5 t per hectare of RP an energy return on energy invested (EROEI) of 1.84 can be achieved for the self-sufficient base case scenario. An SNG production cost of 194 € GJ−1 was obtained for RP. An EROEI of 0.08 was calculated for tubular PBR with a productivity of 75.1 t ha−1 a−1 in the base case scenario and thus was found to be inappropriate for SNG production. EROEI for FPA-PBR with an assumed microalgae productivity of 79 t ha−1 a−1 was found to be 1.01 in the base case scenario and an SNG production cost of 266 € GJ−1. With significantly more optimistic assumptions concerning microalgae productivity, energy input and capital requirement with respect to microalgae cultivation, an EROEI of 3.6–5.8 and SNG production costs of 53–90 € GJ−1 were found for RP, whereas for FPA-PBR an EROEI of 2–3.7 and SNG production costs of 30–103 € GJ−1 were obtained.

Disposable Flat Panel Airlift Photobioreactors

AbstractSingle‐use technology based on disposable bioreactors is gaining more and more importance in biotechnology due its high flexibility, easy handling and ready‐to‐use character when supplied. A disposable, low‐cost, transparent flat panel airlift photobioreactor with optimized flow regime for the efficient cultivation of microalgae and production of microalgal metabolites is presented and possible applications are discussed.