Microalgae Cultivation In Photobioreactor Research Articles

Robust control designs for microalgae cultivation in continuous photobioreactors

Abstract Microalgae are used to produce renewable biofuels and high-value components and in bioremediation and CO2 sequestration tasks. These increasing applications, in conjunction with a desirable constant large-scale productivity, motivate the development and application of practical controllers. This paper addresses the application of robust control schemes for microalgae cultivation in continuous photobioreactors. Due to the model uncertainties and external perturbations, robust control designs are required to guarantee the desired microalgae productivity. Furthermore, simple controller designs are desirable for practical implementation purposes. Therefore, two robust control designs are applied and evaluated in this paper for two relevant case studies of microalgae cultivation in photobioreactors. The first control design is based on an enhanced simple-input output model with uncertain estimation. The second control design is the robust nonlinear model predictive control considering different uncertain scenarios. Numerical simulations of two case studies aimed at lipid production and CO2 capture under different conditions are presented to evaluate the robust closed-loop performance.

Design and techno-economic analysis of direct CO2 capturing with integrated photobioreactors as a building façade

While urban areas are one of the sources of carbon emissions, a lack of research exists on carbon capture, storage, and utilization (CCUS) technologies in the urban. Among different CCUS technologies, direct air capture by microalgae to use in the cultivation process can be one of the possible solutions that can be integrated with the building facade. This research investigates the techno-economic analysis of direct carbon-capturing by microalgae cultivation in photobioreactors integrated into the building façade to produce value-added products. The case study is a commercial building in one of the polluted zones in Tehran. The building facade was designed considering the flat panel photobioreactor with 2 × 1 × 0.05 m3. The microalgae selected in this study is Chlorella Vulgaris. The results show that the designed system produced 2.2 tons of microalgae and captured 3.9 tons of CO2 yearly. In this study, the cost of cultivated microalgae is $10 per kg. Also, the scenarios defined in the sensitive analysis section show that increasing the cultivation capacity or changing the specie of microalgae decreases the cost of producing microalgae.

Novel 3D-printed buoyant structures for improvement in flue gas CO2-derived microalgal biomass production by enhancing anti-biofouling on vertical polymeric photobioreactor

Novel 3D-printed buoyant structures can be applied in various environmental processes because of their considerable advantages. Microalgae cultivation in photobioreactors, directly supplemented by industrial CO2, enables environmental pollution mitigation/cleanup and sustainable energy production. However, in photobioreactor systems, biofilm formation due to gas bubbling decreases microalgal productivity. Therefore, in this study, we aimed to develop a novel 3D-printed buoyant structure to suppress biofilm formation. The 10 mm-sized spherical buoyant structure reduced the height and area of the biofilm by 58.3% and 82.5%, respectively. The structure decreased space where bubble burst occurred and controlled the bubble size, reducing the overall biomass loss by 58.7%. It did not reduce photobioreactor performance noticeably during semi-continuous cultivation, indicating the possibility of long-term applicability. In large-scale outdoor microalgae cultivation using flue gas CO2, the buoyant structure improved the cell density and biodiesel production potential without contamination. This study provides a promising strategy to contribute to biological CO2 mitigation through the utilization of flue gas CO2 for enhanced microalgal production, paving the way for energy and environmental sustainability.

Alternative Source of Nutrients for Microalgae Cultivation in a Photobioreactor System

Photobioreactors are used to produce microalgae biomass for many purposes in industries and agriculture. This research was aimed at investigating alternative source of nutrients with lower cost to produce a sustainable microalgae biomass production in a photobioreactor system other than using F2 nutrient medium, the most commonly microalga nutrient compositions used in laboratories. Firstly, two nutrient sources were used to cultivate Chlorella sp. in the laboratory, i.e., a commercial fertilizer (GrowMore™) and a common fertilizer (NPK) having nitrogen equals to 12.3 mg/L. Chlorella sp. biomass grown in the medium containing the commercial fertilizer or NPK was not significantly different to that of F2 medium, as well as when the dose of the commercial fertilizer was doubled. The commercial fertilizer was then selected as a source of nutrients in the cultivation of microalgae in 135 litre photobioreactors. Chlorella sp. biomass grown in the photobioreactors for 14 days using F2 medium was 0.80 mg/L, while using the commercial fertilizer was 0.75 g/L. Cost analyses of microalga production using these commercial fertilizers showed that it was 80% less than the cost of using F2 nutrient composition. These results showed that the commercial fertilizer could be used as alternative nutrient source for microalgae cultivation in photobioreactors.

A computer-based tool to simulate raceway photobioreactors for design, operation and control purposes

Microalgae cultivation in photobioreactors is a complex process where environmental, biological and/or design variables affect the system. Due to this great complexity, it is important to have tools to analyze the system in a clear and simple way. This work presents a new software tool for the simulation of microalgae production in raceway photobioreactors. The tool allows to simulate the process on different seasons of the year, using different microalgal strains and considering different reactor designs. Moreover, open-loop and closed-loop control strategies can be analyzed, such as On/Off or PI controllers for pH and dissolved oxygen variables, in order to increase the system productivity. Thus, estimations for productivity and gas consumption are also given to compare several simulated scenarios. Real data for the weather variables are taken from a semi-industrial raceway to be used as inputs to the simulation tool.

Real-Time Monitoring of Microalgal Biomass in Pilot-Scale Photobioreactors Using Nephelometry

The increasing cultivation of microalgae in photobioreactors warrants efficient and non-invasive methods to quantify biomass density in real time. Nephelometric turbidity assessment, a method that measures light scatter by particles in suspension, was introduced already several decades ago but was only recently validated as a high-throughput tool to monitor microalgae biomass. The light scatter depends on the density of the suspended particles as well as on their physical properties, but so far there are hardly any accounts on how nephelometric assessment relates to classic methods such as dry weight and spectrophotometric measurement across a broad biomass density range for different microalgae species. Here, we monitored biomass density online and in real time during the semi-continuous cultivation of three commercial microalgae species Chloromonas typhlos, Microchloropsis gaditana and Porphyridium purpureum in pilot-scale photobioreactors, and relate nephelometric turbidity to dry weight and optical density. The results confirm a relatively strong (R2 = 0.87–0.93) and nonlinear relationship between turbidity and biomass density that differs among the three species. Overall, we demonstrate how nephelometry can be used to monitor microalgal biomass in photobioreactors, and provide the necessary means to estimate the biomass density of the studied species from turbidity data to facilitate automated biomass monitoring.

Exploring the potential of wastewater reclamation by means of outdoor cultivation of microalgae in photobioreactors

Exploring the potential of wastewater reclamation by means of outdoor cultivation of microalgae in photobioreactors

An experimental application of extremum seeking control to cultures of the microalgae Scenedesmus obliquus in a continuous photobioreactor

SummaryThe cultivation of microalgae in photobioreactors (PBRs) is important in various sectors such as food, bioenergy, pigments, and cosmetics. Productivity optimization can be achieved without the need for a dynamic process model using model‐free extremum seeking control (ESC). This article explores the use of ESC to drive the productivity of a continuous PBR to optimal or suboptimal setpoints. The latter can be achieved by estimating on‐line the gradient of a measurable performance index using a recursive least squares estimator and by regulating this gradient to a specific value (zero corresponding to the optimum) using the dilution rate as a manipulated variable. Experimental results for the cultivation of the microalgae Scenedesmus obliquus in a lab‐scale flat‐panel PBR are discussed. They show the fast convergence of ESC and its robustness towards environmental perturbations, such as changes in the incident light intensity.

Continuous Microalgal Cultivation for Antioxidants Production.

Microalgae, including cyanobacteria, represent a valuable source of natural compounds that have remarkable bioactive properties. Each microalga species produces a mixture of antioxidants with different amounts of each compound. Three aspects are important in the production of bioactive compounds: the microalga species, the medium composition including light supplied and the photobioreactor design, and operation characteristics. In this study, the antioxidant content and productivity performance of four microalgae were assessed in batch and continuous cultures. Biomass productivity by the four microalgae was substantially enhanced under continuous cultivation by 5.9 to 6.3 times in comparison with batch cultures. The energetic yield, under the experimental conditions studied, ranged from 0.03 to 0.041 g biomass kJ−1. Phenols, terpenoids, and alkaloids were produced by Spirulina platensis, Isochrysis galbana, and Tetraselmis suecica, whereas tocopherols and carotenoids were produced by the four microalgae, except for phycocyanin and allophycocyanin, which were only produced by S. platensis and Porphyridium cruentum. The findings demonstrate that the continuous cultivation of microalgae in photobioreactors is a convenient method of efficiently producing antioxidants.

Does growing Nannochloropsis sp. in innovative flat plate photobioreactors result in changes to fatty acid and protein composition?

Solar cultivation of microalgae in photobioreactors is a valuable bioprocess for the sustainable production of commercially useful metabolites. However, the conventional culture temperature control method in solar closed photobioreactors of evaporative cooling is neither economical nor sustainable. In this study, a novel spectrally selective, insulated glazed flat plate (IGP) photobioreactor employing an infrared reflecting system embedded in the illumination surface was used for cultivation of Nannochloropsis sp. The impact of the temperature control technology on protein, lipid, carbohydrate content and fatty acid profile of Nannochloropsis sp. was investigated and compared to closed photobioreactors using passive evaporative cooling (PEC) and an infrared reflecting film (IRF) on the surface as well as an open raceway pond (ORP). Among all cultivation systems tested, the biochemical composition of biomass (mg g−1 organic biomass) showed a general trend of lipid > protein > carbohydrate, with no large variation of each across treatments. However, the areal and volumetric productivities of these constituents were significantly higher in the photobioreactors than in the ORP; results consistent with biomass productivity data. Of the major saturated and monounsaturated fatty acids present, only the proportion of C16:0, which is 24% higher in the photobioreactors than in the ORP, changed significantly among cultivation systems. The highest content of high-value dietary fatty acids, eicosapentaenoic acid (EPA, C20:5n-3; 15.5%) and ϒ-linolenic acid (C18:3n-6; 8%) were found in the ORP but were similar to that produced in the IGP (15.9 and 3.4%, respectively). Among all photobioreactors, the IGP had the least diel temperature changes and an EPA content that was 21% higher than PEC. Photobioreactors constructed with spectrally selective materials effectively allow management of internal reactor temperature with no significant negative impact on biochemical and fatty acid profiles of microalgae.

Special issue in honour of Prof. Reto J. Strasser - Gradual changes in the photosynthetic apparatus triggered by nitrogen depletion during microalgae cultivation in photobioreactor

Changes of photosynthetic activity during natural depletion of nitrogen in a batch culture of Chlorella vulgaris were studied using chlorophyll a fluorescence-based methods. Complex analysis including JIP-test, multiexponential approximation, and the analysis of differential curves was carried out on the recorded fluorescence transients. Three pivot points were detected in dynamics of the JIP-test parameters during culture growing. We associated these time points with different stages of mineral stress progress. Immediately after nitrogen exhaustion in the cultivation medium, a transient increase in the efficiency of PSII was detected. During this short period, the photosynthetic apparatus acclimated to the stress by reducing the number of PSII reaction centers, simultaneously increasing the effective cross section of the light-harvesting antenna per reaction center. However, prolonged nutrient starvation impaired the structure of the photosynthetic apparatus, deactivating the oxygen-evolving complex and impairing the overall electron transport.

A simplistic approach of algal biofuels production from wastewater using a Hybrid Anaerobic Baffled Reactor and Photobioreactor (HABR-PBR) System.

The current technologies of algal biofuels production and wastewater treatment (e.g., aerobic) process are still in question, due to the significant amount of fresh water and nutrients requirements for microalgae cultivation, and negative energy balance in both processes, especially when considered in the context of developing counties around the world. In this research, a simplistic sustainable approach of algal biofuels production from wastewater was proposed using a Hybrid Anaerobic Baffled Reactor (HABR) and Photobioreactor (PBR) system. The study suggests that the HABR was capable of removing most of the organic and solid (>90% COD and TSS removal) from wastewater, and produced a healthy feedstock (high N: P = 3:1) for microalgae cultivation in PBRs for biofuels production. A co-culture of Chlorella vulgaris, Chlorella sorokiniana, and Scenedesmus simris002 showed high lipid content up to 44.1%; and the dominant FAMEs composition (C16-C18) of 87.9% in produced biofuels. Perhaps, this proposed low-cost technological approach (e.g., HABR-PBR system) would connect the currently broken link of sustainable bioenergy generation and wastewater treatment pathway for developing countries.

A numerical model coupling bubble flow, light transfer, cell motion and growth kinetics for real timescale microalgae cultivation and its applications in flat plate photobioreactors

A comprehensive numerical model based on volume-of-fluid (VOF) is established in this paper for the simulation of multiphysical process of the microalgae cultivation in photobioreactors (PBRs). The comprehensive numerical model is composed of the following four models: (1) a free-surface lattice Boltzmann model for VOF simulations of bubble flow, (2) a discrete ordinate model for collimated light transfer, (3) a Lagrangian model for cell motion, and (4) a kinetic model for microalgae growth. A temporal extrapolation scheme is proposed first to extend the simulation in small time steps into the real timescale. The growths of microalgae in a two-dimensional flat plat PBR are then simulated by the model. The effects of the gas hold-up, bubble distribution, light intensity and light/dark history on the growth of microalgae are further studied in detail. The result showed that the average growth rate can be adopted for the whole PBR. The non-uniform distribution of bubbles attenuates the positive effects of bubbles on increasing light intensity inside the PBRs. Meanwhile, the simulation with the average flow field without fluctuations can lead to non-uniform distributions of cells and errors in prediction the microalgae growth due to the centrifugal effects. The residence time distribution (RTD) of cells in the PBR is almost uniform under the condition studied, and the simulation results with RTD are close to the results with cell tracking. Finally, an improved structure of flat plat PBRs with horizontal deflectors is proposed. The results demonstrated that the inserted deflectors can increase the gas hold-ups and the uniformity of gas distributions. These effects lead to higher light intensity in the PBR, which increases the growth rate of the microalgae.

Comparison between different strategies for the realization of flashing-light effects – Pneumatic mixing and flashing illumination

A major limitation for the phototrophic cultivation of microalgae in photobioreactors is the low culture density which results in expensive harvesting and downstream processes. It is often mentioned that intensive mixing of the culture can improve the efficiency of light utilization by realizing flashing-light effects which enhance the growth of cell cultures. Alternatively, flashing light sources could provide a way to realize this effect without the need of supplying additional energy for fast mixing. While the effects of mixing have been investigated at various reactor scales, experiments with flashing illumination have been mostly conducted in small geometries. In addition, few studies have been conducted for photobioreactors with larger light paths, being characteristic for the production scale. By means of numerical simulations, we evaluate pneumatic mixing and flashing illumination with regard to their ability to realize flashing-light effects in a 5 cm diameter bubble column filled with a suspension of Chlamydomonas reinhardtii. To the best of our knowledge, a numerical comparison between the effects of flashing illumination and pneumatic mixing on the culture growth has not been reported in literature.A thorough comparison of the two methods requires a robust numerical tool which integrates the computation of the fluid flow and the light field, as well as the growth kinetics of algal cells. In the present work, we compute the three-dimensional flow field in a bubble column photobioreactor tracking also micrometer particles in order to simulate the movement of algae. The spectral light field is computed by solving the three-dimensional Radiation Transfer Equation (RTE) at different wavelengths and biomass concentrations. The coupling of the flow and light fields enables the computation of the spatio-temporal light exposure of individual algae cells, which is used to estimate the respective dynamic photosynthesis reaction rates under different operating conditions of the bubble column reactor.We found, numerically, that the contribution of pneumatic mixing alone is negligible in comparison to flashing-light effects, for the investigated operating conditions. In contrast, illumination with flashing LED leads to an increase of the growth rate up to a factor of 2.5 at flashing frequencies higher than 50 Hz in a PBR with industrially relevant operating conditions. Thereby, a proper selection of the duty cycle is not only needed to prevent photoinhibition but also to maximize the effects of flashing light at the same time. According to these results, the utilization of flashing LED sources can provide a mostly unused way to improve photobioreactor productivity in the future.

Water footprint of microalgae cultivation in photobioreactor

This work aims to evaluate the water footprint of microalgae cultivation in a closed pilot-scale multi-tubular photobioreactor, taking into account the life cycle stages of reactor construction and its operation for microalgae cultivation, on a gate-to-gate approach. The data was obtained from real production conditions complemented with data from the literature and life cycle inventory databases. Results indicate that the total water footprint lies in the range of 2.4-6.8 m3/kg dry biomass, being the PBR operation stage responsible for the largest contribution (> 60 %), mainly due to water consumption associated to electricity and nutrients production for the PBR operation.

Heterotrophic cultivation of microalgae in photobioreactor using low cost crude glycerol for enhanced biodiesel production

This integrated approach first time investigates the use of crude glycerol (CG), a primary by product of the biodiesel production, for heterotrophic cultivation of microalgae (namely, Chlorella sp.) in photobioreactor (PBR) to enhance biodiesel production. The glycerol was reported 28.5 ± 0.70% in CG, causing nearly two folds increase in biomass productivity (446.50 ± 1.50 mg L−1day−1) and nearly four folds enhanced lipid productivity (165.15 ± 0.55 mg L−1 day−1) for the algal cells cultivated in PBR with addition of CG (PBR+CG) system as compared to Bold's Basal media (BBM) used as control. The total lipid content (34.36 ± 0.51%) was also doubled for the cells grown in PBR+CG system than in BBM. The analysis of fatty acid methyl esters (FAMEs) profile by GC-MS showed reduction in total unsaturated fatty acids and poly unsaturated fatty acids, and increase in total saturated fatty acids (SFAs) for the biodiesel obtained from algal cells grown in PBR+CG system. Moreover, the physical properties estimation also specified higher cetane number (53.89) and average oxidative stability (3.41 h), which are comparable with ASTM D6751 and EN 14214 fuel standards. This study demonstrates the potential of the use of CG for enhancing the low cost biodiesel production for vehicular transportation.

Comparison of Microalgae Cultivation in Photobioreactor, Open Raceway Pond, and a Two-Stage Hybrid System

In the wake of intensive fossil fuel usage and CO2 accumulation in the environment, research is targeted towards sustainable alternate bioenergy that can suffice the growing need for fuel and also that leaves a minimal carbon footprint. Oil production from microalgae can potentially be carried out more efficiently, leaving a smaller footprint and without competing for arable land or biodiverse landscapes. However, current algae cultivation systems and lipid induction processes must be significantly improved and are threatened by contamination with other algae or algal grazers. To address this issue, we have developed an efficient two-stage cultivation system using the marine microalga Tetraselmis sp. M8. This hybrid system combines exponential biomass production in positive pressure air lift-driven bioreactors with a separate synchronized high-lipid induction phase in nutrient deplete open raceway ponds. A comparison to either bioreactor or open raceway pond cultivation system suggests that this process potentially leads to significantly higher productivity of algal lipids. Nutrients are only added to the closed bioreactors while open raceway ponds have turnovers of only a few days, thus reducing the issue of microalgal grazers.

Recovery of nutrients from swine wastewater using ultrafiltration: Applications for microalgae cultivation in photobioreactors

The large-scale production of microalgae poses a number of challenges, including a costly fertilizer demand. While wastewater provides a concentrated source of nutrients, the presence of biological contaminants and the expense of heat treatment are challenging for large-scale production. The goal of this study was to use ultrafiltration to purify wastewater for use in the cultivation of microalgae. Swine waste was filtered, and the resulting permeate was utilized in a Porphyridium cruentum culture. Fluxes remained relatively constant during operation, and the complete rejection of bacteria was observed. The permeate contained high concentrations of total nitrogen (695.6mgL−1) and total phosphorus (69.1mgL−1). Higher biomass productivity and lipid contents were observed in the microalgae cultivated in the waste medium compared to that of a control medium. This suggests that, by using ultrafiltration as an alternative to heat treatment, agricultural wastewaters could be utilized as a nutrient source for microalgae.

Oil production from pilot-scale microalgae cultivation: An economics evaluation

ABSTRACTOf the potential sources of renewables, the most promising one is microalgae, which are viewed as a biofuel feedstock and technological solution for CO2 sequestration. Among microalgae-derived biofuels, biodiesel is one of the best application choices since microalgae have substantial amounts of oils. Nonetheless, microalgal biodiesel production is still in its infancy, since there is no commercial production on a large scale. The economic data and evaluation related to microalgal biodiesel production are incomplete and out of date, although lots of technological research has been carried out. This article demonstrates the pilot scale cultivation of microalgae in photobioreactors and evaluates the economics of producing oil from microalgae, including net energy ratio and cost-effectiveness evaluation. The contribution of this article lies in the presentation of an updated analysis of economic conditions for pilot scale microalgal oil production, which can provide the direction for future development of this important biofuel process.

DETERMINATION OF OPTIMAL ALGAE CONCENTRATION FOR CONTINUOUS GROWTH OF Scenedesmus sp. IN CHU AND MODIFIED CHU MEDIA

The high cost of production in the cultivation of microalgae in photobioreactors is a barrier to be overcome, therefore is necessary to search to reduce the cost of cultivation. The objectives of this work are to compare the growth of the microalgae Scenedesmus sp. being cultivated in CHU medium and in an alternative medium composed mainly of fertilizer and to establish guidelines for growth under continuous mode. The research approach consisted on growing the microalgae in Erlenmeyer type photobioreactors and producing thegrowth curves for both media. The comparisons between the two, were made based upon three parameters: dry biomass, absorbance, and total lipid quantity. The experiments were made in triplicates in order to quantify the uncertainty in the measurements. The comparison of the cultivation in both media suggest that the modification of the chemical nutrients by fertilizer did not produce significant changes in the number of cells, dry biomass and lipids. In spite of that, the modification resulted in 20% cost reduction in the growth medium. In this way, the substitution of conventional nutrients by fertilizers, constitute an interesting alternative for the production of biomass from microalgae leading to a cost reduction. The results are used to develop strategies for continuous growth.