CO2 capture from air by Chlorella vulgaris microalgae in an airlift photobioreactor

Unleashing the power of microalgae: a pioneering path to sustainability and achieving the sustainable development goals

This study explores the remarkable potential of algae in addressing global sustainability challenges. Microalgae, in particular, emerge as sustainability champions. Their applications span an impressive array of industries and processes, including food and feed production, biofuels, cosmetics, pharmaceuticals, and environmental remediation. This versatility positions algae as key players in achieving over 50% of UN Sustainable Development Goals (SDGs) simultaneously, addressing issues such as climate action, clean water and sanitation, affordable and clean energy, and zero hunger. From sequestering carbon, purifying wastewater, and producing clean energy to combating malnutrition, algae demonstrates unparalleled potential. Their ability to flourish in extreme conditions and their rapid growth rates further enhance their appeal for large-scale cultivation. As research advances, innovative applications continue to emerge, such as algae-based bioplastics and dye-sensitized solar cells, promising novel solutions to pressing global issues. This study illuminates how harnessing the power of algae can drive us towards a more resilient, sustainable world. By leveraging algae's multifaceted capabilities, we can tackle climate change, resource scarcity, and economic development concurrently. The research highlights the critical role of algae in promoting circular economy principles and achieving a harmonious balance between human needs and environmental preservation, paving the way for a greener, more sustainable future.

CFD modeling of CO2 fixation by microalgae cultivated in a lab scale photobioreactor

The green microalga Chlorella vulgaris has been studied for efficient fixation of carbon dioxide, CO2, from elevated concentrations of in-feed gas flows. In this work, a Computational Fluid Dynamics (CFD) model for algae cultivation including the dependence on CO2 concentration in liquid has been derived based on cultivation experiments of Chlorella vulgaris. The experiments were performed in a laboratory scale cylindrical stirred tank reactor with a range of enriched CO2 concentrations in-feed (0.04-50 %). The model provided by Béchet et al. (2015) for algae cultivation considering dependence on local light, algae concentration and temperature has been implemented for CFD and modified for comprising CO2 dependence. The model has been verified with experimental results for the algae productivity showing the proper trends for dissolved CO2 concentrations and algae concentrations. Thus, the developed CFD model can serve as a tool for evaluating photobioreactor performance for CO2 capture.

Usage of Chlorella and diverse microalgae for CO2 capture - towards a bioenergy revolution

To address climate change threats to ecosystems and the global economy, sustainable solutions for reducing atmospheric carbon dioxide (CO2) levels are crucial. Existing CO2 capture projects face challenges like high costs and environmental risks. This review explores leveraging microalgae, specifically the Chlorella genus, for CO2 capture and conversion into valuable bioenergy products like biohydrogen. The introduction section provides an overview of carbon pathways in microalgal cells and their role in CO2 capture for biomass production. It discusses current carbon credit industries and projects, highlighting the Chlorella genus's carbon concentration mechanism (CCM) model for efficient CO2 sequestration. Factors influencing microalgal CO2 sequestration are examined, including pretreatment, pH, temperature, irradiation, nutrients, dissolved oxygen, and sources and concentrations of CO2. The review explores microalgae as a feedstock for various bioenergy applications like biodiesel, biooil, bioethanol, biogas and biohydrogen production. Strategies for optimizing biohydrogen yield from Chlorella are highlighted. Outlining the possibilities of further optimizations the review concludes by suggesting that microalgae and Chlorella-based CO2 capture is promising and offers contributions to achieve global climate goals.

Interchangeable modular design and operation of photo-bioreactors for Chlorella vulgaris cultivation towards a zero-waste biorefinery

This study explores diverse cultivation modes for Chlorella vulgaris within a biorefinery at pilot scale that produces both biodiesel by transesterification of waste frying oils and syngas by gasification of organic wood waste. Given microalgae's comparatively modest biofuel yield relative to principal biorefinery products, the microalgae cultivation process is designed on the biofuels production rates. Liquid and gaseous waste streams are recycled inside the biorefinery: crude glycerol is mixed with wood to enhance the quality of syngas, wastewater is fed to microalgae so as flue gas. Also, the oil extracted from microalgae contributes to produce biodiesel and the waste cells are gasified. Considering that the optimal fit for each cultivation mode varies with the shape of the reactor, we propose a modular approach to assemble them in batteries of tubular, bubble flow, and airlift reactors, and present an operating design criterion that can fulfill the mass balance of the plant by adding/transforming the number of units inside the different batteries. Methods to adjust the operating conditions and control the operating parameters are also discussed. The designed configurations were operated recycling nominal waste streams of about 30 L d-1 of wastewater and 90 Nm3 h-1 of flue gas. Results confirm that the most advantageous one, in terms of volume per recycled waste streams, is a battery of 16 airlift reactors, operating in mixotrophic mode, with growing rate of 0.427 d-1, yield of 3.06, glycerol conversion 39 %, CO2 removal 64 % of inlet 6-10 %(mol) concentration. The same nominal waste streams can also be managed by 40 tubular reactors in almost heterotrophic conditions coupled with 12 bubble columns in autotrophic conditions; working respectively at growing rates of 0.395 d-1 and 0.362 d-1 and yields of 2.94 and 2.84. The battery of tubular reactors reached a glycerol conversion of 45 % and the array of bubble columns removed about 51 % of inlet 12-20 %(mol) CO2 concentration. A complete comparison is reported also in terms of dimensionless numbers and pumping/mixing requirements.

Photobioreactor configurations in cultivating microalgae biomass for biorefinery

Microalgae, highly prized for their protein, lipid, carbohydrate, phycocyanin, and carotenoid-rich biomass, have garnered significant industrial attention in the context of third-generation (3G) biorefineries, seeking sustainable alternatives to non-renewable resources. Two primarily cultivation methods, open ponds and closed photobioreactors systems, have emerged. Open ponds, favored for their cost-effectiveness in large-scale industrial production, although lacking precise environmental control, contrast with closed photobioreactors, offering controlled conditions and enhanced biomass production at the laboratory scale. However, their high operational costs challenge large-scale deployment. This review comprehensively examines the strength, weakness, and typical designs of both outdoor and indoor microalgae cultivation systems, with an emphasis on their application in terms of biorefinery concept. Additionally, it incorporates techno-economic analyses, providing insights into the financial aspects of microalgae biomass production. These multifaceted insights, encompassing both technological and economic dimensions, are important as the global interest in harnessing microalgae's valuable resources continue to grow.

Utilization of microalgal-bacterial energy nexus improves CO2 sequestration and remediation of wastewater pollutants for beneficial environmental services

Carbon dioxide (CO2) emissions from the combustion of fossil fuels and coal are primary contributors of greenhouse gases leading to global climate change and warming. The toxicity of heavy metals and metalloids in the environment threatens ecological functionality, diversity and global human life. The ability of microalgae to thrive in harsh environments such as industrial wastewater, polluted lakes, and contaminated seawaters presents new, environmentally friendly, and less expensive CO2 remediation solutions. Numerous microalgal species grown in wastewater for industrial purposes may absorb and convert nitrogen, phosphorus, and organic matter into proteins, oil, and carbohydrates. In any multi-faceted micro-ecological system, the role of bacteria and their interactions with microalgae can be harnessed appropriately to enhance microalgae performance in either wastewater treatment or algal production systems. This algal-bacterial energy nexus review focuses on examining the processes used in the capture, storage, and biological fixation of CO2 by various microalgal species, as well as the optimized production of microalgae in open and closed cultivation systems. Microalgal production depends on different biotic and abiotic variables to ultimately deliver a high yield of microalgal biomass.

Eco-friendly approach for tannery effluent treatment and CO2 sequestration using unicellular green oleaginous microalga Tetradesmus obliquus TS03

The present study explored the process of bioremediation, sequestration of carbon dioxide, and biofuel production using multifarious potent freshwater microalgae Tetradesmus obliquus TS03. The heavy metals were reduced, viz., 8.34 mg of cadmium (95.13%), 4.56 mg of chromium (97.28%), 1.34 mg of copper (98.67%), 1.24 mg of cobalt (98.19%), 1.93 mg of lead (96.72%), 2.31 mg of nickel (97.14%), and 2.23 mgL^-1 of zinc (96.59%) using photobioreactor microalgal treatment method. The heavy metal biosorption capacity rate (q_max) was 98.90% determined by the Langmuir and Freundlich isotherm kinetics model at 10 days of effluent treatment using Tetradesmus obliquus TS03. The microalgae T. obliquus TS03 utilized 98.34% of carbon dioxide (CO₂) enhanced by acetyl CoA carboxylase and RuBisCO enzymes. The biodiesel was extracted from microalga and identified 32 fatty acid methyl ester major compounds viz., tetradecanoate methyl ester, hexadecanoic acid methyl ester, tridecanoic acid methyl ester, heptadecatrienoic acid methyl ester, octadecanoic acid methyl ester, eicosanoic acid methyl ester, pentadecanoic acid methyl ester, and cis-methylicosanoate using gas mass chromatography (GCMS). The biodiesel functional groups were identified, viz., amides, phenols, alcohols, alkynes, carboxylic acids, carbonyls, and ketones groups using Fourier transformation infrared (FTIR). The bioethanol was identified using high-performance liquid chromatography (HPLC) and determined the peak presented at RT of 4.35 min (75,693.1046 µV s^-1).

Hydrodynamics and mass transfer of concentric-tube internal loop airlift reactors: A review

The concentric-tube internal loop airlift reactor is a typical reactor configuration which has been adopted for a myriad of chemical and biological processes. The reactor hydrodynamics (including mixing) and the mass transfer between the gas and liquid phases remarkably affect the operational conditions and thus are crucial to the overall reactor performance. Hence, this study aims at providing a thorough description of the basic concepts and a comprehensive review of the relevant reported studies on the hydrodynamics and mass transfer of the concentric-tube internal loop airlift reactors, taking microalgae cultivation as an exemplary application. In particular, the reactor characteristics, geometry, CFD modeling, experimental characterization, and scale up considerations are elucidated. The research gaps for future research and development are also identified.

Current perspective on wastewater treatment using photobioreactor for Tetraselmis sp.: an emerging and foreseeable sustainable approach

Urbanization is a revolutionary and necessary step for the development of nations. However, with development emanates its drawback i.e., generation of a huge amount of wastewater. The existence of diverse types of nutrient loads and toxic compounds in wastewater can reduce the pristine nature of the ecosystem and adversely affects human and animal health. The conventional treatment system reduces most of the chemical contaminants but their removal efficiency is low. Thus, microalgae-based biological wastewater treatment is a sustainable approach for the removal of nutrient loads from wastewater. Among various microalgae, Tetraselmis sp. is a robust strain that can remediate industrial, municipal, and animal-based wastewater and reduce significant amounts of nutrient loads and heavy metals. The produced biomass contains lipids, carbohydrates, and pigments. Among them, carbohydrates and lipids can be used as feedstock for the production of bioenergy products. Moreover, the usage of a photobioreactor (PBR) system improves biomass production and nutrient removal efficiency. Thus, the present review comprehensively discusses the latest studies on Tetraselmis sp. based wastewater treatment processes, focusing on the use of different bioreactor systems to improve pollutant removal efficiency. Moreover, the applications of Tetraselmis sp. biomass, advancement and research gap such as immobilized and co-cultivation have also been discussed. Furthermore, an insight into the harvesting of Tetraselmis biomass, effects of physiological, and nutritional parameters for their growth has also been provided. Thus, the present review will broaden the outlook and help to develop a sustainable and feasible approach for the restoration of the environment.

Integrating eco-technological approach for textile dye effluent treatment and carbon dioxide capturing from unicellular microalga Chlorella vulgaris RDS03: a synergistic method

A pilot-scale treatment method was used in the present study to test the biosorption of textile dye from textile effluent and carbon dioxide using Chlorella vulgaris RDS03. The textile dye effluent treatment achieved that textile dye biosorption capacity (q_max) rate of 98.84% on 15 days of treatment using Chlorella vulgaris RDS03. The Langmuir and Freundlich isotherm kinetics model indicated that the higher R² value 0.98. The microalga Chlorella vulgaris RDS03 was captured-96.86% of CO₂ analyzed by CO₂ utilization and biofixation kinetics, 4.65 mgmL^-1 of biomass, 189.26 mgg^-1 of carbohydrate, 233.89 mgg^-1 of lipid, 4.3 mLg^-1 of bioethanol and 4.9 mLg^-1 of biodiesel produced. We performed fatty acid methyl ester (FAME) profiling using gas chromatography-mass spectrometry (GCMS). We found 40 types of biodiesel compounds, specifically myristic acid, pentadecanoic acid, octadecanoic acid, palmitic acid, and oleic acid. The high-performance liquid chromatography (HPLC) validated and analyzed the produced bioethanol.Novelty of the Research• Unicellular microalga Chlorella vulgaris RDS03 was isolated from the freshwater region and investigated their biosorption efficiency against hazardous synthetic azo textile dyes.• Chlorella vulgaris RDS03 ability to biosorption 96.86% of environmental polluted carbon dioxide• Treated biomass was used to produce ecofriendly, unpolluted and green energy such as biofuels (biodiesel and bioethanol).

Investigation of Hydrodynamic Parameters in an Airlift Photobioreactor on CO2 Biofixation by Spirulina sp

The rise of CO2 concentration on Earth is a major environmental problem that causes global warming. To solve this issue, carbon capture and sequestration technologies are becoming more and more popular. Among them, cyanobacteria can efficiently sequestrate CO2, which is an eco-friendly and cost-effective way of reducing carbon dioxide, and algal biomass can be harvested as valuable products. In this study, the hydrodynamic parameters of an airlift photobioreactor such as gas holdup, mean bubble diameter and liquid circulation velocity were measured to investigate CO2 biofixation by Spirulina sp. The total gas holdup was found to increase linearly with the increase in the gas velocity from 0.185 to 1.936 cm/s. The mean bubble velocities in distilled water only and in the cyanobacterial culture on the first and sixth days of cultivation were 109.97, 87.98, and 65.89 cm/s, respectively. It was found that shear stress at gas velocities greater than 0.857 cm/s led to cyanobacterial death. After 7 days of batch culture, the maximum dry cell weight reached 1.62 g/L at the gas velocity of 0.524 cm/s, whereas the highest carbon dioxide removal efficiency by Spirulina sp. was 55.48% at a gas velocity of 0.185 cm/s, demonstrating that hydrodynamic parameters applied in this study were suitable to grow Spirulina sp. in the airlift photobioreactor and remove CO2.

Two-stage hybrid microalgal electroactive wetland-coupled anaerobic digestion for swine wastewater treatment in South China: Full-scale verification

Constructed wetlands have been widely used for organic wastewater treatment owing to low operating costs and simple maintenance. However, there are some disadvantages such as unstable efficiency in winter. In this study, a microalgal electroactive biofilm-constructed wetland was coupled with anaerobic digestion for full-scale treatment of swine wastewater. In a 12-month outdoor trial, the overall removal efficiencies of chemical oxygen demand, ammonium, nitrate, total nitrogen, total phosphorus, and nitrite reached 98.26%/95.14%, 97.96%/92.07%, 85.45%/66.04%, 95.07%/91.48%, 91.44%/91.52%, and 85.45%/84.67% in summer/winter, respectively. Hydrolytic bacteria were dominant in the anaerobic digestion part, and Cyanobium, Shewanella, and Azoarcus were enriched in the microalgal electroactive biofilm. The operating cost of the entire system was approximately 0.118 $/m³ of wastewater. These results confirm that the microalgal electroactive biofilm significantly enhances the efficiency and stability of constructed wetlands. In conclusion, the anaerobic digestion-microalgal electroactive biofilm-constructed wetland is technically and economically feasible for the treatment of swine wastewater.

Biochemical and Morphological Changes Triggered by Nitrogen Stress in the Oleaginous Microalga Chlorella vulgaris

Oleaginous microalgae have been considered promising sources of biodiesel due to their high lipid content. Nitrogen limitation/starvation is one of the most prominent strategies to induce lipid accumulation in microalgae. Nonetheless, despite numerous studies, the mechanism underlying this approach is not well understood. The aim of this study was to investigate the effect of nitrogen limitation and starvation on biochemical and morphological changes in the microalga Chlorella vulgaris FACHB-1068, thereby obtaining the optimal nitrogen stress strategy for maximizing the lipid productivity of microalgal biomass. The results showed that nitrogen limitation (nitrate concentration < 21.66 mg/L) and starvation enhanced the lipid content but generally decreased the biomass productivity, pigment concentration, and protein content in algal cells. Comparatively, 3-day nitrogen starvation was found to be a more suitable strategy to produce lipid-rich biomass. It resulted in an increased biomass production and satisfactory lipid content of 266 mg/L and 31.33%, respectively. Besides, nitrogen starvation caused significant changes in cell morphology, with an increase in numbers and total size of lipid droplets and starch granules. Under nitrogen starvation, saturated fatty acids (C-16:0, C-20:0, and C-18:0) accounted for the majority of the total fatty acids (~80%), making C. vulgaris FACHB-1068 a potential feedstock for biodiesel production. Our work may contribute to a better understanding of the biochemical and morphological changes in microalgae under nitrogen stress. Besides, our work may provide valuable information on increasing the lipid productivity of oleaginous microalgae by regulating nitrogen supply.

Microalgae as a solution of third world energy crisis for biofuels production from wastewater toward carbon neutrality: An updated review

The boost of the greenhouse gases (GHGs, largely carbon dioxide - CO₂) emissions owing to anthropogenic activity is one of the biggest global threats. Bio-CO₂ emission reduction has received more and more attention as an environmentally sustainable approach. Microalgae are very popular in this regard because of excellent speed of growth, low costs of production, and resistance to extreme environments. Besides, most microalgae can undergo photosynthesis, where the CO₂ and solar energy can be converted into sugar, and subsequently become biomass, providing a renewable and promising biofuel strategy with a few outstanding benefits. This review focuses on presenting CO₂ sequestration by microalgae towards wastewater treatment and biodiesel production. First, the CO₂ fixation mechanism by microalgae viz., sequestration and assimilation of CO₂ in green microalgae as well as cyanobacteria were introduced. Besides, factors affecting CO₂ sequestration in microalgae, containing microalgae species and cultivation conditions, such as light condition, photobioreactor, configuration, pH, CO₂ concentration, temperature, and medium composition, were then comprehensively discussed. Special attention was given to the production of biodiesel as third-generation biofuel from various wastewater (CO₂ biofixation), including processing steps of biodiesel production by microalgae, biodiesel production from wastewater, and improved methods. Furthermore, current life cycle assessment (LCA) and techno-economic analysis (TEA) used in biodiesel production were discussed. Finally, the research challenges and specific prospects were considered. Taken together, this review provides useful and updated information to facilitate the development of microalgal "green chemistry" and "environmental sustainability".

Role of Microalgae in Global CO2 Sequestration: Physiological Mechanism, Recent Development, Challenges, and Future Prospective

The rising concentration of global atmospheric carbon dioxide (CO2) has severely affected our planet’s homeostasis. Efforts are being made worldwide to curb carbon dioxide emissions, but there is still no strategy or technology available to date that is widely accepted. Two basic strategies are employed for reducing CO2 emissions, viz. (i) a decrease in fossil fuel use, and increased use of renewable energy sources; and (ii) carbon sequestration by various biological, chemical, or physical methods. This review has explored microalgae’s role in carbon sequestration, the physiological apparatus, with special emphasis on the carbon concentration mechanism (CCM). A CCM is a specialized mechanism of microalgae. In this process, a sub-cellular organelle known as pyrenoid, containing a high concentration of Ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco), helps in the fixation of CO2. One type of carbon concentration mechanism in Chlamydomonas reinhardtii and the association of pyrenoid tubules with thylakoids membrane is represented through a typical graphical model. Various environmental factors influencing carbon sequestration in microalgae and associated techno-economic challenges are analyzed critically.

A Review on the Effect from Steel Slag on the Growth of Microalgae

As a by-product from the metallurgical industry, steel slag contains a large amount of metal elements. In many developing countries, the output of steel slag is huge and the comprehensive utilization rate is low, hence the development of a novel application method for steel slag is of great significance to increase its utilization rate to improve the environment. This paper reviewed the dissolution behavior of Fe, P, Ca and silicate of steel slag under seawater and acidic solutions as an application in the cultivation of different microalgae, such as diatoms, spirulina, and chlorella. This review clarifies that proper pre-treatment of steel slag can effectively increase the dissolved elements of steel slag in the solution and provide more nutrients for the growth of microalgae. Microalgae cultivated with steel slag as a nutrient can be used to produce biodiesel which has a very broad application prospects for cleaner production and environmental protection.

Strengthening mass transfer with the Tesla-valve baffles to increase the biomass yield of Arthrospira platensis in a column photobioreactor

In this study, the Tesla-valve (TV) baffles were used to optimize the flow field in a column photobioreactor (PBR) in order to promote mass transfer of CO₂ gas in the solution. The TV baffles were composed of many tilted plates with central holes and curved arcs facing downwards, installed along inner rising section of the column PBR. Many clockwise and anti-clockwise vortices were generated during the rising flow while passing through proposed TV baffles. An optimum TV baffle structure (30° plate angle, 8 cm arc width) decreased mixing time by 36.4% and increased the mass transfer coefficient by 50%. The TV baffles supported the movement of the A.platensis cells between light and dark regions to enhance their photochemical efficiency ϕPSII by 24.6% and Fv/Fm by 12.7%. Therefore, the biomass yield increased by 28.1% and exhibited an increased helix pitch and trichome length in comparison with traditional column PBR without baffles.

Impacts of Endocrine Disruptor di-n-Butyl Phthalate Ester on Microalga Chlorella vulgaris Verified by Approaches of Proteomics and Gene Ontology

Di-n-butyl phthalate (DBP) is an extensively used plasticizer. Most investigations on DBP have been concentrated on its environmental distribution and toxicity to humans. However, information on the effects of plasticizers on algal species is scarce. This study verified the impacts of endocrine disruptor di-n-butyl phthalate ester on microalga Chlorella vulgaris by approaches of proteomics and gene ontology. The algal acute biotoxicity results showed that the 24h-EC50 of DBP for C. vulgaris was 4.95 mg L^-1, which caused a decrease in the chlorophyll a content and an increase in the DBP concentration of C. vulgaris. Proteomic analysis led to the identification of 1257 C. vulgaris proteins. Sixty-one more proteins showed increased expression, compared to proteins with decreased expression. This result illustrates that exposure to DBP generally enhances protein expression in C. vulgaris. GO annotation showed that both acetolactate synthase (ALS) and GDP-L-fucose synthase 2 (GER2) decreased more than 1.5-fold after exposure to DBP. These effects could inhibit both the valine biosynthetic process and the nucleotide-sugar metabolic process in C. vulgaris. The results of this study demonstrate that DBP could inhibit growth and cause significant changes to the biosynthesis-relevant proteins in C. vulgaris.

Pilot scale wastewater treatment, CO2 sequestration and lipid production using microalga, Neochloris aquatica RDS02

In present investigation carried out large-scale treatment of tannery effluent by the cultivation of microalgae, Neochloris aquatica RDS02. The tannery effluent treatment revealed that significant reduction heavy metals were chromium-3.59, lead-2.85, nickel-1.9, cadmium-10.68, zinc-4.49, copper-0.95 and cobalt-1.86 mg/L on 15th day of treatment using N. aquatica RDS02. The microalgal biosorption capacity q _max rate was Cr-88.66, Pb-75.87, Ni-87.61, Cd-60.44, Co-52.86, Zn-84.90 and Cu-54.39, and isotherm model emphasized that the higher R ² value 0.99 by Langmuir and Freundlich kinetics model. The microalga utilized highest CO₂ (90%) analyzed by CO₂ biofixation and utilization kinetics, biomass (3.9 mg/mL), lipid (210 mg mL^-1), carbohydrate (102.75 mg mL^-1), biodiesel (4.9 mL g^-1) and bioethanol (4.1 mL g^-1). The microalgal-lipid content was analyzed through Nile red staining. Gas chromatography mass spectrometric (GCMS) analysis confirmed that the presence of a biodiesel and major fatty acid methyl ester (FAME) profiling viz., tridecanoic acid methyl ester, pentadecanoic acid methyl ester, octadecanoic acid methyl ester, myristic acid methyl ester, palmitic acid methyl ester and oleic acid methyl ester. Fourier transform infrared (FTIR) analysis confirmed that the presence of a functional groups viz., phenols, alcohols, alkynes, carboxylic acids, ketones, carbonyl and ester groups. The bioethanol production was confirmed by high-performance liquid chromatography (HPLC) analyze.

Chlorella vulgaris cultivation in airlift photobioreactor with transparent draft tube: effect of hydrodynamics, light and carbon dioxide on biochemical profile particularly ω-6/ω-3 fatty acid ratio

Chlorella vulgaris is used for food and feed applications due to its nutraceutical, antioxidant and anticancer properties. An airlift photobioreactor comprising transparent draft tube was used for C. vulgaris cultivation. The effect of reactor parameters like hydrodynamics (0.3-1.5 vvm), light intensity (85-400 μmol m^-2 s^-1), photoperiod (12-24 h) and gas-phase carbon dioxide (CO₂) concentration (5-15% v/v) were evaluated on microalgae and associated bacterial growth, biochemical profile; with special emphasis on ω-3, ω-6 fatty acids, and vitamin B₁₂. The optimal growth of C. vulgaris without CO₂ supplementation was observed at 1.2 vvm, which was associated with higher algal productivity, chlorophyll, vitamin B₁₂ content, and bacterial load along with 72% of nitrate removal. The higher light intensity (400 μmol m^-2 s^-1) and photoperiod (24:0) increased biomass productivity and ω-3 fatty acid content (in lipid) up to 2-3 fold. The elevated levels of gas-phase CO₂ concentration (15% v/v) enhanced EPA content up to 7% and biomass productivity up to 171 mg L^-1 day^-1. However, the increase in CO₂ concentration lowered vitamin B₁₂ content (up to 30%) and bacterial load (2-3 log). Also, all the cultivation conditions favoured desirable ω-6/ω-3 ratio(in the range of 1-2).

Carbon biofixation and lipid composition of an acidophilic microalga cultivated on treated wastewater supplied with different CO2 levels

This study evaluated productivity, CO₂ biofixation, and lipid content in biomass of the acidophilic microalga Chlamydomonas acidophila LAFIC-004 cultivated with five different carbon dioxide concentrations. The influence of carbon dioxide concentration on nutrient removal and pH was also investigated. Treated wastewater (secondary effluent) was used as culture medium. Five experimental setups were tested: T-0% - injection of atmospheric air (0.038% CO₂), T-5% (5% CO₂), T-10% (10% CO₂), T-15% (15% CO₂) and T-20% (20% CO₂). The T-5% and T-10% experiments showed the highest values of productivity and CO₂ biofixation, and maximum biomass dry weight was 0.48 ± 0.02 and 0.51 ± 0.03 g L^-1, respectively. This acidophilic microalga proved to be suitable for carbon biofixation and removal of nutrients from secondary effluent of wastewater treatment plants with high CO₂ concentration. All assays were performed without pH control. This microalga species presented high lipid content. However, fatty acid methyl esters (FAME) are not suitable for biodiesel use.

Carbon dioxide fixation coupled with ammonium uptake by immobilized Scenedesmus obliquus and its potential for protein production

In this study, immobilized Scenedesmus obliquus (S. obliquus) was proposed to simultaneously alleviate the carbon dioxide (CO₂) and ammonium (NH₄⁺-N). Two trophic modes of autotrophy and mixotrophy were conducted by batch experiments with a period of 5 days. The results shown that NH₄⁺-N could be removed more efficiently if algal cells were immobilized, and the trophic mode change had no significant effect on immobilized S. obliquus to NH₄⁺-N removal under 5% CO₂ sparging. Specifically, immobilized S. obliquus could remove NH₄⁺-N completely at initial concentrations of 30 and 50 mg/L and reached about 80% removal rate of NH₄⁺-N at the concentration of 70 mg/L under both trophic modes. The protein synthesis was its main removal mechanism and the dominant amino acid components including glutamic acid (Glu), cystine (Cys), arginine (Arg), methionine (Met) and lysine (Lys) were sensitive to NH₄⁺-N assimilation.

Bio-fixation of flue gas from thermal power plants with algal biomass: Overview and research perspectives

Rate of energy production is reflecting growth of nations and most of energy produced from the coal and natural gas-based thermal power plants (TPPs). Flue gas (point sources of emission) are main exhaustible form of gases that come from thermal power plants and are continuously promoting climate change and various environmental problems in global scenario. The present available technologies of flue gas treatment are energy and cost-intensive process. Among the available techniques for fixation of flue-gases at sustainable part, microalgal bio-fixation of flue gas is an alternative promising and competent technology with assurance of eco-friendly path of low energy and low-cost solution for pollution abetment with production of value added products. According to mechanism involves during photosynthetic process of microalgae, it utilizes atmospheric CO₂ and CO₂ from flue gases for their growth. Past, present and future treatment technologies for flue gas with their challenges are discussed. Recent experimental studies and commercially available bioreactors are very particular for bio-fixation of flue gas from thermal power plants are also reviewed with their future perspectives. The commercial viability of process with specific microalgal strains and utilized biomass for further value-added products are suggested with future limitations.

A critical review on designs and applications of microalgae-based photobioreactors for pollutants treatment

The development of the photobioreactors (PBs) is recently noticeable as cutting-edge technology while the correlation of PBs' engineered elements such as modellings, configurations, biomass yields, operating conditions and pollutants removal efficiency still remains complex and unclear. A systematic understanding of PBs is therefore essential. This critical review study is to: (1) describe the modelling approaches and differentiate the outcomes; (2) review and update the novel technical issues of PBs' types; (3) study microalgae growth and control determined by PBs types with comparison made; (4) progress and compare the efficiencies of contaminants removal given by PBs' types and (5) identify the future perspectives of PBs. It is found that Monod model's shortcoming in internal substrate utilization is well fixed by modified Droop model. The corroborated data also remarks an array of PBs' types consisting of flat plate, column, tubular, soft-frame and hybrid configuration in which soft-frame and hybrid are the latest versions with higher flexibility, performance and smaller foot-print. Flat plate PBs is observed with biomass yield being 5 to 20 times higher than other PBs types while soft-frame and membrane PBs can also remove pharmaceutical and personal care products (PPCPs) up to 100%. Looking at an opportunity for PBs in sustainable development, the flat plate PBs are applicable in PB-based architectures and infrastructures indicating an encouraging revenue-raising potential.

Technology Evolution in Membrane-Based CCS

In recent years, many CO2 capture technologies have been developed due to growing awareness about the importance of reducing greenhouse gas emissions. In this paper, publications from the last decade addressing this topic were analyzed, paying special attention to patent status to provide useful information for policymakers, industry, and businesses and to help determine the direction of future research. To show the most current patent activity related to carbon capture using membrane technology, we collected 2749 patent documents and 572 scientific papers. The results demonstrated that membranes are a developing field, with the number of applications growing at a steady pace, exceeding 100 applications per year in 2013 and 2014. North American assignees were the main contributors, with the greatest number of patents owned by companies such as UOP LLC, Kilimanjaro Energy Inc., and Membrane Technology and Research Inc., making up 26% of the total number of published patents. Asian countries (China, Japan, and Korea) and international offices were also important knowledge sources, providing 29% and 24% of the documents, respectively. Furthermore, this paper highlights 10 more valuable patents regarding their degree of innovation and citations, classified as Y02C 10/10 according to the Cooperative Patent Classification (CPC) criteria.

Generating cycle flow between dark and light zones with double paddlewheels to improve microalgal growth in a flat plate photo-bioreactor

Double paddlewheels were proposed to generate cycle flow for increasing horizontal fluid velocity between dark and light zones in a flat plate photo-bioreactor, which strengthened the mass transfer and the mixing effect to improve microalgal growth with 15% CO₂. Numerical fluid dynamics were used to simulate the cycle flow field with double paddlewheels. The local flow field measured with particle image velocimetry fitted well with the numerical simulation results. The horizontal fluid velocity in the photo-bioreactor was markedly increased from 5.8 × 10^-5 m/s to 0.45 m/s with the rotation of double paddlewheels, resulting in a decreased dark/light cycle period. Therefore, bubble formation time and diameter reduced by 24.4% and 27.4%, respectively. Meanwhile, solution mixing time reduced by 31.3% and mass transfer coefficient increased by 41.2%. The biomass yield of microalgae Nannochloropsis Oceanic increased by 127.1% with double paddlewheels under 15% CO₂ condition.

Airlift bioreactor system for simultaneous removal of hydrogen sulfide and ammonia from synthetic and actual waste gases

The effectiveness of an airlift reactor system in simultaneously removing hydrogen sulfide (H₂S) and ammonia (NH₃) from synthetic and actual waste gases was investigated. The effects of various parameters, including the ratio of inoculum dilution, the gas concentration, the gas retention time, catalyst addition, the bubble size, and light intensity, on H₂S and NH₃ removal were investigated. The results revealed that optimal gas removal could be achieved by employing an activated inoculum, using a small bubble stone, applying reinforced fluorescent light, adding Fe₂O₃ catalysts, and applying a gas retention time of 20 s. The shock loading did not substantially affect the removal efficiency of the airlift bioreactor. Moreover, more than 98.5% of H₂S and 99.6% of NH₃ were removed in treating actual waste gases. Fifteen bands or species were observed in a profile from denaturing gradient gel electrophoresis during waste gas treatment. Phylogenetic analysis revealed the phylum Proteobacteria to be predominant. Six bacterial strains were consistently present during the entire operating period; however, only Rhodobacter capsulatus, Rhodopseudomonas palustris, and Arthrobacter oxydans were relatively abundant in the system. The photosynthetic bacteria R. capsulatus and R. palustris were responsible for H₂S oxidation, especially when the reinforced fluorescent light was used. The heterotrophic nitrifier A. oxydans was responsible for NH₃ oxidation. To our knowledge, this is the first report on simultaneous H₂S and NH₃ removal using an airlift bioreactor system. It clearly demonstrates the effectiveness of the system in treating actual waste gases containing H₂S and NH₃.

Computational fluid dynamics modeling of carbon dioxide capture from air using biocatalyst in an airlift reactor

In this work, a novel three-phase and three dimensional CFD model based on the Euler-Euler approach developed for modeling the hydrodynamic, mass transfer and CO₂ fixation using microalgae in an internal loop airlift reactor with internal sparger. The main objective of this work was to development of CFD codes in order to simulate the CO₂ biofixation process under different input gas velocity during 11 days of culture time and simulate the unsteady state mass transfer based on Surface-Renewal-Stretch (SRS) model. The CFD results were compared with the our previous experimental work and they showed good agreement with a margin of less than 10%. This paper illustrated the ability of the CFD in complex process simulation such as CO₂ biofixation in the airlift reactor and provided a useful basis for further study.

Impact of Flue Gas Compounds on Microalgae and Mechanisms for Carbon Assimilation and Utilization

To shift the world to a more sustainable future, it is necessary to phase out the use of fossil fuels and focus on the development of low-carbon alternatives. However, this transition has been slow, so there is still a large dependence on fossil-derived power, and therefore, carbon dioxide is released continuously. Owing to the potential for assimilating and utilizing carbon dioxide to generate carbon-neutral products, such as biodiesel, the application of microalgae technology to capture CO₂ from flue gases has gained significant attention over the past decade. Microalgae offer a more sustainable source of biomass, which can be converted into energy, over conventional fuel crops because they grow more quickly and do not adversely affect the food supply. This review focuses on the technical feasibility of combined carbon fixation and microalgae cultivation for carbon reuse. A range of different carbon metabolisms and the impact of flue gas compounds on microalgae are appraised. Fixation of flue gas carbon dioxide is dependent on the selected microalgae strain and on flue gas compounds/concentrations. Additionally, current pilot-scale demonstrations of microalgae technology for carbon dioxide capture are assessed and its future prospects are discussed. Practical implementation of this technology at an industrial scale still requires significant research, which necessitates multidisciplinary research and development to demonstrate its viability for carbon dioxide capture from flue gases at the commercial level.