Large-scale biodiesel production using flue gas from coal-fired power plants with Nannochloropsis microalgal biomass in open raceway ponds

Advances in microalgae-based carbon sequestration: Current status and future perspectives

The advancement in carbon dioxide (CO2) sequestration technology has received significant attention due to the adverse effects of CO2 on climate. The mitigation of the adverse effects of CO2 can be accomplished through its conversion into useful products or renewable fuels. In this regard, microalgae is a promising candidate due to its high photosynthesis efficiency, sustainability, and eco-friendly nature. Microalgae utilizes CO2 in the process of photosynthesis and generates biomass that can be utilized to produce various valuable products such as supplements, chemicals, cosmetics, biofuels, and other value-added products. However, at present microalgae cultivation is still restricted to producing value-added products due to high cultivation costs and lower CO2 sequestration efficiency of algal strains. Therefore, it is very crucial to develop novel techniques that can be cost-effective and enhance microalgal carbon sequestration efficiency. The main aim of the present manuscript is to explain how to optimize microalgal CO2 sequestration, integrate valuable product generation, and explore novel techniques like genetic manipulations, phytohormones, quantum dots, and AI tools to enhance the efficiency of CO2 sequestration. Additionally, this review provides an overview of the mass flow of different microalgae and their biorefinery, life cycle assessment (LCA) for achieving net-zero CO2 emissions, and the advantages, challenges, and future perspectives of current technologies. All of the reviewed approaches efficiently enhance microalgal CO2 sequestration and integrate value-added compound production, creating a green and economically profitable process.

Universal and unique strategies for the production of polyunsaturated fatty acids in industrial oleaginous microorganisms

Polyunsaturated fatty acids (PUFAs), especially docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA) and arachidonic acid (ARA), are beneficial for reducing blood cholesterol and enhancing memory. Traditional PUFA production relies on extraction from plants and animals, which is unsustainable. Thus, using microorganisms as lipid-producing factories holds promise as an alternative way for PUFA production. Several oleaginous microorganisms have been successfully industrialized to date. These can be divided into universal and specialized hosts according to the products range of biosynthesis. The Yarrowia lipolytica is universal oleaginous host that has been engineered to produce a variety of fatty acids, such as γ-linolenic acid (GLA), EPA, ARA and so on. By contrast, the specialized host are used to produce only certain fatty acids, such as ARA in Mortierella alpina, EPA in Nannochloropsis, and DHA in Thraustochytrids. The metabolic engineering and fermentation strategies for improving PUFA production in universal and specialized hosts are different, which is the subject of this review. In addition, the widely applicable strategies for microbial lipid production that are not specific to individual hosts were also reviewed.

Solar bioreactors used for the industrial production of microalgae

Microalgae are excellent sources of biomass containing several important compounds for human and animal nutrition-proteins, lipids, polysaccharides, pigments and antioxidants as well as bioactive secondary metabolites. In addition, they have a great biotechnological potential for nutraceuticals, and pharmaceuticals as well as for CO2 sequestration, wastewater treatment, and potentially also biofuel and biopolymer production. In this review, the industrial production of the most frequently used microalgae genera-Arthrospira, Chlorella, Dunaliella, Haematococcus, Nannochloropsis, Phaeodactylum, Porphyridium and several other species is discussed as concerns the applicability of the most widely used large-scale systems, solar bioreactors (SBRs)-open ponds, raceways, cascades, sleeves, columns, flat panels, tubular systems and others. Microalgae culturing is a complex process in which bioreactor operating parameters and physiological variables closely interact. The requirements of the biological system-microalgae culture are crucial to select the suitable type of SBR. When designing a cultivation process, the phototrophic production of microalgae biomass, it is necessary to employ SBRs that are adequately designed, built and operated to satisfy the physiological requirements of the selected microalgae species, considering also local climate. Moreover, scaling up microalgae cultures for commercial production requires qualified staff working out a suitable cultivation regime. KEY POINTS: • Large-scale solar bioreactors designed for microalgae culturing. • Most frequently used microalgae genera for commercial production. • Scale-up requires suitable cultivation conditions and well-elaborated protocols.

An integration of algae-mediated wastewater treatment and resource recovery through anaerobic digestion

Eutrophication is one of the major emerging challenges in aquatic environment. Industrial facilities, including food, textile, leather, and paper, generate a significant amount of wastewater during their manufacturing process. Discharge of nutrient-rich industrial effluent into aquatic systems causes eutrophication, eventually disturbs the aquatic system. On the other hand, algae provide a sustainable approach to treat wastewater, while the resultant biomass may be used to produce biofuel and other valuable products such as biofertilizers. This review aims to provide new insight into the application of algal bloom biomass for biogas and biofertilizer production. The literature review suggests that algae can treat all types of wastewater (high strength, low strength, and industrial). However, algal growth and remediation potential mainly depend on growth media composition and operation conditions such as light intensity, wavelength, light/dark cycle, temperature, pH, and mixing. Further, the open pond raceways are cost-effective compared to closed photobioreactors, thus commercially applied for biomass generation. Additionally, converting wastewater-grown algal biomass into methane-rich biogas through anaerobic digestion seems appealing. Environmental factors such as substrate, inoculum-to-substrate ratio, pH, temperature, organic loading rate, hydraulic retention time, and carbon/nitrogen ratio significantly impact the anaerobic digestion process and biogas production. Overall, further pilot-scale studies are required to warrant the real-world applicability of the closed-loop phycoremediation coupled biofuel production technology.

Development of Microalgae Biodiesel: Current Status and Perspectives

Microalgae are regarded as a promising source of biodiesel. In contrast with conventional crops currently used to produce commercial biodiesel, microalgae can be cultivated on non-arable land, besides having a higher growth rate and productivity. However, microalgal biodiesel is not yet regarded as economically competitive, compared to fossil fuels and crop-based biodiesel; therefore, it is not commercially produced. This review provides an overall perspective on technologies with the potential to increase efficiency and reduce the general costs of biodiesel production from microalgae. Opportunities and challenges for large-scale production are discussed. We present the current scenario of Brazilian research in the field and show a successful case in the research and development of microalgal biodiesel in open ponds by Petrobras. This publicly held Brazilian corporation has been investing in research in this sector for over a decade.

New Angled Twin-layer Porous Substrate Photobioreactors for Cultivation of Nannochloropsis oculata

An angled twin-layer porous substrate photobioreactor (TL-PSBR) using LED light was designed to cultivate Nannochloropsis oculata. Flocculation and sedimentation by modification of pH to 11 were determined as the optimal method for harvesting the N. oculata inoculum. The following optimised parameters were found: tilt angle 15°, Kraft 220 g m^-2 paper as substrate material, initial inoculum density of 12.5 g m^-2, 140 µmol photons m^-2 s^-1 light intensity, and a light/dark cycle of 6:6 (h). Test cultivation for 14 days was performed under optimised conditions. The total dried biomass standing crop was 75.5 g m^-2 growth area with an average productivity of 6.3 g m^-2 d^-1, the productivity per volume of used culture medium was 126.2 mg/L d^-1, total lipid content 21.9% (w/w), and the highest productivity of total lipids was 1.33 g m^-2 d^-1. The dry algal biomass contained 3% eicosapentaenoic acid (w/w), 3.7% palmitoleic acid (w/w), and 513 mg kg^-1 vitamin E. The optimisation of N. oculata cultivation on an angled TL-PSBR system yielded promising results, and applications for commercial products need to be further explored.

Enhancement of violaxanthin accumulation in Nannochloropsis oceanica by overexpressing a carotenoid isomerase gene from Phaeodactylum tricornutum

Nannochloropsis has been considered as a promising feedstock for the industrial production of violaxanthin. However, a rational breeding strategy for the enhancement of violaxanthin content in this microalga is still vacant, thereby limiting its industrial application. All-trans-lycopene locates in the first branch point of carotenogenesis. The carotenoid isomerase (CRTISO), catalyzing the lycopene formation, is thus regarded as a key enzyme for carotenogenesis. Phaeodactylum tricornutum can accumulate high-level carotenoids under optimal conditions. Therefore, it is feasible to improve violaxanthin level in Nannochloropsis by overexpression of PtCRTISO. Protein targeting analysis of seven PtCRTISO candidates (PtCRTISO1–6 and PtCRTISO-like) demonstrated that PtCRTISO4 was most likely the carotenoid isomerase of P. tricornutum. Moreover, the transcriptional pattern of PtCRTISO4 at different cultivation periods was quite similar to other known carotenogenesis genes. Thus, PtCRTISO4 was transformed into N. oceanica. Compared to the wild type (WT), all three transgenic lines (T1–T3) of N. oceanica exhibited higher levels of total carotenoid and violaxanthin. Notably, T3 exhibited the peak violaxanthin content of 4.48 mg g^–1 dry cell weight (DCW), which was 1.68-folds higher than WT. Interestingly, qRT-polymerase chain reaction (PCR) results demonstrated that phytoene synthase (NoPSY) rather than ζ-carotene desaturase (NoZDS) and lycopene β-cyclase (NoLCYB) exhibited the highest upregulation, suggesting that PtCRTISO4 played an additional regulatory role in terms of carotenoid accumulation. Moreover, PtCRTISO4 overexpression increased C18:1n-9 but decreased C16:1n-7, implying that C18:1 may serve as a main feedstock for xanthophyll esterification in Nannochloropsis. Our results will provide valuable information for the violaxanthin production from Nannochloropsis.

Screening and application of Chlorella strains on biosequestration of the power plant exhaust gas evolutions of biomass growth and accumulation of toxic agents

To use microalgae for the biosequestration of carbon dioxide (CO₂) emitted from the coal-fired power plants, the screening of high CO₂ tolerant microalgae and their accumulation of toxic agents have attracted significant research attention. This study evaluated 10 Chlorella strains for high CO₂ tolerance using combined growth rates and growth periods subjected to logistic parameters. We selected LAMB 31 with high r (0.89 ± 0.10 day^-1), high k (6.51 ± 0.19), and medium Tp (5.17 ± 0.15 day) as a candidate for CO₂ biosequestration. Correspondingly, six genes involving carbon fixation and metabolism processes were upregulated in LAMB 31 under high CO₂ conditions, verifying its high CO₂ tolerant ability. LAMB 31 cultures exposed to exhaust gas of power plant under different flow rates grew well, but the high flow rate (0.6 L/h) showed inhibition effects compared with low flow rates (0.2 and 0.3 L/h) at the end of the culturing period. The toxic agents in the exhaust gas including sulfur, arsenic, and mercury accumulated in LAMB 31 biomass but were deemed safe for use in the production of both human food and animal feed based on the National Food Safety Standard in China. This study showed a complete process involving high CO₂ tolerant microalgae screening, high CO₂ tolerant verification, and in situ application in a power plant. Data results provide valuable information as the basis for future research studies in microalgae application on CO₂ mitigation at emission sources.

Biomitigation of CO2 from flue gas by Scenedesmus obtusiusculus AT-UAM using a hybrid photobioreactor coupled to a biomass recovery stage by electro-coagulation-flotation

The microalga Scenedesmus obtusiusculus AT-UAM efficiently captured CO₂ from two flue gas streams in a hybrid photobioreactor located in a greenhouse. Uptake rates of CO₂, NO, and SO₂ from a formulated gas stream were 160.7 mg L^-1 day^-1, 0.73 mg L^-1 day^-1, and 1.56 mg L^-1 day^-1, respectively, with removal efficiencies of 100% for all gases. Exhaust gases of a motor generator were also removed with uptake rates of 111.4 mg L^-1 day^-1, 0.42 mg L^-1 day^-1, and 0.98 mg L^-1 day^-1, obtaining removal efficiencies of 77%, 71%, and 53% for CO₂, NO_x, and SO₂, respectively. On average, 61% of the CO₂ from both flue gas streams was assimilated as microalgal biomass. The maximum CO₂ uptake rate of 182 mg L^-1 day^-1 was achieved for formulated flue gas flow rate above 100 mL min^-1. The biomass recovery of 88% was achieved using a 20-L electro-coagulation-flotation chamber coupled to a settler with a low specific power consumption of 0.27 kWh kg^-1. The photobioreactor was operated for almost 7 months without contamination of invasive species or a decrease in the activity. It is a very encouraging result for long-term operation in flue gas treatment.

CO2 utilization: Turning greenhouse gas into fuels and valuable products

This study critically reviews the recent developments and future opportunities pertinent to the conversion of CO₂ as a potent greenhouse gas (GHG) to fuels and valuable products. CO₂ emissions have reached an alarming level of around 410 ppm and have become the primary driver of global warming and climate change leading to devastating events such as droughts, hurricanes, torrential rains, floods, tornados and wildfires across the world. These events are responsible for thousands of deaths and have adversely affected the economic development of many countries, loss of billions of dollars, across the globe. One of the promising choices to tackle this issue is carbon sequestration by pre- and post-combustion processes and oxyfuel combustion. The captured CO₂ can be converted into fuels and valuable products, including methanol, dimethyl ether (DME), and methane (CH₄). The efficient use of the sequestered CO₂ for the desalinization might be critical in overcoming water scarcity and energy issues in developing countries. Using the sequestered CO₂ to produce algae in combination with wastewater, and producing biofuels is among the promising strategies. Many methods, like direct combustion, fermentation, transesterification, pyrolysis, anaerobic digestion (AD), and gasification, can be used for the conversion of algae into biofuel. Direct air capturing (DAC) is another productive technique for absorbing CO₂ from the atmosphere and converting it into various useful energy resources like CH₄. These methods can effectively tackle the issues of climate change, water security, and energy crises. However, future research is required to make these conversion methods cost-effective and commercially applicable.

Microalgae based biorefinery promoting circular bioeconomy-techno economic and life-cycle analysis

Microalgae are source of third generation biofuel having the key advantage of high lipid productivity. In recent times, biorefinery is seen as promising option to further reduce the production cost of microalgae biofuel. However, exact energy balance analysis has not been performed on important biorefinery routes. In this aspect, three biorefinery routes, all based on lipid based biorefinery route are evaluated for economical production of microalgal biofuel and valorised products. Biorefinery route 1 involves production of biodiesel, pigments, and animal feed. Biorefinery route 2 involves biogas and pigments production and two stage fermentation, and third biorefinery route involves bio-hydrogen and pigments production. Finally, the technoeconomic assessment of three biorefinery routes were reviewed, net energy savings, and life-cycle costing approaches to economize microalgal biorefinery are suggested.

Nannochloropsis oceanica Cultivation in Pilot-Scale Raceway Ponds—From Design to Cultivation

Raceways ponds are the microalgal production systems most commonly used at industrial scale. In this work, two different raceway configurations were tested under the same processing conditions to compare their performance on the production of Nannochloropsis oceanica. Biomass productivity, biochemical composition of the produced biomass, and power requirements to operate those reactors were evaluated. Water depths of 0.20 and 0.13 m, and culture circulation velocities of 0.30 and 0.15 m s−1 were tested. A standard configuration, which had a full channel width paddlewheel, proved to be the most energy efficient, consuming less than half of the energy required by a modified configuration (had a half channel width paddlewheel). The later showed to have slightly higher productivity, not enough to offset the large difference in energetic consumption. Higher flow velocity (0.30 m s−1) led to a 1.7 g m−2 d−1 improvement of biomass productivity of the system, but it increased the energy consumption twice as compared to the 0.15 m s−1 flow velocity. The latter velocity showed to be the most productive in lipids. A water depth of 0.20 m was the most suitable option tested to cultivate microalgae, since it allowed a 54% energy saving. Therefore, a standard raceway pond using a flow velocity of 0.3 m s−1 with a 0.20 m water depth was the most efficient system for microalgal cultivation. Conversely, a flow velocity of 0.15 m s−1 was the most suitable to produce lipids.

Ultrahigh-cell-density heterotrophic cultivation of the unicellular green microalga Scenedesmus acuminatus and application of the cells to photoautotrophic culture enhance biomass and lipid production

Although production of biodiesels from microalgae is proved to be technically feasible, a commercially viable system has yet to emerge. High‐cell‐density fermentation of microalgae can be coupled with photoautotrophic cultivation to produce oils. In this study, by optimizing culturing conditions and employing a sophisticated substrate feed control strategy, ultrahigh‐cell‐density of 286 and 283.5 g/L was achieved for the unicellular alga Scenedesmus acuminatus grown in 7.5‐L bench‐scale and 1,000‐L pilot‐scale fermenters, respectively. The outdoor scale‐up experiments indicated that heterotrophically grown S. acuminatus cells are more productive in terms of both biomass and lipid accumulation when they are inoculated in photobioreactors for lipid production as compared to the cells originally grown under photoautotrophic conditions. Technoeconomic analysis based on the pilot‐scale data indicated that the cost of heterotrophic cultivation of microalgae for biomass production is comparable with that of the open‐pond system and much lower than that of tubular PBR, if the biomass yield was higher than 200 g/L. This study demonstrated the economic viability of heterotrophic cultivation on large‐scale microalgal inocula production, but ultrahigh‐productivity fermentation is a prerequisite. Moreover, the advantages of the combined heterotrophic and photoautotrophic cultivation of microalgae for biofuels production were also verified in the pilot‐scale.

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.

Exploration of green integrated approach for effluent treatment through mass culture and biofuel production from unicellular alga, Acutodesmus obliquus RDS01

This study deals with the open pond (OP) pilot scale treatment of cassava effluent and enhancement of Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) enzyme through CO₂ utilization by the microalga, Acutodesmus obliquus RDS01. The cassava effluent treatment (ET) revealed maximum reduction of ammonia (96.8%), calcium (94.6%), chloride (95.2%), chlorine (98.5%), inorganic phosphate (94.6%), magnesium (96.8%), nitrate (96.89%), organic carbon (95.9%), organic phosphorus (96.3%), potassium (97.9%), sodium (97.1%), and sulfate (95.4%) on 15^th day using A. obliquus. The microalga produced highest RuBisCO enzyme activity (90%), CO₂ utilization efficiency (95%), biomass (8.9 gL^-1), lipid (176.65 mg mL^-1), carbohydrate (96.78 mg mL^-1), biodiesel (4.1 mL g^-1), and bioethanol (3.7 mL g^-1) during OP treatment. The isolated RuBisCO gene (rbcL) was used to construct the protein model by homology modeling. The microalgal-lipid content was analyzed through thin layer chromatography, the biodiesel produced was analyzed using Fourier-transform infrared spectroscopy and gas chromatography mass spectrometry (GCMS). The bioethanol production was confirmed by high performance liquid chromatography and GCMS analyses. A. obliquus produced of 98.75% biodiesel and 96.83% bioethanol in the OP pilot scale treatment A. obliquus. Overall, the microalga A. obliquus could act as an effective CO₂ capturing and bioremediation agent in the cassava ET, and also for the biodiesel and bioethanol can be produced.

Integrated lipid production, CO2 fixation, and removal of SO2 and NO from simulated flue gas by oleaginous Chlorella pyrenoidosa

CO₂, SO₂, and NO are the main components of flue gas and can cause serious environmental issues. Utilization of these compounds in oleaginous microalgae cultivation not only could reduce air pollution but could also produce feedstock for biodiesel production. However, the continuous input of SO₂ and NO inhibits microalgal growth. In this study, the toxicity of simulated flue gas (15% CO₂, 0.03% SO₂, and 0.03% NO, balanced with N₂) was reduced through automatic pH feedback control. Integrated lipid production and CO₂ fixation with the removal of SO₂ and NO was achieved. Using this technique, a lipid content of 38.0% DW was achieved in Chlorella pyrenoidosa XQ-20044. The lipid composition and fatty acid profile indicated that lipid production by C. pyrenoidosa XQ-20044 cultured with flue gas is suitable as a biodiesel feedstock; 81.2% of the total lipids were neutral lipids and 99.5% of the total fatty acids were C16 and C18. The ratio of saturated fatty acids to monounsaturated fatty acids in the microalgal lipid content was 74.5%. In addition, CO₂, SO₂, and NO from the simulated flue gas were fixed and converted to biomass and lipids with a removal efficiency of 95.9%, 100%, and 84.2%, respectively. Furthermore, the utilization efficiencies of CO₂, SO₂, and NO were equal to or very close to their removal efficiencies. These results provide a novel strategy for combining biodiesel production with biofixation of flue gas.

The Role of Malic Enzyme on Promoting Total Lipid and Fatty Acid Production in Phaeodactylum tricornutum

To verify the function of malic enzyme (ME1), the ME1 gene was endogenously overexpressed in Phaeodactylum tricornutum. Overexpression of ME1 increased neutral and total lipid content and significantly increased saturated fatty acids (SFAs) and polyunsaturated fatty acids (PUFAs) in transformants, which varied between 23.19 and 25.32% in SFAs and between 49.02 and 54.04% in PUFAs, respectively. Additionally, increased ME1 activity was accompanied by elevated NADPH content in all three transformants, indicating that increased ME1 activity produced additional NADPH comparing with that of WT. These results indicated that ME1 activity is NADP-dependent and plays an important role in the NADPH levels required for lipid synthesis and fatty acid desaturation in P. tricornutum. Furthermore, our findings suggested that overexpression of endogenous ME1 represents a valid method for boosting neutral-lipid yield in diatom.

Coupled microalgal cultivation with the treatment of domestic sewage and high-level CO2

The ability of Scenedesmus quadricauda SDEC-13 to accumulate biomass and remove nutrients in domestic sewage from campus when incorporated with 15% CO₂ was explored. The maximum specific growth rate, biomass productivity, biomass concentration, and CO₂ fixation rate were 0.14 d^-1, 0.08 g/L/d, 0.69 g/L, and 0.076 g-CO₂/L/d, respectively. The lipid content of SDEC-13 at different culture phases was also evaluated and it increased following nutrient limitation. The removal efficiencies of total nitrogen, total phosphorus, nitrate, and ammonium were all above 90%. A coupled system was designed with hydraulic retention time of 8.33 d and biomass harvest ratio of 12%, which could yield 0.54 g/L biomass and 25% lipid content with efficient domestic sewage treatment.

Boosting Nannochloropsis oculata growth and lipid accumulation in a lab-scale open raceway pond characterized by improved light distributions employing built-in planar waveguide modules

Aiming at alleviating the adverse effect of poor light penetrability on microalgae growth, planar waveguide modules functioned as diluting and redistributing the intense incident light within microalgae culture more homogeneously were introduced into a lab-scale open raceway pond (ORP) for Nannochloropsis oculata cultivation. As compared to the conventional ORP, the illumination surface area to volume ratio and effective illuminated volume percentage in the proposed ORP were respectively improved by 5.53 times and 19.68-172.72%. Consequently, the superior light distribution characteristics in the proposed ORP contributed to 193.33% and 443.71% increase in biomass concentration and lipid yield relative to those obtained in conventional ORP, respectively. Subsequently, the maximum biomass concentration (2.31 g L^-1) and lipid yield (1258.65 mg L^-1) was obtained when the interval between adjacent planar waveguide modules was 18 mm. The biodiesel produced in PWM-ORPs showed better properties than conventional ORP due to higher MUFA and C18:1 components proportions.

Feasibility of biodiesel production and CO2 emission reduction by Monoraphidium dybowskii LB50 under semi-continuous culture with open raceway ponds in the desert area

BACKGROUND

Compared with other general energy crops, microalgae are more compatible with desert conditions. In addition, microalgae cultivated in desert regions can be used to develop biodiesel. Therefore, screening oil-rich microalgae, and researching the algae growth, CO₂ fixation and oil yield in desert areas not only effectively utilize the idle desertification lands and other resources, but also reduce CO₂ emission.

RESULTS

Monoraphidium dybowskii LB50 can be efficiently cultured in the desert area using light resources, and lipid yield can be effectively improved using two-stage induction and semi-continuous culture modes in open raceway ponds (ORPs). Lipid content (LC) and lipid productivity (LP) were increased by 20% under two-stage industrial salt induction, whereas biomass productivity (BP) increased by 80% to enhance LP under semi-continuous mode in 5 m² ORPs. After 3 years of operation, M. dybowskii LB50 was successfully and stably cultivated under semi-continuous mode for a month during five cycles of repeated culture in a 200 m² ORP in the desert area. This culture mode reduced the supply of the original species. The BP and CO₂ fixation rate were maintained at 18 and 33 g m^-2 day^-1, respectively. Moreover, LC decreased only during the fifth cycle of repeated culture. Evaporation occurred at 0.9-1.8 L m^-2 day^-1, which corresponded to 6.5-13% of evaporation loss rate. Semi-continuous and two-stage salt induction culture modes can reduce energy consumption and increase energy balance through the energy consumption analysis of life cycle.

CONCLUSION

This study demonstrates the feasibility of combining biodiesel production and CO₂ fixation using microalgae grown as feedstock under culture modes with ORPs by using the resources in the desert area. The understanding of evaporation loss and the sustainability of semi-continuous culture render this approach practically viable. The novel strategy may be a promising alternative to existing technology for CO₂ emission reduction and biofuel production.

Carbon dioxide capture strategies from flue gas using microalgae: a review

Global warming and pollution are the twin crises experienced globally. Biological offset of these crises are gaining importance because of its zero waste production and the ability of the organisms to thrive under extreme or polluted condition. In this context, this review highlights the recent developments in carbon dioxide (CO2) capture from flue gas using microalgae and finding the best microalgal remediation strategy through contrast and comparison of different strategies. Different flue gas microalgal remediation strategies discussed are as follows: (i) Flue gas to CO2 gas segregation using adsorbents for microalgal mitigation, (ii) CO2 separation from flue gas using absorbents and later regeneration for microalgal mitigation, (iii) Flue gas to liquid conversion for direct microalgal mitigation, and (iv) direct flue gas mitigation using microalgae. This work also studies the economic feasibility of microalgal production. The study discloses that the direct convening of flue gas with high carbon dioxide content, into microalgal system is cost-effective.

Lipid Production from Nannochloropsis

Microalgae are sunlight-driven green cell factories for the production of potential bioactive products and biofuels. Nannochloropsis represents a genus of marine microalgae with high photosynthetic efficiency and can convert carbon dioxide to storage lipids mainly in the form of triacylglycerols and to the ω-3 long-chain polyunsaturated fatty acid eicosapentaenoic acid (EPA). Recently, Nannochloropsis has received ever-increasing interests of both research and public communities. This review aims to provide an overview of biology and biotechnological potential of Nannochloropsis, with the emphasis on lipid production. The path forward for the further exploration of Nannochloropsis for lipid production with respect to both challenges and opportunities is also discussed.

Installation of flow deflectors and wing baffles to reduce dead zone and enhance flashing light effect in an open raceway pond

To reduce the dead zone and enhance the flashing light effect, a novel open raceway pond with flow deflectors and wing baffles was developed. The hydrodynamics and light characteristics in the novel open raceway pond were investigated using computational fluid dynamics. Results showed that, compared with the control pond, pressure loss in the flow channel of the pond with optimized flow deflectors decreased by 14.58%, average fluid velocity increased by 26.89% and dead zone decreased by 60.42%. With wing baffles built into the raceway pond, significant swirling flow was produced. Moreover, the period of average L/D cycle was shortened. In outdoor cultivation of freshwater Chlorella sp., the biomass concentration of Chlorella sp. cultivated in the raceway pond with wing baffles was 30.11% higher than that of the control pond.

CO2 , NOx and SOx removal from flue gas via microalgae cultivation: a critical review

Flue gas refers to the gas emitting from the combustion processes, and it contains CO2 , NOx , SOx and other potentially hazardous compounds. Due to the increasing concerns of CO2 emissions and environmental pollution, the cleaning process of flue gas has attracted much attention. Using microalgae to clean up flue gas via photosynthesis is considered a promising CO2 mitigation process for flue gas. However, the impurities in the flue gas may inhibit microalgal growth, leading to a lower microalgae-based CO2 fixation rate. The inhibition effects of SOx that contribute to the low pH could be alleviated by maintaining a stable pH level, while NOx can be utilized as a nitrogen source to promote microalgae growth when it dissolves and is oxidized in the culture medium. The yielded microalgal biomass from fixing flue gas CO2 and utilizing NOx and SOx as nutrients would become suitable feedstock to produce biofuels and bio-based chemicals. In addition to the removal of SOx , NOx and CO2 , using microalgae to remove heavy metals from flue gas is also quite attractive. In conclusion, the use of microalgae for simultaneous removal of CO2 , SOx and NOx from flue gas is an environmentally benign process and represents an ideal platform for CO2 reutilization.