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Modulating cultivation regimes of Messastrum gracile SVMIICT7 for biomass productivity integrated with resource recovery via hydrothermal liquefaction
Biomass offers a promising alternative for producing biofuels and chemicals through hydrothermal liquefaction, a process known for its ability to convert complex organic materials into valuable liquid products. Optimizing hydrothermal liquefaction for large-scale application involves understanding the underlying mechanisms and addressing key scientific and technical issues. We review hydrothermal liquefaction of biomass-derived chemicals, focusing on the breakdown and depolymerization of cellulose, hemicellulose, lignin, lipids, and proteins under hydrothermal conditions. We examine critical parameters such as reaction temperature, pressure, solvent selection, and catalyst choice, and their impact on product yield and quality. Catalytic routes transform key intermediates, such as 5-hydroxymethylfurfural and levulinic acid, into high-value liquid fuels and chemicals, offering significant potential for sustainable fuel production. Recent advances in process optimization are discussed.
The dairy industry is becoming one of the biggest sectors within the global food industry, and these industries use almost 34% of the water. The amount of water used is governed by the production process and the technologies employed in the plants. Consequently, the dairy industries generate almost 0.2–10 L of wastewater per liter of processed milk, which must be treated before being discharged into water bodies. The cultivation of microalgae in a mixotrophic regime using dairy wastewater enhances biomass growth, productivity, and the accumulation of value-added product. The generated biomass can be converted into biofuels, thus limiting the dependence on petroleum-based crude oil. To fulfill the algal biorefinery model, it is important to utilize every waste stream in a cascade loop. Additionally, the harvested water generated from algal biomass production can be recycled for further microalgal growth. Economic and sustainable wastewater management, along with proper reclamation of nutrients from dairy wastewater, is a promising approach to mitigate the problem of water scarcity. A bibliometric study revealing limited work on dairy wastewater treatment using microalgae for biofuel production. And, limited work is reported on the pretreatment of dairy wastewater via physicochemical methods before microalgal-based treatment. There are still significant gaps remains in large-scale cultivation processes. It is also crucial to discover robust strains that are highly compatible with the specific concentration of contaminants, as this will lead to increased yields and productivity for the targeted bio-product. Finally, research on reutilization of culture media in photobioreactor is necessary to augument the productivity of the entire process. Therefore, the incorporation of the microalgal biorefinery with the wastewater treatment concept has great potential for promoting ecological sustainability.
Single‐chambered electrofermentation (EF) bioreactors are operated using spent wash to evaluate the effect of applied voltage (EF/AV; −0.6 V) against closed‐circuit (EF/CC; 100 Ω) and control (EF/C; no voltage/resistance) operations using deoiled microalgae biomass‐derived biochar electrodes (anode) on increasing acidogenesis rate/efficiency. Higher total volatile fatty acids (VFA) production (mg L⁻¹) is observed in EF/AV (2866), depicting 31% and 57% increments than EF/CC (1984) and EF/C (1224), respectively. The VFA profiles (C2–C4) in EF/AV are higher with acetic/butyric/propionic acids (1922/704/240 mg L⁻¹) and biogas co‐generation [H2 (22%) and CH4 (3%)] than EF/CC and EF/C. The applied voltage (EF/AV) helps in electron flux regulation for increasing substrates conversion and VFA generation. Residual VFA‐rich effluents from EF process is utilized as substrate to mixotrophic microalgae cultivation (nutrient‐ and stress‐phase) that resulted in biomass production with simultaneous wastewater treatment. EF/AV depicts higher biomass growth (0.72 g L⁻¹; 4th day) and substrate removal (63%) than other operations. Integrations of electrofermentation with photosynthetic processes operated with the residual resources in effluents towards product valorization and wastewater polishing, accounts for low‐carbon economy.
This study demonstrates the combination of wastewater treatment and green microalgae cultivation for the low-cost production of lipids as a feedstock for biodiesel production. Three green microalgal species were used: Chlamydomonas reinhardtii, Monoraphidium braunii, and Scenedesmus obliquus. Nutrient, heavy metals and minerals removal, biomass productivity, carbohydrate , protein, proline, lipid, and fatty acids methyl ester (FAMEs) contents besides biodiesel properties were evaluated. The results showed that all algal species were highly efficient and had the potential to reduce nitrate, ammonia, phosphate, sulfate, heavy metals (Zn 2+ , Cu 2+ , Mn 2+ , and Fe 2+), calcium, magnesium, sodium, and potassium after 10 days of algal treatment compared to initial concentrations. The removal efficiency of these parameters ranged from 12 to 100%. The growth rates of M. braunii and S. obliquus cultivated in wastewater were significantly decreased compared to the control (synthetic medium). In contrast, C. reinhardtii showed the highest growth rate when cultivated in sewage water. Wastewater could decrease the soluble carbohydrates and protein content in all tested algae and increase the proline content in M. braunii and S. obliquus. In wastewater culture, M. braunii had the highest lipid productivity of 5.26 mg L −1 day −1. The fatty acid profiles of two studied species (C. reinhardtii and M. braunii) revealed their suitability as a feedstock for biodiesel production due to their high content of saturated fatty acids, representing 80.91% and 68.62% of the total fatty acid content, respectively, when cultivated in wastewater. This study indicated that wastewater could be used to modify biomass productivity, lipid productivity, and the quantity of individual fatty acids in some algae that affect biodiesel quality to achieve international biodiesel standards.
Herein, we report the catalytic hydrothermal treatment of dried microalgae biomass (DAB) of Scenedesmus sp. SVMIICT1 for biocrude production employing various alkali and metal-based catalysts at 200 °C and 100 bar pressure under N2 and H2 atmospheres with twelve designed experiments. The specific influence of inert and reducing conditions together with the catalyst on the yields of bio-oil as well as the composition, off-gases and energy recovery were explored. The results depicted that H2 atmosphere and catalyst loading resulted in bio-oil with higher heating ratios (HHV: 29-37 MJ kg⁻¹) and yields by hydrodeoxygenation (H/C: 1.52-1.64; O/C: 0.51-0.08). Among the catalysts, Pt/C (H2) afforded bio-oil with the highest calorific value (∼37 MJ kg⁻¹), which is compositionally equivalent to fossil-based hydrocarbon fuels. H2-Pt/C condition increased the saturated hydrocarbon (biodiesel) content by 38.7-89.3% of the total bio-oil. Aliphatic/aromatic hydrocarbons were predominantly observed in the bio-oil followed by carboxylic acids, furan derivatives, indanone derivatives, ketones, and substituted benzenes. The by-products of HTL (aqueous fraction, off-gases, and solid residues) were analyzed for their specific characteristics. Further, the aqueous fraction was valorized through dark-fermentation to produce bio-hydrogen and bio-methane, wherein maximum biogas yield (H2, 186 mL g⁻¹ TOC conversion and CH4, 131 mL g⁻¹ TOC conversion) were observed under NaOH-catalyzed conditions. This study depicts the comprehensive utilization of algal biomass for energy and chemical intermediates in a circular chemistry context. This journal is
Microalgal biomass has been proved to be a sustainable source for biofuels including bio-oil, biodiesel, bioethanol, biomethane, etc. One of the collateral benefits of integrating the use of microalgal technologies in the industry is microalgae’s ability to capture carbon dioxide during the application and biomass production process and consequently reducing carbon dioxide emissions. Although microalgae are a feasible source of biofuel, industrial microalgae applications face energy and cost challenges. To overcome these challenges, researchers have been interested in applying the bio-refinery approach to extract the important components encapsulated in microalgae. This review discusses the key steps of microalgae-based biorefinery including cultivation and harvesting, cell disruption, biofuel and value-added compound extraction along with the detailed technologies associated with each step of biorefinery. This review found that suitable microalgae species are selected based on their carbohydrate, lipid and protein contents and selecting the suitable species are crucial for high-quality biofuel and value-added products production. Microalgae species contain carbohydrates, proteins and lipids in the range of 8% to 69.7%, 5% to 74% and 7% to 65% respectively which proved their ability to be used as a source of value-added commodities in multiple industries including agriculture, animal husbandry, medicine, culinary, and cosmetics. This review suggests that lipid and value-added products from microalgae can be made more economically viable by integrating upstream and downstream processes. Therefore, a systematically integrated genome sequencing and process-scale engineering approach for improving the extraction of lipids and co-products is critical in the development of future microalgal biorefineries.
Microalgae are multifaceted photosynthetic microorganisms with emerging business potential. They are present ubiquitously in terrestrial and aquatic environments with rich species diversity and are capable of producing significant biomass. Traditionally, microalgal biomass is being used as food and feed in many countries around the globe. The production of microalgal-based bioactive compounds at an industrial scale through biotechnological interventions is gaining interest more recently. The present review provides a detailed overview of the key algal metabolites, which plays a crucial role in nutraceutical, functional foods, and animal/aquaculture feed industries. Bioactive compounds of microalgae known to exhibit antioxidant, antimicrobial, antitumor, and immunomodulatory effects were comprehensively reviewed. The potential microalgal species and biological extracts against human pathogens were also discussed. Further, current technologies involved in upstream and downstream bioprocessing including cultivation, harvesting, and cell disruption were documented. Establishing microalgae as an alternative supplement would complement the sustainable and environmental requirements in the framework of human health and well-being.
Herein, four kinds of low-cost biochars (i.e. straw, rice husk, peanut shell and sawdust biochar) were used as the catalysts and their influence on the HTL of microalgae was investigated. It indicated that the biochars could improve the bio-oil quality. Particularly, the sawdust biochar increased the higher heating value of the bio-oil from 35.8 MJ/kg (without biochar) to 37.7 MJ/kg. The straw biochar increased the low boiling fraction (< 200 o C) of bio-oil by about 15% compared with that of the bio-oil without catalysts. Overall, the biochar is beneficial for the bio-oil used as transportation fuels in the future.
Biobased products (biobased materials, bioenergy/biofuels, and biobased chemicals) are the futuristic replacement of fossil-based chemicals and considerably the best way of transiting towards a low-carbon economy (LCE) and which also can simultaneously mitigate the global challenges associated with the depletion of abiotic resources and climate change. Endurance with the biobased products is leading to the development of a biobased economy (BBE). Assessment of environmental sustainability achieved by the production of biobased products is important to understand their decarbonization potential, and all associated impacts with their life cycle and can be measured with life cycle assessment (LCA) tool. Life cycle sustainability assessment (LCSA) combines LCA with life cycle costing (LCC) and social life cycle assessment (sLCA). The framework of this paper is thus designed to provide an insight into biobased products with an overview of LCA as critical tools to measure sustainability. LCA methodology and its importance in determining sustainability for biobased products with associated limitations and probable solutions are also discussed. Circular economy (CE) integrating cradle-to-cradle (C2C) approach is also discussed as a promising endeavor to achieve LCE.
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In recent years, researchers have highlighted the role of low cost-efficient agro-industrial by-products used as supplements in algal culture media. The aim of the study was to identify and characterize the basic metabolic pathways in Tetradesmus obliquus cells induced by supplementation with beet molasses in photoheterotrophic and mixotrophic culture conditions. To assess the impact of the nutritional strategy in unicellular algae, growth curves were plotted and lipid, carbohydrate, and protein levels were determined. Fourier Transform Infrared Spectroscopy was applied to measure the Tetradesmus obliquus cell composition. Additionally, the C16-C18 fatty acid profile of Tetradesmus obliquus was determined by gas chromatograph/mass spectrometry. The switch from autotrophy to photoheterotrophy and mixotrophy contributes to shortening of the adaptation growth phase. The highest protein content was obtained in the mixotrophic growth. This study has demonstrated high content of 18:1, cisΔ9, 18:2, cisΔ9,12, ω6, and 18:3, cisΔ9,12,15, ω3 in photoheterotrophic and mixotrophic culture conditions. High levels of proteins and essential fatty acids make Tetradesmus obliquus cell biomass important for human and animals health.
Sunlight energy largely exceeds the energy required by anthropic activities, and therefore its exploitation represents a major target in the field of renewable energies. The interest in the mass cultivation of green microalgae has grown in the last decades, as algal biomass could be employed to cover a significant portion of global energy demand. Advantages of microalgal vs. plant biomass production include higher light-use efficiency, efficient carbon capture and the valorization of marginal lands and wastewaters. Realization of this potential requires a decrease of the current production costs, which can be obtained by increasing the productivity of the most common industrial strains, by the identification of factors limiting biomass yield, and by removing bottlenecks, namely through domestication strategies aimed to fill the gap between the theoretical and real productivity of algal cultures. In particular, the light-to-biomass conversion efficiency represents one of the major constraints for achieving a significant improvement of algal cell lines. This review outlines the molecular events of photosynthesis, which regulate the conversion of light into biomass, and discusses how these can be targeted to enhance productivity through mutagenesis, strain selection or genetic engineering. This review highlights the most recent results in the manipulation of the fundamental mechanisms of algal photosynthesis, which revealed that a significant yield enhancement is feasible. Moreover, metabolic engineering of microalgae, focused upon the development of renewable fuel biorefineries, has also drawn attention and resulted in efforts for enhancing productivity of oil or isoprenoids.
Chlorella sp. is both an important model for green algal photosynthesis and is produced using industrial scale photobioreactors. In photobioreactors, cells travel through steep gradients of illumination at rates determined by photobioreactor design, mixing rates, culture density and surface irradiance levels. We used non-invasive, rapid fluorescence measures to show that Chlorella vulgaris tolerates short-term exposures to super-saturating irradiance by transiently accelerating electron transport away from Photosystem II. This capacity lasts for only 10–20 s, and longer exposures to supersaturating irradiance induced down-regulation of electron transport through slowing of down-stream electron sinks, induction of non-photochemical quenching and net Photosystem II photoinactivation. Cells previously acclimated to high growth light were able to partially recover from the down regulation within 300–600 s, but cells previously acclimated to low growth light suffered more sustained down-regulation after exposure to super-saturating light. These metrics can be used to guide and constrain culture density, mixing rate and irradiance regime decisions in photobioreactors.
Bio‐oils, produced by biomass pyrolysis, have become promising candidates for feedstocks of high value‐added chemicals and alternative sources for transportation fuels. Bio‐oil is such a complicated mixture that contains nonpolar hydrocarbons and polar components which cover almost all kinds of organic oxygenated compounds such as carboxylic acids, alcohols, aldehydes, ketones, esters, furfurals, phenolic compounds, sugar‐like material, and lignin‐derived compounds. Comprehensive characterization of bio‐oil and its subfractions could provide insight into the conversion process of biomass processing, as well as its further utilization as transportation fuels or chemical raw materials. This review focuses on advanced analytical strategies on in‐depth characterization of bio‐oil, which is concerned with gas chromatography, high‐resolution mass spectrometry, FTIR spectroscopy and NMR spectroscopy, offering complementary information for previous reviews.
The majority of the sludge from the treatment of wastewater in milk processing plants is land spread. The drawbacks of land spreading include local oversupply due to high transport costs, which results in sludge being spread on lands in the vicinity of the dairy factories. Local oversupply can lead to accumulation of certain substances in soil through annual application over many years. Therefore, in the long term, there is a need for alternative methods to recover energy and nutrients from increasing volumes of sludge generated from dairy processing. Pyrolysis offers a potential alternative to land spreading, which can reduce health and environmental risks, while providing an avenue for the recovery of energy and nutrients. Pyrolysis allows energy recovery in the form of a high calorific value pyrolysis gas and a char which may be used as a soil amendment. In this study pyrolysis of dried dairy sludge was carried out at pilot scale. The results indicate that a dried biological sludge can be successfully pyrolysed and when mixed with wood the resulting char meets European Biochar Certificate criteria regarding carbon content. Most of the initial energy content of the feedstock was retained in the pyrolysis gas prior to cleaning, 53%, compared to 34.5% in the char and 1.5% in the tar. For the pyrolysis gas after cleaning (mainly cracking in presence of air) the initial energy content of the feedstock retained in the gas was only slightly higher than that retained in the char, 39.2% versus 34.5%, while the tar accounted for 0.8% of the initial energy content.
Microalgae are gaining commercial interests in the areas food, feed and biofuel sector. They have intrinsic ability to harness energy from sunlight and photosynthetically valorize CO2 into various bio-based products viz., triacylglycerols (TAGs), mono/poly-unsaturated fatty acids (MUFA, PUFA), pigments etc. Microalgae have adapted to grow in various nutritional environments due to their metabolic versatility and resilience. Strategic evaluation of newly isolated strain Chlorella sp. from a residential lake was performed. The strain was investigated by varying the nutritional modes to gain insights into biomass and fatty acids production. Maximum biomass (3.59 g/L) was observed in mixotrophic condition followed by heterotrophic (1.58 g/L) and autotrophic condition (0.59 g/L). The maximum lipid yield (670 mg/g DCW) was observed in mixotrophic condition whereas maximum total lipid content (36%) was observed in heterotrophic condition. Significant correlation was noticed between fluorescence parameters measured by OJIP and non-photochemical quenching (NPQ) with the function of nutritional mode variations. Autotrophic condition showed higher photosynthetic activity which was well correlated with high fluorescence intensity as represented by OJIP, NPQ1, and NPQ2 curves. Good balance of saturated fatty acids (SFA) and unsaturated fatty acids was observed in autotrophic mode, whereas polyunsaturated fatty acids (PUFA) and mono unsaturated fatty acid (MUFA) content were relatively higher in mixotrophic and heterotrophic conditions.
Photosynthetic organisms have evolved numerous photoprotective mechanisms and alternative electron sinks/pathways to fine‐tune the photosynthetic apparatus under dynamic environmental conditions, such as varying carbon supply or fluctuations in light intensity. In cyanobacteria flavodiiron proteins (FDPs) protect the photosynthetic apparatus from photodamage under fluctuating light (FL). In Arabidopsis thaliana, which does not possess FDPs, the PGR5‐related pathway enables FL photoprotection. The direct comparison of the pgr5, pgrl1 and flv knock‐out mutants of Chlamydomonas reinhardtii grown under ambient air demonstrates that all three proteins contribute to the survival of cells under FL, but to varying extents. The FDPs are crucial in providing a rapid electron sink, with flv mutant lines unable to survive even mild FL conditions. In contrast, the PGRL1 and PGR5‐related pathways operate over relatively slower and longer time‐scales. Whilst deletion of PGR5 inhibits growth under mild FL, the pgrl1 mutant line is only impacted under severe FL conditions. This suggests distinct roles, yet a close relationship, between the function of PGR5, PGRL1 and FDP proteins in photoprotection.
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Monoraphidium sp. KMC4 was cultivated mixotrophically for simultaneous treatment of dairy wastewater and biomass production. The KMC4 was cultivated with varying chemical oxygen demand concentrations of simulated synthetic dairy wastewater. Monoraphidium sp. KMC4 outperformed in 50% strength with biomass concentration of 1.47 g L-1. A significant change in biomass of 3.69 g L-1 was achieved after maintaining the pH of algal culture. The nutrient consumption promoted microalgal growth in the form of biomass productivity (122 mg L-1 day-1), accumulation of carbohydrate (28.73±1.6 wt%), protein (48.50±1.3 wt%), and lipid (20.29±2.3 wt%). This strain showed efficacious performance in treating simulated synthetic dairy wastewater obtaining biomass for various applications. The algal biomass derived from wastewater reported a significant volatile matter content and higher heating value. The biomass demonstrates satisfactory thermal degradation behavior which reveals its feasibility as feedstock for thermochemical conversion to biocrude. The integration of biomass production in high-scale raceway pond along with biocrude production is a promising pathway toward the generation of green energy for replacing traditional fossil fuels.
Developing an efficient photobioreactor (PBR) and reducing freshwater dependence are among the significant challenges for generating 3rd generation biomass feedstock. Addressing these, the present study focused on developing a modified airlift (MoAL) PBR. Its performance was further evaluated and compared with the traditional airlift PBR by cultivating microalgae in dark fermentation spent wash. Lower mixing time and higher interfacial mass transfer coefficient was observed in the MoAL PBR having a perforated draft tube. Experimentally, the MoAL exhibited the maximum biomass concentration of 3.18 g L-1, which was 30% higher than that of the conventional airlift PBR. The semi-continuous operation of the MoAL (with water recycling) achieved the maximum biomass productivity of 0.83 g L-1 d-1, two folds superior to that of batch culture. The comprehensive biomass characterization (proximate, ultimate, and thermochemical) further confirmed its potential for bioenergy application. Considering that, hydrothermal liquefaction of the biomass resulted in a maximum biocrude yield of 31% w/w with a higher heating value (HHV) of 36.6 MJ kg-1. In addition, the biocrude comprised 66.6% w/w lighter fraction (<343 °C), including 21.5% w/w of heavy naphtha, 20.5% w/w of kerosene, and 24.6% w/w of diesel. The results can help develop sustainable technology for simultaneous wastewater remediation and biocrude production.
High photosynthetic rates and high lipid content make microalgae a promising source for the industrial production of edible oils. Enhancement of oleic acid production in microalgae is an important area in the algae-based edible oils research. This study employed the mixotrophic mode of nutrition with a nutrient-rich growth phase (GP) followed by two different stress phases (SPs), including nitrate depletion (single stress phase; SSP) and nitrate depletion with salinity (dual stress phase; DSP). In DSP, higher oleic acid (C18:1) content was observed and considered the same for autotrophic mode. In addition to DSP, it evaluated the role of nitrate to phosphate ratio in GP towards fatty acid biosynthesis. The relative and absolute quantities of fatty acids were measured in mixotrophic and autotrophic cultivations. Higher oleic acid was obtained at 250 mM NaCl DSP condition in mixotrophic cultivation compared to other conditions. Whereas autotrophic mode microalgae cultivated in dual stress phase (DSP for A-GP-1N1P, DSP for A-GP-1N2P, DSP for A-GP-1N4P, and DSP for A-GP-1N6P) resulted in ~30 % (w/w) of total fatty acids with 55 % of Oleic acid in absolute quantity. Overall, the study depicted a distinct influence of nitrate, nitrate to phosphate ratio, nitrate depletion (single stress), and dual stress on the oleic acid yield. The application of dual stress to microalgae cultivation is a new approach to oleic acid enhancement.
Polyhydroxybutyrates (PHBs) are naturally occurring biopolymeric compounds that accumulate in a variety of microorganisms, including microalgae as energy and carbon storage sources. The present study was designed to evaluate nature-based PHB production using microalgae in biphasic (growth (GP) and stress phase (SP)). Microalgal PHB accumulation was driven by nutrient constraint, with a maximal production of 29.5% of PHB from 0.94 gm L⁻¹ of biomass. The biopolymer obtained was PHB homopolymer with carbonyl (C=O) stretch of the aliphatic ester moiety. PHB granules were observed in the cytoplasm of the cell by fluorescence microscopy, and NMR and GC-MS spectra suggested a structural link. Chlorophyll a fluorescence transients inferred through OJIP, exhibited significant variation in photosynthetic process concerning nutritional mode. The transition cultivation modes showed synergistic effects on fatty acid composition and biomass productivity. Mining bio-based products from microalgae enable greener routes as alternatives to conventional plastics.
This study investigated the hydrothermal liquefaction (HTL) of extracted major macromolecules from Tetraselmis sp. biomass. The carbohydrate fraction was first recovered from Tetraselmis biomass using pressurized heated water. The crude lipid fraction was then extracted from carbohydrate-free biomass by hexane. The remaining biomass was considered as protein extract. HTL runs were conducted from 275 to 350 °C and 30 min for each extract; the maximum biocrude yields for carbohydrate, lipid, and proteins were obtained at 325, 325, and 350 °C, respectively. Next, HTL runs of these macromolecules were conducted at 350 °C for 10–60 min. The highest biocrude yields from carbohydrate, lipid, and protein extracts were obtained at 45, 20, and 45 min, respectively. The optimal energy recovery, as biocrude, from carbohydrate, lipid, and protein extracts were 41, 85, and 81%, respectively. Therefore, microalgae biomass with low carbohydrate content or carbohydrate-extracted biomass could be used as feedstock for biocrude production.
Hydrothermal liquefaction of algal biomass is growing as a promising technology for the production of liquid biofuels in the renewable energy sector. The potential of this technology under varied process conditions has been extensively demonstrated worldwide. Recent developments have revealed that the feedstock quality and extraction solvents have a considerable effect on the biocrude quality and yield. Algal lipids and proteins contribute significantly to biocrude formation, while carbohydrates contribute mainly to the production of gaseous products. However, the challenges associated with high N/C and O/C ratios in biocrude derived from proteinaceous biomass need to be addressed. Recently, co-liquefaction of carbonaceous biomass with different biochemical compositions has proven promising in biocrude upgradation. The most contemporary research also revealed that solvents as a co-solvent and/or extraction agent have the most significant effect on biocrude quality and yield in the HTL mixture. These solvents could also intensely influence the properties and distribution of energy-dense compounds in the biocrude. The present paper extensively reviews the effect of feedstock composition on HTL biocrude formation and its characteristics based on the information collected from more than 150 scientific resources published between 2010 and 2021. Furthermore, the role of solvents as reaction medium and extraction agent on biocrude yield and composition has been critically reviewed. Future perspectives and challenges in the use of approaches for improving biocrude formation are also discussed.
Tetradesmus sp. SVMIICT4 was isolated and cultivated mixotrophically in a flat-panel photobioreactor (FP-PBR) for concurrent dairy wastewater treatment, carbon fixation, and biomass production. Integrated wastewater treatment showed good COD and nutrients removal efficiency accounting for biomass with an accumulation of carbohydrate (21.48 mg g⁻¹) and protein (19.52 mg g⁻¹). Chlorophyll a fluorescence transients (Fv/Fm, ETo/RC, TRo/RC, and Abs/RC) deduced through OJIP curve fittings, showed consistent improvement in photosynthetic activity throughout the cultivation period. The absorption flux per reaction centre corroborated with increased chlorophyll content (18.94 mg g⁻¹), resulting in higher electron transport (ET/Rc) and non-photochemical quenching (NPQ). The fatty acid profile showed high content of unsaturated, followed by saturated fatty acids, which will have multiple applications in food, feed, and fuel industries, enabling a bio-based economy.
Waste is an inherent and unavoidable part of any process which can be attributed to various factors such as process inefficiencies, usability of resources and discarding of not so useful parts of the feedstock. Dairy is a burgeoning industry following the global population growth, resulting in generation of waste such as wastewater (from cleaning, processing, and maintenance), whey, and sludge. These components are rich in nutrients, organic and inorganic materials. Additionally, the presence of alkaline and acidic detergents along with sterilizing agents in dairy waste makes it an environmental hazard. Thus, sustainable valorization of dairy waste requires utilization of biological methods such as microbial treatment. This review brings forward the current developments in utilization and valorization of dairy waste through microbes. Aerobic and anaerobic treatment of dairy waste using microbes can be a sustainable and green method to generate biofertilizers, biofuels, power, and other biobased products.
This review article focuses on recent updates on remediation of industrial wastewater (IWW) through microalgae cultivation. These include how adding additional supplements of nutrient to some specific IWWs lacking adequate nutrients improving the microalgae growth and remediation simultaneously. Various pretreatments strategy recently employed for IWWs treatment other than dealing with microalgae was discussed. Various nutrient-rich IWW could be utilized directly with additional dilution, supplement of nutrients and without any pretreatment. Recent advances in various approaches and new tools used for cultivation of microalgae on IWW such as two-step cultivation, pre-acclimatization, novel microalgal-bioelectrical systems, integrated catalytic intense pulse-light process, sequencing batch reactor, use of old stabilized algal-bacterial consortium, immobilized microalgae cells, microalgal bacterial membrane photobioreactor, low-intensity magnetic field, BIO_ALGAE simulation tool, etc. are discussed. In addition, biorefinery of microalgal biomass grown on IWW and its end-use applications are reviewed.
The isolated Messastrum gracile SVMIICT7 was mixotrophically cultivated in flat panel photobioreactor (FP-PBR) towards understanding the photosynthetic transient and product profile. Biomass productivity attained a maximum of 45 mg L⁻¹d⁻¹, with COD, nitrates and phosphates removal of 83.3%, 84.05%, and 74.98% respectively. Messastrum sp. showed good assimilation of proteins (124 mg g⁻¹), carbohydrates (119 mg g⁻¹) and lipids (26%). The myristoleic acid (C14:1-39.1%) and heptadecanoic acid (C17:0-29.1%) are abundant fatty acids with therapeutic, food and feed applications. The cellular ultrastructure studies revealed facile arrangement of chloroplast and starch covered pyrenoids supporting increased carbohydrate accumulation. Photosystem II (PSII) [Y(II), ETR(II), Y(NPQ), and Y(NO)] and photosystem I (PSI) [Y(I), ETR(I), Y(NA), and Y(ND)] transients showed improved photosynthetic efficiency directing microalgae growth and biomass productivity. Higher Fv/Fm values indicates relatively good water splitting and carbon fixation at PSII and PSI facilitating improved photosynthetic electron transport and synthesis of value-added products thereby enabling bioeconomy.
To study the biomass and photosynthetic efficiency difference of freshwater microalga Scenedesmus obliquus under autotrophic and mixotrophic cultivation mode, the biomass production, photosynthetic pigments, and photosynthetic efficiency were investigated by lab-scale cultivation experiments. Specifically, S. obliquus was cultured in BG11 medium supplemented with different concentrations of exogenous sodium acetate to examine the physiological and biochemical effects on the oil rich freshwater algae S. obliquus. Results showed that exogenous sodium acetate exerted a “low promotion and high suppression” effect on the biomass production and photosynthetic pigments accumulation, and the biomass production and pigment contents peaked at a sodium acetate concentration of 10.00 mg L⁻¹. Meanwhile, photosynthetic efficiency was significantly increased under low concentration of sodium acetate, however, when adding the high concentrations of sodium acetate, the oxygen evolving complex in donor side of reaction center was damaged, and electron transport at the donor and receptor sides of the reaction center was also suppressed, accompanied by the changes of the absorption, transfer, and application of light energy. These findings provide a strategy to boost biomass accumulation of S. obliquus by mixotrophic cultivation mode with appropriate concentration of sodium acetate, and would be conducive to further accelerate the industrialization of this strain.
The present study is aimed to understand the photosynthetic transients of Chlorella sorokiniana SVMBIOEN2 during treatment of dairy wastewater under different light intensities (100, 150, and 200 µmol m⁻²s⁻¹) in mixotrophic mode. Light intensities showed marked influence on photosystem behavior, lipid profile, and organic pollutant removal. Analysis of Chlorophyll a fluorescence transient including Fv/Fm, ETo/RC, TRo/RC, and Abs/RC showed better photosystem efficiency at 100 µmol m⁻²s⁻¹ operations. OJIP curve fitting depicted a positive L-band at 150 µmol m⁻²s⁻¹ indicating lower kinetic energy of PSII reaction centres at high light intensities. Better photosynthetic activity at 100 µmol m⁻²s⁻¹ operations resulted in good assimilation of biomass (2.3 g L⁻¹), carbohydrates (10.2 mg g⁻¹), and proteins (14 mg g⁻¹) with a significant reduction in chemical oxygen demand (85%). Phycoremediation of dairy wastewater accumulates predominantly monounsaturated fatty acids followed by polyunsaturated fatty acids showing the application of C. sorokiniana in nutraceutical and food industries.
Hydrothermal liquefaction (HTL) of biomass results in the formation of bio-oil, aqueous phase (HTL-AP), bio-char, and gaseous products. Safer disposal of HTL-AP is difficult on an industrial scale since it comprises low molecular acid compounds. This review provides a comprehensive note on the recent articles published on the effective usage of HTL-AP for the recovery of valuable compounds. Thermo-chemical and biological processes are the preferred techniques for the recovery of biofuel, platform chemicals from HTL-AP. From this review, it was evident that the composition of HTL-AP and product recovery are the integrated pathways, which depend on each other. Substitute as reaction medium in HTL process, growth medium for algae and microbes are the most common mode of reuse and recycle of HTL-AP. Future research is needed to depict the mechanism of HTL process when HTL-AP is used as a reaction medium on an industrial scale. Need to find a solution for the hindrance in commercializing HTL process and recovery of value-added compounds from HTL-AP from lab scale to industry level. Integrated pathways on reuse and HTL-AP recycle helps in reduced environmental concerns and sustainable production of bio-products.
Hydrothermal liquefaction (HTL) of wastewater microalgae is constrained by the high ash content in the biomass. High ash content causes slagging, fouling, and catalyst deactivation. In this study, batch experiments for hydrothermal liquefaction (HTL) of filamentous algae were carried out in a 1.8-L autoclave reactor to measure the effects of reaction conditions and demineralization on the yields and quality of the products. Nitric acid was the most efficient demineralization acid, halving ash content (27.1 to 13.5 wt%). HTL of untreated algae showed that, as temperature increased from 310 to 350 • C, total bio-crude oil (light + heavy) yield slightly increased (25.4 to 28.0 wt%, dry ash-free). HTL of demineralized algae gave higher total bio-crude oil yields (27.2 to 43.4 wt%, dry ash-free), greater energy recovery (29.5 to 50.9%), higher char yields (22.1 to 32.5 wt%, dry ash-free), and decreased energy consumption ratios (from 1.0 to 0.6) compared to HTL of untreated algae. Fourier transform infrared spectroscopy (FT-IR) of the feedstocks revealed slightly higher carbohydrate content in the acid-treated biomass compared to the untreated biomass. Both FT-IR and FT ion cyclotron resonance mass spectroscopy of the bio-crude oils showed no significant difference in the functional groups and heteroatoms in the hexane-soluble bio-crude oil derived from untreated and acid-treated biomass. These results demonstrate that demineralization of wastewater algae is relevant for improving the feasibility of algal wastewater-treatment-to-biofuel systems.
Microalgae (including cyanobacteria) have recently gained interest as a potential food and feed source due to their high nutritional value. Some microalgae and cyanobacteria have been traditionally consumed worldwide. Under the European Union’s legislation, they are considered, with only few exceptions, as novel foods, and therefore in order to enter the EU market it is required primarily to be authorized after risk assessment to ensure that they do not display any risk to public health. This chapter gives an overview of the current legislative situation of microalgae as food in the European Union and discusses the potential health risks associated with the consumption of microalgae and cyanobacteria.
Hydrothermal liquefaction (HTL) process is a wet-thermochemical conversion technology that was used to convert biomass into bio-crude oil. Commercialization of the HTL technology faces drawbacks due to product stability, by-products generation and working area. In order to overcome these challenges, this review especially focuses on possible pathways to valorise and recover nutrients from post-hydrothermal liquefaction wastewater (PHWW) that was obtained during the HTL process. Numerous studies were reported on bio-oil production from biomasses like algae, forest and agriculture residues, etc at a temperature range of 240–320 °C at time of 30–60 min. Apart from bio-oil, nearly 25 to 50 wt% of aqueous phase was generated, disposal of this aqueous phase is hectic since it comprised of low molecular weight acid compounds. In this study, the composition of PHWW and possible routes (biological and thermochemical pathways) to valorise it were discussed in detail. In addition, recycle and reutilization of PHWW were reviewed with recent findings. From the review, the use of anaerobic digestion as a detoxify step prior to microalgae cultivation resulted in decreasing the fresh water dilution from 20 × to 4 × and improved the energy output from 3.44 to 20.7 kJ g⁻¹ COD. This review will provide new insights towards closed circular approach opportunities in thermochemical pathways during sustainable energy production.
As a highly effective nanocomposite for hydrothermal liquefaction (HTL) of microalgae, the recycled biochar synthesized from Spirulina platensis and impregnated into CeO2 has been demonstrated. The order of in situ > ex situ > biochar nanocomposites for higher bio-oil. The highest bio-oil conversion of 33% was achieved at the optimum temperature of 250 °C. The use of the biochar nanocomposite also resulted in a decrease in the oxygen and nitrogen content of the bio-oil and an increase in its heating value, which was found to be high at 35.64 MJ/kg. With the inclusion of the in situ biochar nanocomposite, energy recovery was increased by up to 65.34%. The current study has shown that low biochar nanocomposite concentrations (0.20 g), low temperature (250 °C), and short residence time (30 min) are essential for improved bio-oil yield and quality of bio-oil.
The present study attempts to integrate phyco-remediation and enhanced lipid productivity using microalgae-bacterial consortium enriched from wastewater fed aquaculture pond. Metagenomic analyses and microscopic images of the consortium revealed the presence of Chlorella variabilis, Parachlorella kessleri, Thermosynechococcus elongatus, Chlamydomonas, Phaeodactylum tricornutum, Oscillatoriales, Synechocystis sp., Microcystis aeruginosa, Nostocales, Naviculales, Stramenopiles, other members of Chlorophyceae, Trebouxiophyceae, and Chroococcales along with potential bacterial bioremediants. During a 30 days trial run (15 days stabilization and 14 days remediation studies) for phyco-remediation drastic reduction in the nutrient and COD content from the tested wastewater samples was seen. There was up to 93% and 87.2% reduction in chemical oxygen demand (COD) and ammonium concentration, respectively. Further, almost 100% removal of nitrates and phosphates from the dairy wastewater upon 48 h of treatment with polyculture under ambient temperature (25 ± 2 °C) with 6309 lux illumination and mild aeration, was observed for all the seven cycles. Interestingly, the nutrient and COD concentrations in the treated water were below the discharge standards as per Central Pollution Control Board (CPCB) norms. In additions, biomass (reported as dry cell weight) was enhanced by 67% upon treatment with ammonia-rich dairy wastewater exhibiting 42% lipid, 55% carbohydrate, and 18.6% protein content enhancement. The polyculture mainly grown as attached biofilm to the surface, offered an easy harvesting and separation of grown biomass from the treated wastewater. Overall, dairy wastewater was found to be a potential nutrient source for microalgae-bacteria cultivation thereby making the treatment process sustainable and eco-friendly.
Production of low carbon biofuels and biochemical from renewable feedstock’s using a biological process is being considered as one of sustainable alternatives to fossil-based linear economy. This study demonstrated the pilot-scale (10 m3) production of biohydrogen (bio-H2) and volatile fatty acids/carboxylic acids (VFA) through acidogenic fermentation (AF) using renewable food waste (FW) as feedstock. Bio-H2 production of 54,288 L (155.10 LH2/kg COD) along with 25.77 g/L of VFA (composed of acetic (HAc: 15.2±1.98 g/L), propionic (HPr: 4.89±1.26 g/L) and butyric acid (HBu 5.67±0.96 g/L) was achieved within 48 h of the fermentation period. COD removal of 58% with 49.7% of bio-H2 conversion efficiency (HCE) was observed. The further acidogenic process was integrated with biorefinery platform (methanogenesis + photosynthesis) in a circular loop strategy which assisted to derive multiple biobased products (CH4, algal biomass, O2 and treated water for reuse) from fatty acids rich acidogenic effluent and untreated COD of AF. The whole bio-manufacturing unit (acidogenesis + methanogenesis + photosynthesis) converted the renewable feedstock (waste/wastewater) into fuels and platform chemicals, in analogy to a conventional oil refinery with maximizing resource recovery. Life cycle assessment (LCA) tool was employed to study the environmental impact of both for bio-H2 (standalone, ST) as well as for waste biorefinery (WB) processes and the results depicted that WB approach offers relatively lower impact (approx. 3.5 folds less than ST). The approach offered to determine the flow of carbon and its conversion to products which aided in the reduction of carbon emissions as well as minimizing the burden on natural resources with the biosynthesis of green H2 and other value-added products addressing carbon neutrality with bioeconomy.
Hydrothermal liquefaction (HTL) is a promising technique for crude bio-oil (biocrude) production from microalgae. Instead of traditional direct HTL at one temperature with a residence time, the present work explored two temperature steps (TTS) and more temperature steps (MTS) of microalgae (Chlorella) HTLs for the first time in mini-batch reactors. Specifically, the reactions for the TTS of HTL were performed at a relatively low temperature (150–300 °C) for 10–40 min and then at a high temperature (350 °C) for 10–20 min. In the MTS of HTL, three or four temperature steps were adopted and each temperature stage (within 150–300 °C) was kept for 10 min. The results show that the low temperature pre-reaction stage significantly affected the yield, elemental composition, higher heating value, energy recovery and molecular component of biocrude. The biocrude derived from the TTS of HTL of 250*20–350 (i.e., a pre-reaction at 250 °C for 20 min followed by a HTL at 350 °C for 10 min) had the highest H content (9.00 wt%) and the lowest S content (0.55 wt%). Its yield, elemental composition, higher heating value, and energy recovery were comparable with those of the biocrude from the direct HTL at 350 °C for 30 min (with the highest yield and the best quality in tests). In comparison to direct HTL, a proper TTS of microalgae HTL was able to reduce reaction temperature on the premise of ensuring similar biocrude properties, so might be applicable in the algal bio-oil production.
Some wastewater sources, such as agricultural waste and runoff, and industrial sewage, can degrade water quality. This study summarises the sources and corresponding mechanisms that trigger eutrophication in lakes. Additionally, the trophic status index and water quality index (WQI) which are effective tools for evaluating the degree of eutrophication of lakes, have been discussed. This study also explores the main nutrients (nitrogen and phosphorus) driving transformations in the water body and sediment. Lake Erhai was used as a case study, and it was found to be in a mesotrophic state, with N and P co-limitation before 2006, and only P limitation since 2006. Finally, effective measures to maintain sustainable development in the watershed are proposed, along with a framework for an early warning system adopting the latest technologies (geographic information systems (GIS), remote sensing (RS)) for preventing eutrophication.
This review examines in detail the production and characteristics of biochar resulting from hydrothermal liquefaction. Specifically, the impact of feedstocks and different process parameters on the properties and yield of biochar by hydrothermal liquefaction has been thoroughly studied. Hydrothermal liquefaction derived biochars, relative to biochars from high-temperature thermochemical processes retain critical functional groups during carbonization and are therefore promising for a wide range of applications. Most of the review's efforts are to study possible hydrothermal liquefaction biochar applications in various fields, including fuel, metal and dye adsorption, pollutant reduction, animal feed, and biogas catalyst. The feasibility of biochar through the hydrothermal liquefaction process has been analysed via life-cycle assessment and energy evaluation. The article concludes with a brief discussion on possible issues and strategies for the sustainable development of hydrothermal liquefaction-based biochar.
Microalgal biomass as bioenergy feedstock is gaining wide attention for biocrude production through hydrothermal liquefaction (HTL). However, the availability of feedstock in all seasons is a major challenge. Hence, to ensure a consistent supply of feedstock and transform waste to energy, the present study investigates co-HTL of domestic wastewater treatment derived microalgal biomass (Monoraphidium sp. KMC4) and domestic sewage sludge (DSS) as bioenergy feedstocks. The effects of temperature, feedstock ratio, and residence time were studied and optimised for maximum biocrude yield. The study showed that, co-HTL at optimum operating conditions of 325 °C, 75:25 wt% (KMC4:DSS), and 45 min produced 39.38 wt% biocrude yield at a conversion rate of 83.96 wt%. The optimum biocrude yield was 16% and 79% higher than the individual HTL of KMC4 and DSS respectively. The comprehensive characterizations of co-HTL biocrude showed 76.77%, 10.6%, 8.85%, 3.38% of C, H, N, O and 39.47 MJ Kg⁻¹ of HHV with an energy recovery rate of 77.53%. Meanwhile, co-HTL enhanced the distillation profile of biocrude which had 10.13% of heavy naphtha, 23.92% of kerosene, and 27.09% of gas oil. The FTIR and GC–MS analysis confirmed that the co-HTL biocrude had superior hydrocarbons such as alcohols and esters with limited nitrogen and oxygen heterocyclic compounds. In addition, ICP-AES confirmed a significant decrease in transfer of mineral elements from the co-HTL feedstock to biocrude. This validates the sustainability of the co-HTL process to produce high energy density biocrude with the potential to substitute fossil fuels.
To explore the feasibility of scaling up hydrothermal liquefaction (HTL) of algal biomass, a pilot-scale continuous flow reactor (CFR) was operated to produce bio-crude oil from algal biomass cultivated in urban wastewater. The CFR system ran algal slurry (5 wt.% solids loading) at 350 °C and 17 MPa for 4 h without any clogging issues. Bio-crude oil chemistry was characterized by high-resolution Fourier transform mass spectroscopy (FT-MS), proton nuclear magnetic resonance spectroscopy (1H NMR), bomb calorimetry, and elemental analysis. Bio-crude oil yield of 28.1 wt% was obtained with higher heating values of 38-39 MJ/kg. The quality of light bio-crude oil produced from the CFR system was comparable in terms of molecular structures to bio-crude oil produced in a batch reactor.
In plants, solar energy is converted into chemical energy by the complex process of photosynthesis. Crop production is strongly dependent on the photosynthetic rates. In higher plants, photosynthetically active radiation (PAR), the fraction of sunlight with wavelengths from 400 to 700 nm is harvested by the photosynthetic pigments of photosystems PSI and PSII. PAR intensity is an important factor that determines the rate of photosynthesis. Too high or too low PAR intensities adversely affect the photosynthetic machinery. At low light intensities above the light compensation point (LCP), photosynthetic rate increases proportionally to the light intensity and reaches a maximum. Generally, plants try to maintain photosynthetic efficiency under changing light intensities by balancing conversion of radiation energy and protecting any damage to photosynthetic apparatus by photoinhibition and repairing damage. Some of the photoprotective mechanisms to dissipate surplus energy are quenching of chlorophyll fluorescence, modification of light‐harvesting and energy‐transfer processes. Alterations in leaf anatomy and chloroplast movements regulate the amount of light absorption by enhancing light capture in shade conditions and preventing light absorption in excessive light conditions in order to maximize photosynthetic efficiency. Through a genetic, molecular, physiological, biochemical, and functional genomics approach, significant developments have been made in identifying genes and molecular mechanisms underlying the relationship of light intensity and photosynthesis. Understanding these mechanisms has a practical value in improving photosynthetic efficiency in crop species growing under differential light intensities.
Microalgae due to its metabolic versatility have received a focal attention in the biorefinery and bioeconomy context. Microalgae products have broad and promising application potential in the domain of renewable fuels/energy, nutraceutical, pharmaceuticals and cosmetics. Biorefining of microalgal biomass in a circular loop with an aim to maximize resource recovery is being considered as one of the sustainable option that will have both economical and environmental viability. The expansive scope of microalgae cultivation with self-sustainability approach was discussed in this communication in the framework of blue-bioeconomy. Microalgae based primary products, cultivation strategies, valorization of microalgae biomass for secondary products and integrated biorefinery models for the production of multi-based products were discussed. The need and prospect of self-sustainable models in closed loop format was also elaborated.
Two strains of Galdieria sulphuraria algae, 5587.1 and SOOS, were grown on municipal wastewater to develop energy-positive treatment systems. Hydrothermal liquefaction (HTL) of 5-10 wt.% algal biomass solids was conducted at 310-350 °C for 5-60 min. to produce bio-crude oil. HTL product yields and energy recovery were compared to those from previous studies using G. sulphuraria grown on a modified Cyanidium medium. Total bio-crude oil yields were lower (11.2-23.0 wt.%) and char yields were higher (22.6-36.4 wt.%) for HTL of algae grown on actual wastewater compared that grown on media (31.4 wt.% and 4.8 wt.%, respectively), indicating a potential limitation for using yields from media-based studies. High-resolution mass spectroscopy of bio-crude oil provides new insights into differences in composition based on growth media Energy recovery in total bio-crude oil and char at 350°C was 17-28% and 14-19%, respectively, for the 5587.1 strain, and 23-27% and 14-25%, respectively, for the SOOS strain.
An effluent from the dairy industry, waste scum oil containing triglycerides of fatty acids from C 4 - C 18 was selected as a potential feedstock for biodiesel production in the presence of nano calcium oxide obtained from modified eggshell. The eggshell waste was calcined at 800 °C to facile synthesize nano calcium oxide and was characterized by X-ray diffractometer (XRD), Fourier Transform Infrared spectrometer (FTIR) and Scanning Electron Microscope (SEM). The calcined eggshell contains mainly nano calcium oxide (CaO) in calcite form with the crystallite size ranging from 16 to 22 nm. The transesterification reaction was carried out in a batch reactor and the operating parameters like catalyst loading, the molar ratio of methanol: oil, reaction temperature and time were optimized. Maximum biodiesel yield of 96% was obtained at the molar ratio of methanol: oil of 6:1, catalyst amount of 2.4 wt% and the reaction temperature of 65 °C for 3 h. The quality of biodiesel produced by transesterification of dairy waste scum was tested on variable four-stroke compressible diesel engine and found to be comparable with conventional diesel in brake thermal efficiency and specific fuel consumption. The biodiesel produced possessed high cetane number and low NOx, than conventional diesel. The results proved that nano CaO obtained from eggshell can be effectively reused and recycled as a heterogeneous catalyst for biodiesel production.
The potential of microalgae for the treatment of dairy wastewater (DWW) was studied by integrating with bioethanol production. At the end of treatment, organic carbon removal was observed to be 90% with simultaneous removal of nutrients. Biomass concentration increased from 3 rd day and reached to a maximum of 1.4 g L ⁻¹ by the end of cycle. The biomolecular composition of microalgae comprised of 38% carbohydrates, 15% proteins and 22% lipids. Reducing sugars extracted from deoiled microalgae showed highest percentage of glucose (54.12%) than other monomers. The reducing sugars obtained were utilized for the production of bioethanol via yeast fermentation using Saccharomyces cerevisiae. This resulted in the production of ethanol (3G) upto 116.2 mg g ⁻¹ with simultaneous decrease in reducing sugars upto 92 mg g ⁻¹ . The results obtained indicate potential of microalgae to produce multiple biobased products in a biorefinery framework.
The current study is aimed at understanding the effect of two different concentrations of nitrate (NaNO 3 ) i.e., 2.94 mM (1X) and 8.82 mM (3X) on the productivity of Scenedesmus sp. in terms photosynthetic efficiency, growth, biomass and protein/lipid synthesis. The experiments were conducted by growing the microalgae in mixotrophic mode with a fixed dissolved organic carbon (110 mM). Chlorophyll a fluorescence fast kinetics parameter such as F V /F M F M /F O , P i _Abs, TRo/RC and ABS/RC depicted an improved PSII efficiency in 3X conditions. Higher nitrate concentration in BBM medium favored better assimilation of chlorophyll pigments, carbohydrates (160 mg/g), proteins (524 mg/g) and total lipids along with higher biomass (11.4 g/L). The microalgae cell growth, biomass and biochemical composition are significantly influenced by excess nitrates supplementation in the growth medium.
Hydrothermal liquefaction (HTL) is a promising technique of producing crude bio-oil (biocrude) from wet biomass. This work conducted the co-HTLs of microalgae (chlorella) and sewage sludge (SS) at 340 °C, 18 MPa, 0.3 MPa of initial H 2 addition, 30 min of residence time under different feedstock mass ratios conditions, and explored the effects of three kinds of SS ashes on biocrude properties during microalgae HTL for the first time. Corresponding biocrude yields, elemental compositions, higher heating values, energy recoveries, boiling point distributions, and compound compositions were examined systematically. The results show that there was a certain synergistic effect on the improvement of biocrude yield other than biocrude quality in the co-HTL of microalgae and SS, especially at the 1:1 of mass ratio condition. This co-HTL could improve the actual biocrude yield by 4.7 wt% and decrease the actual solids yield by 3.6 wt% in contrast to corresponding theoretical yields. The pyrolysis-state SS ash could reduce the N and O contents, increase the C and H contents and HHV, and improve the proportion of low-boiling-point (<250 °C) compounds in the biocrude from microalgae HTL, while the oxidation-state or reduction-state SS ash was able to increase biocrude yield by approximately 3.3 wt%.
Algae are a group of ubiquitous photosynthetic organisms comprising eukaryotic green algae and Gram-negative prokaryotic cyanobacteria, which have immense potential as a bioresource for various industries related to biofuels, pharmaceuticals, nutraceuticals and feed. This fascinating group of organisms also has applications in modern agriculture through facilitating increased nutrient availability, maintaining the organic carbon and fertility of soil, and enhancing plant growth and crop yields, as a result of stimulation of soil microbial activity. Several cyanobacteria provide nitrogen fertilization through biological nitrogen fixation and through enzymatic activities related to interconversions and mobilization of different forms of nitrogen. Both green algae and cyanobacteria are involved in the production of metabolites such as growth hormones, polysaccharides, antimicrobial compounds, etc., which play an important role in the colonization of plants and proliferation of microbial and eukaryotic communities in soil. Currently, the development of consortia of cyanobacteria with bacteria or fungi or microalgae or their biofilms has widened their scope of utilization. Development of integrated wastewater treatment and biomass production systems is an emerging technology, which exploits the nutrient sequestering potential of microalgae and its valorisation. This review focuses on prospects and challenges of application of microalgae in various areas of agriculture, including crop production, protection and natural resource management. An overview of the recent advances, novel technologies developed, their commercialization status and future directions are also included.
In this study, bio-oil was produced through hydrothermal liquefaction (HTL) of C. vulgaris biomass cultivated in wastewater and was enriched into transportation fuels. Bio-oil yield was 29.37% wt at 300 °C, 60 min, at 15 g/200 mL biomass loading rate with 3% wt nano ZnO catalyst loading. Applying catalyst reduced oxygen and nitrogen content in bio-oil and increased its calorific value (19.6 ± 0.8 MJ/Kg). Bio-oil was enriched through liquid-liquid extraction (LLE) and higher yield was obtained at 30 °C for dichloromethane solvent (18.2% wt). Compounds of enriched oil were within the petro-diesel range (C8-C21). Bio-char after HTL process was activated and used as adsorbent in wastewater treatment process to remove organic pollutants (COD, NO3, NH3 and PO4). Treated wastewater can be supplied as growth medium for microalgae cultivation in further experiments. Nearly 3-4 times the nanocatalyst can be reused in the HTL process.
Hydrothermal liquefaction (HTL) of renewable microalgae is a potential promising technology for liquid biofuel production. This paper investigated the effect of ultrasonic pre-treatment of microalgae (Spirulina platensis) on HTL products distribution and oil quality. Cell disruption was enhanced with increasing of ultrasonic power and time. Liquefaction was promoted by larger ultrasonic power, but restrained by prolonged pre-processing time. Such promotion was found more pronounced at relatively lower liquefaction temperature. Combined with FT-IR, GC–MS and TG analysis, the ultrasonic pre-treatment increased the compounds at lower boiling point within the biocrude oil. The distillation characters of the hydrothermal oils were more near to the heavy Iraqi crude, with only 22 wt% in distillation zone of diesel.
Algal biomass is known as a promising sustainable feedstock for the production of biofuels and other valuable products. However, since last decade, massive amount of interests have turned to converting algal biomass into biochar. Due to their high nutrient content and ion-exchange capacity, algal biochars can be used as soil amendment for agriculture purposes or adsorbents in wastewater treatment for the removal of organic or inorganic pollutants. This review describes the conventional (e.g., slow and microwave-assisted pyrolysis) and newly developed (e.g., hydrothermal carbonization and torrefaction) methods used for the synthesis of algae-based biochars. The characterization of algal biochar and a comparison between algal biochar with biochar produced from other feedstocks are also presented. This review aims to provide updated information on the development of algal biochar in terms of the production methods and the characterization of its physical and chemical properties to justify and to expand their potential applications.