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The aquaculture industry is an efficient edible protein producer and grows faster than any other food sector. Therefore, it requires enormous amounts of fish feed. Fish feed directly affects the quality of produced fish, potential health benefits, and cost. Fish meal (FM), fis oil (FO), and plant-based supplements, predominantly used in fish feed, face challenges of low availability, low nutritional value, and high cost. The cost associated with aquaculture feed represents 40-75% of aquaculture production cost and one of the key market drivers for the thriving aquaculture industry. Microalgae are a primary producer in aquatic food chains. Microalgae are expanding continuously in renewable energy, pharmaceutical pigment, wastewater treatment, food, and feed industries. Major components of microalgal biomass are proteins with essential amino acids, lipids with polyunsaturated fatty acids (PUFA), carbohydrates, pigments, and other bioactive compounds. Thus, microalgae can be used as an essential, viable, and alternative feed ingredient in aquaculture feed. In recent times, live algae culture, whole algae, and lipid-extracted algae (LEA) have been tested in fish feed for growth, physiological activity, and nutritional value. The present review discusses the potential application of microalgae in aquaculture feed, its mode of application, nutritional value, and possible replacement of conventional feed ingredients, and disadvantages of plant-based feed. The review also focuses on integrated processes such as algae cultivation in aquaculture wastewater, aquaponics systems, challenges, and future prospects of using microalgae in the aquafeed industry.

Aquatic plants are promising green energy feedstocks owing to their high rate of growth, photosynthesis, and CO2-fixing efficiency. They possess a paramount advantage of non-competitiveness with food crops over the first or second generation biofuel feedstocks. Specifically, low lignin content and higher concentrations of polysaccharides make these plants very attractive for biogas and liquid biofuel production. However, a regular supply of biomass is a limitation that can be overcome by employing harvesting techniques with sustainable measures, which ensure rapid regrowth of biomass for the next cycle. Harvesting of both aquatic macrophytes (weeds) as well as macroalgae is achieved by either manual or mechanical means. Following regular supply through effective harvesting, biofuel production can be further restricted due to their complex structural make-up. In order to improve the biofuel production, various pretreatment methods have been explored to disrupt the complex structure of aquatic weeds and macroalgae, thereby increasing the breakdown of biomass material more readily. This review examines traditional and modern techniques for biofuel production using aquatic weeds and macroalgae. It also discusses recent advancements in the harvesting and pretreatment techniques that improve overall efficiency. Choosing an effective pretreatment method can greatly influence biofuel recovery and production.

A catalyst comprised of SnO2 impregnated on ZnO nanowires, which presented remarkable ability to catalyze fatty acid esterification/transesterification reactions, is reported. For optimization of reaction conditions, artificially acidified soybean oil with 10 wt.% oleic acid was used as a model feed. The optimized conditions were: 150 °C, 6 h, 5 g of oil, catalyst concentration of 5%, and methanol:oil molar ratio of 15:1. The catalyst achieved 92% of total fatty acid methyl esters (FAME) content and was used five times without the necessity of catalyst washing from one reaction to the other. Then, such conditions were applied to produce biodiesel from the oil extracted from Scenedesmus sp. microalgae; the system reached 72% of FAME content, without any previous refining or degumming process of the oil. Rietveld refinement, X-ray diffraction, elemental mapping in scanning transmission electron microscopy, X-ray photoelectron spectroscopy, and pyridine-desorption Fourier-transform infrared spectroscopy were used to characterize the material.

Commercial scale microalgal biodiesel production is still challenging due to high cultivation cost and inefficient conversion technologies. This work explicated an economical cultivation strategy using anaerobic centrate as growth medium of Acutodesmus obliquus. Among various modes evaluated maximum lipid productivity of 57.03 mgL−1d−1 was achieved under mixotrophic cultivation condition using anaerobic centrate. Conversion of microalgal lipids to biodiesel using immobilized Candida rugosa lipase was investigated. Reaction parameters viz. temperature, methanol to oil molar and enzyme amount were optimized using response surface methodology. Best conversion of 95.36% was achieved at 50 °C, methanol to oil ratio of 3:1 and 15% enzyme amount based on oil weight. Immobilized C. rugosa lipase can be used for 5 batches without much loss in conversion efficiency (>90%). Most of the fuel properties comply with ASTM and EN standards.

The use of phytohormones in microalgal biofuel production is gaining interest due to their substantial effect on biomass and lipid enrichment under standard and nitrogen-limited conditions. In the present study, the collaborative effect of combination of selected phytohormones viz. zeatin (Z), indole acetic acid (IAA), and gibberellic acid (GA) was studied to optimize biomass and lipid production in microalga Acutodesmus obliquus under nitrogen-limitation using response surface methodology (RSM). Significantly higher biomass and lipid productivity was obtained with the supplementation of combination of Z (0.5 mg L−1), IAA (1.0 mg L−1), and GA (5.0 mg L−1) under nitrogen-limitation with 49.07% and 77.20% increase respectively as compared to optimized nitrogen control (ON). Zeatin was found to be the most significantly influencing phytohormone followed by GA in RSM data analysis. Present study elucidated the synergistic effect of combination of exogenous phytohormones in enhancing biomass and lipid productivity under nitrogen-limitation for microalgal biofuel production.

This study attempted to determine an efficient flocculant that was used for the recovery of microalgal species, namely Scenedesmus. The biomass was grown in a pilot-scale raceway pond receiving domestic wastewater discharges. The microalgae achieved treatment efficiencies of 76.5% NH4+-N, 83.1% PO43−-P, 73.9% biological oxygen demand (BOD), and 42.8% chemical oxygen demand (COD). During microalgal harvesting, the highest biomass recoveries of 99.0%, 97.5%, and 96.5% were obtained by alum 300 mg/L at pH 7, chitosan 100 mg/L at pH 7, and cationic polymer 10 mg/L at pH 10, respectively. The protein, carbohydrate, and lipid yields of polymer-harvested biomass were 28.0%, 12.4%, and 17.4% (dry cell weight), respectively, which were higher than those obtained when applying either alum or chitosan flocculants. Moreover, the cationic polymer provided no detrimental effects on the fractions of fatty acids derived from the harvested microalgal cells. The microalgal biomass subjected to flocculation by either polymer or chitosan were entire and intact, whereas the lysis of cells was noticed during the alum flocculation process.

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.

The green synthesis of carbon dots (CDs) from renewable resources has attracted attention of the scientific community for their use in different industrial applications. CDs derived from microbial biomass can be synthesized using simple and low-cost methods, and have been reported to possess high compatibility, excellent optical behavior and low toxicity. In this study, carbon dots were synthesized using lipids and carbohydrates extracted from microalga Acutodesmus obliquus via microwave hydrothermal digestion method. The synthesized carbon dots showed excitation peaks in the ultraviolet light region (c.a.350 nm) and emission in the visible region (c.a.420 nm). The addition of acetone led to red shifting of excitation wavelength in all the synthesized carbon dots. The transmission electron microscopy analysis depicts the nano-size (1.2-11 nm) range of the synthesized carbon dots, where excitation and emission wavelengths were not dependent on the size of the particles. Carbon dots prepared from microalgal metabolites showed fluorescence under ultraviolet luminescence in different wavelengths after the addition of chemicals. The carbon dots thus generated can be explored for their applications such as dyes, sensors, bioimaging and photosensitizers.

Economical and sustainable microalgal biomass production is crucial for its commercial scale application for energy and other commodities. This work elucidates a novel cultivation strategy where nutrient-rich waste activated sludge (WAS) and final effluent (FE) from a municipal wastewater treatment process is used for microalgal biomass generation. This strategy reduces the use of synthetic nutrients, fertilizers and fresh water. Strategy development included investigation of pre-treatment/extraction methods for effective nutrient release and bacterial load reduction. Evaluation of growth kinetics, photosynthetic performance, nutrient removal efficiencies and biochemical composition of microalgae under mixotrophic (Mixo) and heterotrophic (Hetero) modes of cultivation was performed. Urea supplementation is studied to enhance the biomass productivity. Microalgae cultivation in acid pre-treated WAS + FE with urea supplementation of 1500 mg L− 1 showed biomass productivity of 298.75 mg L− 1 d− 1. Microalgal biomass grown with WAS + FE using developed strategy showed higher lipid and protein productivities and comparable carbohydrate yields to the synthetic media.

Development of novel and scalable strategies for improving lipid and biomass yield is a prerequisite for sustainable biodiesel production from microalgae. Currently, nutrient stress is the widely employed lipid enhancement strategy in microalgae, yet, it is associated with compromised biomass productivity. In this study, an alternative approach for lipid enhancement in Chlorella sorokiniana is proposed to alleviate the constraint of low biomass production under nutrient stress. This is achieved through combining nutrient stress along with selected metals and EDTA stress. A substantial increase in lipid yield (77.03 mg L−1 d−1) was achieved when the nutrient (nitrogen and phosphorus) stress BG11 media was supplemented with a combination of iron (15 mg L−1), magnesium (125 mg L−1), calcium (18 mg L−1) and EDTA (3 mg L−1). This was further validated by quantifying the expression levels of two key genes involved in photosynthesis (rbcL) and lipid biosynthesis (accD) pathways. Under this developed strategy a 5.15-fold increase in rbcL and 9.79-fold increases in accD gene expression were noted in comparison to the culture grown in BG11 medium. A significant correlation could also be drawn between the expression of rbcL and accD genes to biomass yields, photosynthetic performance and lipid productivity. The proposed strategy could be easily applied at various commercial scale microalgal cultivation systems for enhancing lipid content without compromising the biomass yield.

Heterogeneous solid acid catalyzed conversion of microalgal lipids to biodiesel is a scarcely studied area. In this study chromium-aluminum mixed oxide catalyst was investigated for catalytic conversion of microalgal lipids to biodiesel. Lipids from Scenedesmus obliquus grown in an open raceway pond (3000L) was used as feedstock. Reaction variables such as temperature, methanol to oil molar ratio and catalyst amount were optimized using response surface methodology. FAME conversion of 98.28% was achieved using chromium-aluminum catalyst at 80 °C, with methanol to oil molar ratio of 20:1 and catalyst amount of 15%. Catalytic efficiency of this heterogeneous solid acid catalyst was compared to a homogeneous acid catalyst (sulfuric acid). Chromium-aluminum mixed oxide catalyst can be effectively used for 4 batches of conversion reactions without significant loss in its activity.

Despite the significant reduction in biomass production, nitrogen stress is the most feasible strategy used for lipid enhancement in microalgae. The present study evaluates the potential for application of exogenous cytokinins to enhance the lipid accumulation and overcome the biomass production constraint of microalgae under nitrogen stressed conditions. This study was undertaken to investigate the effect of exogenously supplied cytokinins viz . kinetin and zeatin at various stages of growth on biomass and lipid productivities of Acutodesmus obliquus under nitrogen stress (ON). The effect of cytokinins on photosynthetic performance and biochemical composition of biomass was also evaluated. Enhanced biomass and lipid productivities were observed upon supplementation of the microalgae with selected cytokinins. Supplementation with kinetin (1 mg L −1 ) and zeatin (0.1 mg L −1 ) resulted in increased biomass productivity up to 50% and 60.7% respectively as compared to ON control. The highest biomass productivity of 176.79 mg L −1 d −1 was obtained with the addition of 0.1 mg L −1 zeatin (ON-Z i ) at the initial log phase. Cytokinin supplementation resulted in improvement in photosynthetic performance and exhibited higher rETR (relative electron transport rate) values as compared to ON control. The addition of kinetin (ON-K m ) at mid log phase and Zeatin (ON-Z i ) at initial log phase resulted in 64.95% and 63.06% increase in lipid productivity respectively, compared to ON control. Carbohydrate productivity was also found to be increased with zeatin (ON-Z i ) supplementation. The supplementation of cytokinins is an easy and scalable strategy for biomass and lipid enhancement in Acutodesmus obliquus under nitrogen stress.

Abiotic stresses like drought, cold, salinity, heat, oxidative stress and the presence of excess levels of heavy metal in the agroecosystems lead to a decrease in the growth and productivity of major crops worldwide. The majority of stresses are connected with each other and results in elevated adverse impacts on the plants as well as other important components of the environment. The intensity of stresses and associated adverse impacts are increasing substantially in the era of climate change that again triggers to produce abnormalities in the crops. To overcome the effects of abiotic stresses, a number of strategies have been investigated, such as developing and cultivate stress-tolerant varieties, use of organic fertilizers, and the application of high yielding varieties. Application of biochar to mitigate the impacts of major abiotic stresses especially drought, salinity, and heavy metal has been found very effective. Amendment of biochar in stress affected agroecosystems improves the soil physicochemical and biological features and thereby enhances the productivity of crops. In this chapter, efforts have been made to discuss about three major stresses, i.e. drought, salinity, and heavy metals, their impacts on soil as well as plant productivity. Further, the efficiency and mechanism of biochar in reducing the impacts of stresses when using as a soil amendment have also been discussed thoroughly.

Soil quality degradation is one of the major outcomes of environmental pollution that can be characterized by deficient nutrients and concurrent presence of toxic substances in the agroecosystems. To enhance crop productivity, farmers use uncontrolled synthetic fertilizers that further deteriorate the soil health. Several organic components have been developed to reduce the overload of chemical fertilizers. Application of biochar to the soil is one of the potential methods that not only enhance the crop productivity but at the same time improve soil quality parameters. The application of biochar has also been proved to reduce the greenhouse gaseous emission from the agroecosystems. In this chapter, the efforts have been made to explore the production, features, mode of application, and effects of biochar on crop productivity and soil health. Further, the impact of biochar in the mitigation of abiotic stresses has also been discussed.

Cultivation of microalgae utilizing wastewater substrate could form a sustainable biorefinery with double benefit of biomass generation and nutrient remediation. In this study potential of aquaculture wastewater is evaluated for cultivation of Chlorella sorokiniana in heterotrophic mode for generation of high value biomass. Nutrient removal potential is also assessed. Aquaculture wastewater with 400 mgL−1 sodium nitrate supplementation resulted in biomass productivity of 498.14 mgL−1d−1. The biomass generated showed lipid productivity of 150.19 mgL−1d−1, carbohydrate productivity of 172.91 mgL−1d−1 and protein productivity of 141.57 mgL−1d−1. The nutrient removal efficiencies were 75.56% for ammonium, 84.51% for nitrates, 73.35% for phosphates and 71.88% for COD (chemical oxygen demand). The findings of this study underline the potential of aquaculture wastewater for production of valuable microalgal biomass which can be utilized for biofuels or feed application. This biorefinery concept also polished aquaculture wastewater which can be effectively reused.

Downstream step of catalytic conversion is scarcely investigated area in microalgal biodiesel production process. In this study a heterogeneous acid catalyst, tungstated zirconia (WO3/ZrO2) is evaluated for conversion of S. obliquus lipids. Catalytic efficiency of tungstated zirconia catalyst was compared to the homogeneous acid catalyst and enzyme catalyst in terms of conversion efficiency, reaction parameters, energy consumption and reusability. Tungstated zirconia catalyst showed maximum biodiesel conversion of 94.58% at 100 °C temperature, 12:1 methanol to oil molar ratio and 15% of catalyst amount based on oil weight in 3 h. Tungstated zirconia showed comparable biodiesel conversion to homogeneous catalyst and higher conversion than the enzyme catalyst. The time requirement for heterogeneous catalyst was lowest, while, the energy consumption was highest among the selected catalysts. Most of the fuel properties of biodiesel synthesized by tungstated zirconia catalyzed conversion of S. obliquus lipids comply with the specifications set by ASTM and EN standards.

Microalgal cultivation using aquaculture wastewater is a promising biorefinary concept for integrated biomass generation and subsequent nutrient removal. In this study, potential of aquaculture wastewater (AWW) as a nutrient substrate for cultivation of Scenedesmus obliquus , Chlorella sorokiniana and Ankistrodesmus falcatus was investigated. Nutrient removal efficiencies were also investigated for selected microalgal strains. Sodium nitrate supplementation strategy is applied to enhance the productivities of biomass, lipid, carbohydrate and protein. Biomass productivities of A. falcatus (198.46 mg L − 1 d − 1 ) with 400 mg L − 1 sodium nitrate supplementation and C. sorokiniana (157.04 mg L − 1 d − 1 ) with 600 mg L − 1 sodium nitrate supplementation in AWW were comparable to the synthetic medium. Comparable lipid, carbohydrates and proteins productivities were observed in microalgal biomass cultivated using AWW to the productivities in the synthetic medium. Microalgal cultivation in AWW showed removal efficiencies in the range of 86.45–98.21% for ammonia, 75.76–80.85% for nitrate, 98.52–100% for phosphate and 42–69% for COD.

Harvesting of microalgal biomass is still a bottleneck to its commercial scale application, due to small cell size, low culture densities, colloidal stability and thus unfavourable economics. Centrifugation is an efficient technique but the high energy consumption makes it unsuitable for low value microalgal products. Chemical flocculation and filtration are inefficient and time consuming methods for harvesting of small size microalgae. In this study, an electrochemical harvesting (ECH) process was assessed for the harvesting of a small size microalga Ankistrodesmus falcatus by using non-sacrificial carbon electrodes. Harvesting efficiency of ECH was compared to centrifugation and flocculation using alum and chitosan. The highest recovery efficiency was obtained by centrifugation (93% after 15 min) followed by ECH process (91% after 30 min), alum (86% after 60 min) and chitosan (55% after 60 min). However, the energy consumption of ECH process (1.76 kWh kg− 1) was much lower than the centrifugation process (65.34 kWh kg− 1). The biochemical composition of harvested biomass was also assessed, and it was found that the ECH process has no deteriorating effect on the quality of biomass. High recovery efficiency, low energy consumption and the use of non-sacrificial electrodes make ECH a sustainable harvesting technique for small size microalgae.

The commercial realization of microalgal biodiesel production necessitates substantial impulsion towards development of strategies to improve lipid yields upstream. Nitrogen stress is the most widely used lipid enhancement strategy; yet, it is associated with compromised biomass productivity. In this novel approach, combined effect of metals and EDTA on lipid productivity of Acutodesmus obliquus was investigated under nitrogen limited conditions. The effect of metal concentrations, individually and in combination, on microalgal lipids and biomass production is a scarcely exploited area. Combined metal stress alleviates the constraint of low biomass production under nitrogen limitation and improved the overall lipid productivity. Highest lipid productivity of 73.23 mg L −1 d −1 was achieved with a combination of iron 9 mg L −1 , magnesium 100 mg L −1 and calcium 27 mg L −1 at limited nitrogen (750 mg L −1 ). This was 1.72 fold higher than nitrogen stress alone and 1.99 fold higher than BG11 medium. Iron was found to be most significantly influencing metal followed by magnesium in response surface methodology data analysis. The enhanced photosynthetic performance and chlorophyll content further confirmed the significant impact of iron and magnesium on the microalgal biomass. The addition of EDTA to the optimised metal combination further improved the lipid productivity to 80.23 mg L −1 d −1 (2.18 fold). At 3000 L open cultivation pond this strategy has resulted in an increase of 2.08 fold in lipid productivity. Higher biodiesel conversion rates were also observed with this easy, universally applicable and scalable lipid enhancement strategy.

In order to realize the potential of microalgal biodiesel there is a need for substantial impetus involving interventions to radically improve lipid yields upstream. Nutrient stress and alteration to cultivation conditions are commonly used lipid enhancement strategies in microalgae. The main bottleneck of applying conventional strategies is their scalability as some of these strategies incur additional cost and energy. Novel lipid enhancement strategies have emerged to research forefront to overcome these challenges. In this review, the latest trends in microalgal lipid enhancement strategies, possible solutions and future directions are critically discussed. Advanced strategies such as combined nutrient and cultivation condition stress, microalgae–bacteria interactions, use of phytohormones EDTA and chemical additives, improving light conditions using LED, dyes and paints, and gene expression analysis are described. Molecular approaches such as metabolic and genetic engineering are emerging as the potential lipid enhancing strategies. Recent advancements in gene expression studies, genetic and metabolic engineering have shown promising results in enhancing lipid productivity in microalgae; however environmental risk and long term viability are still major challenges.

The conventional approaches in transesterification have several techno-economic and sustainability challenges. To this end, the utilization of waste resources and renewably sourced energy has tremendous appeal. The present study reports the findings on the transesterification of waste cooking oil (WCO) catalyzed by CaO (sourced from waste chicken eggshells) using solar irradiation based thermal energy. A co-solvent (ethyl acetate) was used to enhance the miscibility of transesterification reactants. The process variables (alcohol to oil molar ratio, catalyst concentration, reaction time, and reaction temperature) were optimized using response surface methodology based Box-Behnken design. During the experiments, the intensity of solar radiation varied, and the reactor temperature ranged between 40 and 46 °C. At the predicted optima, a conversion of 90.14% was attained, which was in reasonable agreement with the predicted response (93.64%). A gradual loss of catalytic activity was observed over consecutive batch transesterification runs, and at the end of a 3rd run, 73.13% conversion of WCO was attained. The solar radiation available at the ground level during the experimental period was ≈ 5.15 kW m−2 d−1, while the conventional approach (hot-plate based heating) led to an energy input of 2.5 kWh per run at the optimized reaction condition. With process intensification strategies, solar irradiation is poised to become an increasingly sustainable alternative to the conventional approach. The findings of the study present an economical and environmentally benign approach to biodiesel production.

Whole cell lipase catalysis and microalgal feedstocks make overall biodiesel synthesis greener and sustainable. In this study, a novel approach of whole cell lipase-catalyzed conversion of Scenedesmus obliquus lipids was investigated for biodiesel synthesis. Microalgal biodiesel was characterized for its fuel properties. Optimization of process parameters for immobilized Aspergillus niger whole cell lipase-catalyzed biodiesel synthesis was carried out. Highest biodiesel conversion of 53.76% was achieved from S. obliquus lipids at 35 °C, methanol to oil ratio of 5:1 and 2.5% water content based on oil weight with 6 BSPs (Biomass support particles). Step-wise methanol addition was applied to account for methanol tolerance, which improved biodiesel conversion upto 80.97% and gave 90.82 ± 1.43% yield. Immobilized A. niger lipase can be used for 2 batches without significant loss in conversion efficiency. Most of the fuel properties of biodiesel met the specifications set by international standards.

Conversion of microalgal lipids using biocatalyst is a novel and greener approach to produce biodiesel. Free and immobilized lipases from Candida sp. and Pseudomonas fluorescens along with free lipases from porcine pancreas and wheat germ were screened for biodiesel conversion of Scenedesmus obliquus lipids. Among selected lipases from various sources immobilized lipase from P. fluorescens showed superior biodiesel conversion. Optimization of reaction parameters viz. lipase amount, temperature, methanol to oil molar ratio and water content was carried out using response surface methodology. Best conversion of 66.55% was achieved at 35 °C, methanol to oil ratio of 3:1 with 10% enzyme amount and 2.5% water content based on oil weight. To tackle methanol tolerance step-wise methanol addition was applied, which improved biodiesel conversion upto 90.81%. Immobilized P. fluorescens lipase can be used for 4 batches without much loss in conversion efficiency (>95%). Biodiesel produced has the cetane number of 51.77, Calorific value of 37.67 MJ kg−1. Most of the fuel properties of biodiesel met the specifications set by ASTM and EN standards.

Microalgae lipid-derived biofuels is considered promising candidates for substitution of petroleum-based energy sources. However, the lipid extraction from the algal biomass stands as a challenge due to its low yields and cost-intensive cell disruption procedures. In this study a multivariate optimization of the extraction conditions was suggested, aiming a maximization of the lipid extraction from Scenedesmus sp. microalgae grown using wastewater as a nutrient medium. The extraction method, extraction time, solvent mixture and pretreatment were considered between upper and lower levels in order to access their significance, including their interactions, on the experimental response, while using a reduced number of experiments. The studies were performed using low-cost extraction methods (magnetic stirring and ultrasonication). The optimal extraction condition was obtained using CHCl3:MeOH (2:1) solvent mixture, in a 2-hour extraction period using ultrasonication. Fatty acid profiles of extracted lipids were also evaluated.

Enhancement of lipid accumulation is essential to improve the commercial feasibility of microalgal biodiesel production. An oleaginous microalgal strain, Ankistrodesmus falcatus KJ671624 was evaluated for its potential as a biodiesel feedstock in this study. The collective effect of nutrient (nitrogen, phosphorous and iron) stresses on the lipid productivity of the selected strain was studied by response surface methodology. The highest lipid content of 59.6% and lipid productivity of 74.07 mg L −1 d −1 was obtained under nutrient stress with nitrogen 750 mg L −1 , phosphorus 0 mg L −1 and iron 9 mg L −1 . The photosynthetic behaviour validates the high lipid productivity under combined nutrient stress condition. Saturated fatty acid composition was increased by 38.49% under selected nutrient stress condition compared to BG11 medium. The enhanced lipid accumulation with suitable lipid profile (C16:0, C18:1, C18:2, C18:3) and biodiesel conversion of 91.54 ± 1.43% achieved in A. falcatus KJ671624 further confirm its potential as a promising feedstock for biodiesel production.

Biodiesel, a renewable fuel has a great potential in fulfilling an ever-increasing transport fuel demand. The enzymatic conversion process of feedstock oil to biodiesel is greener when compared to the conventional approach of chemical conversion due to mild reaction conditions and less wastewater generation. Lipases obtained from various microbial sources have been widely applied as catalysts for the conversion of oil to biodiesel. Biodiesel and glycerol obtained by enzymatic conversion have shown a higher purity as compared to that obtained by other conversion techniques. Enzymatic conversion of oil to biodiesel is less energy intensive because of milder reaction conditions and fewer purification steps involved in processing. Lipases, due to their catalytic efficiency and specificity, have emerged as a great tool for converting a wide range of feedstock oils to biodiesel. This manuscript presents an overview of the use of enzymatic conversion for making biodiesel production sustainable and environmentally-friendly. The constraints of enzymatic conversion are the high cost of the enzyme and its inhibition by alcohol and glycerol. The possible solutions to overcome these constraints are discussed. Recent advances to develop an effective process for enzymatic conversion of feedstock oils into biodiesel are critically evaluated. Prospective and challenges in scaling up of this technology are also discussed.

Microalgae are recognized as one of the most powerful biotechnology platforms for many value added products including biofuels, bioactive compounds, animal and aquaculture feed etc. However, large scale production of microalgal biomass poses challenges due to the requirements of large amounts of water and nutrients for cultivation. Using wastewater for microalgal cultivation has emerged as a potential cost effective strategy for large scale microalgal biomass production. This approach also offers an efficient means to remove nutrients and metals from wastewater making wastewater treatment sustainable and energy efficient. Therefore, much research has been conducted in the recent years on utilizing various wastewater streams for microalgae cultivation. This review identifies and discusses the opportunities and challenges of different wastewater streams for microalgal cultivation. Many alternative routes for microalgal cultivation have been proposed to tackle some of the challenges that occur during microalgal cultivation in wastewater such as nutrient deficiency, substrate inhibition, toxicity etc. Scope and challenges of microalgal biomass grown on wastewater for various applications are also discussed along with the biorefinery approach.

The industrialization of integrated algae-aquaculture systems entails appropriate information regarding environmental and economic assessments, field and laboratory analyses, and feasibility studies. Accordingly, Scenedesmus obliquus was cultivated in a raceway pond (300 m3), and the algal biomass was used as a protein source for the growth of Nile tilapia (Oreochromis niloticus). Nile tilapia fish was cultivated in five commercial-scale tanks for 44 weeks, having a productivity of 15–20 kg (live weight)·m−3. Among various algal-based fish meals, the diet containing 7.5% microalgae provided the largest body length (29 cm) and weight (402 g), as well as the optimum growth performance parameters. Scenedesmus obliquus was subjected to lipid extraction, and the defatted biomass was also used as a substitute for fishmeal in diets. Nile tilapia grown using the lipid-extracted algae had improved health status, and the biochemical composition was satisfactory. Further, two scenarios were economically investigated: Case 1, the direct use of algal biomass for fish production, and Case 2, the utilization of algae for biodiesel production followed by the application of residual biomass in fishmeal diets. Based on field experiments and financial information reported in the literature, the two options would offset their initial investment cost within payback periods of 7.5 and 6.8 yr, respectively.

Quantitative assessments of the potential of a novel non-edible feedstock for biodiesel production are crucial for two reasons in addition to the primary concern of food security. The first is to find alternative fuels as replacements for dwindling reserves of fossil fuels. The second is to ascertain whether biodiesel produced from oil extracted from Croton gratissimus has properties close to that for biodiesel from other feedstock. C. gratissimus, a non-food grain, native to Africa, was collected from the Democratic Republic of the Congo. Its oil was extracted and characterized for its fatty acid profile and its acid and saponification values. The efficiencies of homogeneous (sulfuric acid) and heterogeneous (sulfated zirconia oxide) acid catalysts were evaluated. Sulfated zirconia oxide gave a better biodiesel yield of 84.65 ± 0.45% based on the oil weight compared to a biodiesel yield of 80.35 ± 1.05% given by sulfuric acid. The synthesized biodiesel, from C. gratissimus oil, was tested for its thermop...

Microalgal biodiesel has to overcome a cost incurring harvesting bottleneck for its commercial scale production. In this study electrochemical harvesting (ECH) using Chlorella sorokiniana and Scenedesmus obliquus was investigated. Nonsacrificial carbon electrodes were used to overcome the cost and metallic contamination implications. The effect of applied current and addition of electrolyte on harvesting efficiency was investigated. Addition of electrolyte (NaCl) increased the recovery efficiency of C. sorokiniana from 65.99% to 94.52%. ECH process has not shown any deteriorating effect on the lipid extraction process as well as fatty acid composition. ECH process for C. sorokiniana with optimum conditions showed 94.52% recovery efficiency with energy consumption of 1.6 kWh kg−1. This study for the first time validates application of nonsacrificial carbon electrodes in ECH process of microalgae. Attractive high recovery efficiency, low energy consumption and use of nonsacrificial electrodes could make ECH a possible step in commercial microalgal biomass and biodiesel production.

Research findings of the past few decades on the cultivation of microalgae for biodiesel production from laboratory to pilot scale microalgal cultivation have translated into empirical hope of developing an eco-friendly biofuel from algae. As far as economic sustainability is concerned, harvesting of microalgae is one of the most energy extensive processes and thus a major challenge, being faced by this industry. In our study, we designed and developed a quaternary ammonium salt based cationic polymer and evaluated its effectiveness for freshwater microalgae harvesting. An epichlorohydrin-n,n-diisopropylamine-dimethylamine polymer with high viscosity (1040 cps) was synthesized. The flocculation performance of this polyamine polymer was evaluated in terms of biomass recovery efficiency of microalgae ( Scenedesmus sp.), its effect on lipid yield and composition. The results revealed that due to high molecular weight, the biomass recovery efficiency of the polymer was achieved >90% at a very small dose of 8 mg/L whereas similar biomass recovery efficiency of chitosan and alum were achieved at 80 and 250 mg/L respectively. The presence of functional quaternary amine and hydroxyl groups played an important role in electric charge neutralization of microalgal cells, hence the improved microalgal flocculation performance in comparison to the natural flocculants but not affecting the lipid yield and its composition. The approximate cost of harvesting 1 kg of Scenedesmus biomass is approximately 0.5 USD for the polyamine polymer whereas 50 USD for chitosan. Therefore, polymer based harvesting of microalgae for low valued products such as biodiesel, polyamine based polymers would be preferred over the natural polymer.

The potential of nitrogen sources supplementing domestic wastewater for the cultivation of microalgae was assessed. Urea, potassium nitrate, sodium nitrate and ammonium nitrate were evaluated for their effect on cultivation and lipid production of Chlorella sorokiniana. Urea showed the highest biomass yield of 0.220 g L(-1) and was selected for further experimentation. Urea concentrations (0-10 g L(-1)) were assessed for their effect on growth and microalgal physiology using pulse amplitude modulated fluorometry. A concentration of 1.5 g L(-1) urea produced 0.218 g L(-1) biomass and 61.52% lipid by relative fluorescence. Physiological stress was evident by the decrease in relative Electron Transport Rate from 10.45 to 6.77 and quantum efficiency of photosystem II charge separation from 0.665 to 0.131. Gas chromatography analysis revealed that C16:0, C18:0, C18:1, C18:2 and C18:3 were the major fatty acids produced by C. sorokiniana. Urea proved to be an effective nitrogen supplement for cultivation of C. sorokiniana in wastewater.

Copyright: 2014. Elsevier. Due to copyright restrictions, only the abstract is available. For access to the full text item, please consult the publisher's website. The definitive version of the work is published in Chemical Engineering Research and Design, August 2014, Vol. 92, Issue 8, pp. 1503-1511