• Energy Research
• 7. Clean energy close

Abstract Excessive use of fossil fuels to satisfy our rapidly increasing energy demand has created severe environmental problems, such as air pollution, acid rain and global warming. Biofuels are a potential alternative to fossil fuels. First- and second-generation biofuels face criticism due to food security and biodiversity issues. Third-generation biofuels, based on microalgae, seem to be a plausible solution to the current energy crisis, as their oil-producing capability is many times higher than that of various oil crops. Microalgae are the fastest-growing plants and can serve as a sustainable energy source for the production of biodiesel and several other biofuels by conversion of sunlight into chemical energy. Biofuels produced from microalgae are renewable, non-toxic, biodegradable and environment friendly. Microalgae can be grown in open pond systems or closed photobioreactors. Microalgal biofuels are a potential means to keep the development of human activities in synchronization with the environment. The integration of wastewater treatment with biofuel production using microalgae has made microalgal biofuels more attractive and cost effective. A biorefinery approach can also be used to improve the economics of biofuel production, in which all components of microalgal biomass (i.e., proteins, lipids and carbohydrates) are used to produce useful products. The integration of various processes for maximum economic and environmental benefits minimizes the amount of waste produced and the pollution level. This paper presents an overview of various aspects associated with biofuel production from microalgae, including the selection and isolation of microalgal species, various cultivation and harvesting techniques as well as methods for their subsequent conversion into biofuels.

In today's era, we need to replace chemical or physical processes of nanoparticle synthesis with biosynthesis processes to avoid environmental damage. These bioderived nanoparticles can help in addressing the problems of wastewater treatment and biofuels production. This review gives an insight into solving multiple problems using a nano-biorefinery approach in conjugation with wastewater treatment. The major advantage of using a bio-derivative method in nanoparticle synthesis is its low toxicity towards the environment. The current review discusses the development of nanoscience and its biogenic importance. It covers the usage of microalgae for (A) Nanoparticle's biosynthesis (B) Mechanism of nanoparticle biosynthesis (C) Nanoparticles in bio-refinery processes (D) Wastewater treatment with microalgae and bio-derived nanoparticles (E) A hypothetical mechanistic approach, which utilizes the photothermal effect of metallic nanoparticles to extract lipids from the cells without cell damage. The term "cell milking" has been around for quite some time, and the hypothesis discussed in the present study can help in this context. The current hypothesized process can pave ways for futuristic endeavors to conjugate nanoparticles and microalgae for viable and commercial production of biofuel, nanoparticles, and many other molecules.

Abstract Microalgae are explored as the sustainable source of biodiesel production considering their multi restorative effects on water and air. In spite of rapid growth and high lipid content, production of microalgal biodiesel is energy intensive and costly affair. Efficient oil extraction is one of the major hurdles; which needs special attention for making the microalgae biodiesel production process economical. Current review paper discusses various types of extraction methods starting from conventional organic solvent extraction method to advance methods like microwave irradiation, ultrasound, supercritical fluids and ionic liquids for oil extraction and also the direct transesterification. Modern methods give better yield in comparison to conventional methods. The variation of properties of microalgae biodiesel with respect to fatty acid methyl ester (FAME) composition is also highlighted and the properties are compared against established standards. Also, the previous studies on performance and emission characteristics using microalgae have been reviewed and it is found that microalgae biodiesel is on comparable scale to that of first and second generation biofuel. Lots of research scope and further developments in harvesting, drying, oil extraction and direct transesterification stages can potentially establish microalgae as next generation source for transport fuel production.

A novel earthen separator-based dual-chambered unplanted core of constructed wetland coupled with microbial fuel cell was developed for studying the microbe-material interaction and their effect on treatment performance and electricity generation. The constructed wetland integrated microbial fuel cell was evaluated for the degradation of high molecular weight diazo Congo red dye as a model pollutant. The system exhibited 89.99 ± 0.04% of dye decolorization and 95.80 ± 0.71% of chemical oxygen demand removal efficiency from an initial concentration of 50 ± 10 mg/L and 750 ± 50 mg/L, respectively. Ultraviolet-Visible spectrophotometric and gas chromatography-mass spectrometric analysis revealed naphthalene and phenol as mineralized products. The developed system achieved high power density and current density generation of 235.94 mW/m3 and 1176.4 mA/m3, respectively. Results manifested that dual-chambered constructed wetland coupled with microbial fuel cell has a high capability of dye decolorization and toxicity abatement with appreciable simultaneous bioelectricity generation owing to the significantly low internal resistance of 100 Ω.

The present study investigates the interactive effect of important growth media components (organic carbon, nitrate, and phosphate) on Scenedesmus sp. Interestingly, simultaneous enhancement of energy storage components and biomass was observed. Six carbon sources taken into consideration in this study affected the microalgal growth profile distinctively. As compared to other studied organic carbon sources, the addition of glucose (5 g L−1) resulted in the highest total biomass (3.7 ± 0.2 g L−1). Further, the combination of 5 g L−1 glucose supplementation with phosphate and nitrate starvation fetched the highest total lipids (331.5 ± 29.4 µg mg−1 and 281.6 ± 9.4 µg mg−1, respectively). Additionally, phosphate starvation with 5 g L−1 glucose supplementation fetched the highest total fatty acid content (118.35 µg mg−1). Carbohydrate content was the highest (1025.5 ± 83.9 µg mg−1) at 5 g L−1 of glucose, 1.5 g L−1 of NaNO3, and 0.04 g L−1 of K2HPO4 without compromising biomass accumulation. Principal component analysis confirmed that carbohydrate and protein concentrations in the algal biomass were closely related to enhanced biomass and chlorophyll concentration and were heavily impacted by NaNO3 in the presence of high organic carbon. This study supported in fetching the nutrient combinations, which increased biomass with wide range of fuel, as well as non-fuel applications.

Microalgae have garnered much contemplation as candidates to fix CO2 into valuable compounds. Although microalgae have been studied to produce various metabolites, they have not yet proved successful for commercialization. Since, handling such problems practically requires satisfying multiple parameters simultaneously, we put forth a multi-parameter optimization strategy to manipulate the carbon metabolism of Scenedesmus sp. to improve biomass production and enhance CO2 fixation to increase the production of fuel-related metabolites. The Box-Behnken design method was applied with CO2 concentration, CO2 sparging time and glucose concentration as independent variables; biomass and total fatty acid methyl ester (total FAME) content were analyzed as response variables. The strain is supplemented with both CO2 and glucose with an aim to enhance carbon flux and rechannel it towards carbon fixation. As per the results obtained in this study, Scenedesmus sp. could effectively exploit high CO2 concentration (15%) for longer duration under high concentration of glucose supplementation (9 g/L) producing a biomass of 635.24 ± 39.9 μg/mL with a high total fatty acid methyl ester (FAME) content of 71.29 ± 4.2 μg/mg, significant acetyl-CoA carboxylase enzyme activity and a favorable fatty acid profile: 35.8% palmitic acid, 10.5% linoleic acid and 30.6% linolenic acid. The carbohydrate content was maximum at 10% CO2 sparged for the longest duration of 90 min under glucose concentration of 9 g/L. This study puts forth an optimal design that can provide evidence on comprehending the carbon assimilation mechanism to enhance production of biomass and biofuels and provide conditions to microalgal species to tolerate CO2 rich flue gas.