Economically viable microalgal biodiesel production is unrealistic and unsustainable owing to expensive harvesting or dewatering techniques. Hence, immense and meticulous exploration of harvesting process is essential to identify knowledge leads by which suitable harvesting technique could be ascertained for lucrative biodiesel production. With this in view, this review aims to collate and highlight the spectrum of harvesting techniques applied to microalgae, i.e., conventional – modern, high cost- inexpensiveness, energy efficient- energy consuming process. At the outset, global energy outlook and demand had been critically addressed, and the scientific ways to tackle or satiate the fuel demand had also been highlighted in this reveiw. This review manuscript has thrown widespread light on the physical harvesting methods namely centrifugation, sedimentation, filtration, flotation and technical advantages thereof. Due to the energy-intensive and cost barrier of physical harvesting techniques, chemical methods entailing organic, inorganic, and electroflocculation have come to limelight and in this regard, microalgae used, floc recovery and the dose of flocculants have been compared and presented in detail. Further, state of the art harvesting techniques viz., bioflocculation by microalgae/bacteria, flocculation by pH adjustment, and magnetic nanocomposite based microalgal harvesting had been critically articulated. Besides discussing the several methods, this paper has summarized the key challenges in conventional and advanced harvesting techniques and also provided the scope thereof. Hence, the key suggestions and findings given in this manuscript would positively offer a well-defined roadmap in choosing foreseeable harvesting technology for cost-effective microalgal biofuel development.

A comprehensive review on harvesting of microalgae for biodiesel – Key challenges and future directions

The energy demands and costs of harvesting microalgal biomass make it unrealistic and unsustainable for economically feasible microalgal biofuel production Therefore, meticulous exploration of the harvesting processes is essential to identify appropriate harvesting techniques for potentially commercialized microalgal biodiesel production Flocculation may be a superior method when considering harvesting efficiency, economic cost, energy consumption and technical feasibility This review sheds some light on the recent progresses of physical/chemical flocculation and bioflocculation applied in the microalgal biomass harvesting processes Physical flocculation techniques are energy-intensive and require special equipment, creating the cost barrier for microalgal biomass harvesting Magnetic particle flocculation is much more efficient and is also recyclable In contrast, chemical flocculation that involves the application of organic and inorganic flocculants, is now in the limelight The microalgae species applied, the dosages of flocculants as well as flocculation recovery efficiencies are compared and presented in detail in this review In addition, bioflocculation as a harvesting techniques is critically described, in particular the mechanisms of autoflocculation, a promising bioflocculation by co-cultivation of microalgae with microorganisms, are explored This review also disclosed the effects of flocculant application on downstream processes, especially when chemical flocculants are applied This review intends to provide guidance for the long-term adoption of these economically beneficial mature flocculation recovery technologies in the biofuel industry Despite of considerable progress, key challenges such as further reduction of costs and the minimization of downstream product pollution risks in conventional and advanced harvesting techniques, must be addressed This article also suggests the directions for future research in microalgae harvesting and argues that the production of microalgae biofuels under potential policy intervention should be carried out in a healthy and sustainable way

A review on flocculation as an efficient method to harvest energy microalgae: Mechanisms, performances, influencing factors and perspectives

Microalgae are considered as a potential and sustainable feedstock for the production of biofuels, fine chemicals, nutraceuticals, and cosmetics. This is accredited to their high lipid and carbohydrate content, fast growth and rapid CO2 sequestration ability. However, large volumes of feedstock are required to extract and process biochemicals from microalgal biomass due to the small biomass to liquid ratio. This produces substantial challenges in attaining a sustainable energy balance in microalgae-based products process operations. Additionally, the small size of microalgal cells along with their negatively charged cell surface and cell density similar to the growth medium produces challenges in microalgae harvesting. The high cost associated with microalgae harvesting is a major bottleneck for commercialization of algae-based industrial products. Hence, microalgae harvesting is recognized as an area that needs to be explored and developed. This article aims to collate and present an overview of current harvesting strategies such as physical, chemical, biological, electrical and magnetic methods along with their future prospects. This review also highlights the evolution of microalgal harvesting and elucidates the fundamental phenomena of each technology in relation to key physical parameters such as morphology, size, density and surface charge. Besides throwing widespread light on various harvesting methods, this review article has also presented their advantages and disadvantages. Life cycle assessment (LCA) and technoeconomic analysis (TEA) was reviewed to assess the feasibility of various harvesting system for commercial application based on the environmental and technoeconomic impacts. Hence, the vital proposals provided in this review article would undeniably pave the way for choosing the appropriate harvesting strategy.

A comprehensive review on microalgal harvesting strategies: Current status and future prospects

Flavonoids are a group of plant constituents called phenolic compounds and correspond to the nonenergy part of the human diet. Flavonoids are found in vegetables, seeds, fruits, and beverages such as wine and beer. Over 7000 flavonoids have been identified and they have been considered substances with a beneficial action on human health, particularly of multiple positive effects because of their antioxidant and free radical scavenging action. Although several studies indicate that some flavonoids have provident actions, they occur only at high doses, confirming in most investigations the existence of anti-inflammatory effects, antiviral or anti-allergic, and their protective role against cardiovascular disease, cancer, and various pathologies. Flavonoids are generally removed by chemical methods using solvents and traditional processes, which besides being expensive, involve long periods of time and affect the bioactivity of such compounds. Recently, efforts to develop biotechnological strategies to reduce or eliminate the use of toxic solvents have been reported, reducing processing time and maintaining the bioactivity of the compounds. In this paper, we review, analyze, and discuss methodologies for biotechnological recovery/extraction of flavonoids from agro-industrial residues, describing the advances and challenges in the topic.

/pdf/conventional-and-emerging-extraction-processes-of-flavonoids-24pbxfgpje.pdf

Conventional and Emerging Extraction Processes of Flavonoids

In this study, non-conventional methods of supercritical extraction with CO2 as well as with ethanol or ethyl acetate cosolvent, pressurized-liquid extraction (PLE), and ultrasound-assisted extraction (UAE) were employed to evaluate the extraction of vetiver (Chrysopogon zizanioides) oil in comparison with hydrodistillation and Soxhlet techniques. The highest yields by supercritical CO2 extraction were obtained at 60 °C and 20 MPa, 2.23% (m/m), and 2.66% (m/m) with 5% (V/V) of ethanol cosolvent, while PLE and UAE methods allowed a reduction of the time and solvent volume, obtaining up to 83% and 70% of the Soxhlet yields, respectively. The main identified compound in the oil (supercritical extraction, Soxhlet, PLE, and UAE with n-hexane) was sesquiterpene khusimol (29.19% to 32.89%), as well as isovalencenol, α-vetivone, and zizanoic acid. The highest antioxidant activity was 71.44 μmolTrolox gextract−1 for the sample obtained by Soxhlet extraction with ethanol.

Extraction of vetiver (Chrysopogon zizanioides) root oil by supercritical CO2, pressurized-liquid, and ultrasound-assisted methods and modeling of supercritical extraction kinetics

The supercritical fluid extraction (SFE) of Greek sage leaves, i.e. Salvia fruticosa, was investigated at various operation conditions. Specifically, the effect of pressure, temperature and solvent flow rate on the extraction yield was examined. Thus, it was shown that as pressure increases, yield increases as well, while the effect of temperature depends on the extraction pressure. As for the flow rate, its increase mainly enhances the extraction rate. The SFE extracts were also compared with the essential oil obtained with hydrodistillation in terms of composition, revealing practically the absence of thujone and the presence of more biologically active compounds in SFE extracts. Finally, the experimental data were correlated successfully by a mass balance model based on Lack’s plug flow model.

Supercritical CO2 extraction of Salvia fruticosa

The harvesting of Nannochloropsis oceanica was studied using magnetic separation with naked iron oxide particles. Emphasis was given to the effect of pH on harvesting efficiency, while mixing time ...

Konstantinos Magoulas

Papers

Supercritical CO2 extraction of Salvia fruticosa

Magnetic harvesting of marine algae Nannochloropsis oceanica