Harvesting techniques applied to microalgae: A review
Introduction
It is well known that microalgae have a huge potential in a wide variety of applications. Concerning environmental ones, microalgae can play an important role in bioremediation of wastewater and carbon dioxide sequestration [1], [2], [3], [4], [5], [6]. Furthermore, these photosynthetic microorganisms have been considered as a potential renewable fuel source [7], [8], [9]: they can be used as raw material for the production of biodiesel, biomethane, bioethanol, biohydrogen and biobutanol. These biofuels are viewed as the most promising alternative to fossil fuels, being able to provide up to 25% of global required energy [9], [10], [11]. One of these energetic products is 3rd generation biodiesel obtained from the transesterification of microalgal lipids, which, in appropriate culture conditions, may represent a significant fraction of their biomass [5], [7], [9], [11], [12], [13]. Microalgal based biofuels present numerous economic and ecological advantages comparing to the terrestrial crop based ones: (i) continuous growth in waters with a wide range of salinities and chemical compositions; (ii) growth in any location (arable and non-arable land) without the need for herbicides or pesticides; and (iii) high specific production yields and photosynthetic efficiency [5], [8], [9], [10], [11], [14]. Residual biomass, which is poor in lignin and very rich in proteins and other compounds of commercial interest, can then be used in animal feed production and in the synthesis of different high-valued compounds, such as nutritional supplements, cosmetics and pharmaceuticals in a biorefinery-based production [9], [11], [15], [16], [17], [18], [19].
Despite the wide range of applications, microalgal production is not economically viable yet. Several research pathways have been explored to improve the economics of the process [10], [20]. Firstly, microalgae may be cultivated using wastewater as culture medium. This approach aims to reduce both production costs and freshwater requirement. Microalgae have the ability to grow on these environments, assimilating nutrients and metals from the wastewater [9], [21]. Hence, microalgae would play an important remediation role during the tertiary wastewater treatment phase [9]. Secondly, a biorefinery-based production is a strategy that lowers the overall production costs by taking credit of all products of commercial interest that can be obtained from microalgae [7], [9]. Finally, a low cost harvesting process should be studied, as this production step represents 20–30% of the biomass production costs [9], [10], [22], [23], [24]. The main reasons for high process costs are the small size of microalgae and their growth in very dilute cultures (mass concentration less than 1 g L−1) with densities close to that of the water [22], [23]. In addition, microalgal surface is negatively charged and the cells carry algogenic organic matter (AOM), which keeps stable their dispersed state [25]. At this moment, there is no microalgal harvesting method that is both economically viable and efficient. Lowering harvesting costs is thus considered a key factor for the development of sustainable full-scale production of microalgal biomass [10]. Accordingly, this study aims to review the recent research concerning harvesting methods applied to microalgae, discussing their advantages and disadvantages in terms of applicability, environmental impacts and cost-effectiveness.
Section snippets
Microalgal harvesting methods
An ideal harvesting process should be effective for the majority of microalgal strains and should allow the achievement of high biomass concentrations, while requiring moderate costs of operation, energy and maintenance [22]. The initial harvesting stage is generally costly and determines the following downstream processing. One of two methodologies is generally applied: (i) a two-step concentration where the suspension is primarily thickened to a slurry of about 2–7% of total suspended solids,
Research needs
The optimization of a pre-concentration step before the dewatering process is the most promising approach towards lowering microalgal harvesting costs. At the same time, environmental sustainability must be taken into account. Regarding biologically based harvesting methods, better understanding and control of auto and bioflocculation processes could improve their performance and reduce operational costs. Biological production of flocculants shows great potential, given the absence of microbial
Conclusions
The harvesting methods described in this review constitute efficient ways to recover microalgal biomass from the culture medium. However, there is not a universal method that can be applied to harvest all microalgal strains with the same efficiency. An efficient method should be designed basing on microalgal properties, such as cell morphology and size and cell surface properties, on the properties of the culture medium and on the quality and value of the end product. Additionally, to improve
Acknowledgements
Ana L. Gonçalves and José C.M. Pires are grateful to Foundation for Science and Technology (FCT), POPH-QREN and European Social Fund (FSE) for their fellowships SFRH/BD/88799/2012 and SFRH/BPD/66721/2009, respectively.
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