process; therefore, it is necessary to choose an effective procedure to concentrate the
biomass with low energy to minimize separation costs. The main technologies and
emerging options for microalgae recovery are shown in Fig. 1 and Table 2.
Harvesting includes physical (centrifugation, sedimentation, and filtration), chemical (flocculation, flotation, auto-flocculation, and bioflocculation), and electric
(electro-coagulation–filtration or electrochemical harvesting) alternatives. Japar
et al. (2017) established that filtration, flocculation, bioflocculation, and
electro-coagulation-filtration and further drying using solar heat to process the algal
cake are the most feasible solutions to remove water due to their high harvesting
efficiencies, moderate operational and logistic costs, no negative impacts on the
environment, and the shortest harvesting time. However, in a biorefinery, the
combination of separation processes is recommended: a first step where the biomass
is concentrated with a mechanical or chemical process to obtain a final concentration around 2–7% of total suspended solids, and a second, dewatering step to
produce a microalgal cake (Barros et al. 2015; Gerardo et al. 2015). The
pre-concentration step reduces the energy necessary to separate biomass from
water. However, the dewatering process must be established depending on the
strain and the final product requirements, so more research is required to reduce the
energy requirement and lower microalgal harvesting costs (Barros et al. 2015). New
emerging technologies include ultrasound, magnetophoretic procedures, the use of
polymers to absorb water, and co-culture with fungi to form flocs (Xia et al. 2011;
Zhou et al. 2013) that favor the removal of solids; however, additional research is
still required.
3.1.2 Cell Disruption
Microalgae store most of their valuable components inside the cell, behind a thick
and resistant cell wall. Therefore, energy - or solvent- consuming steps are needed
to alter this physical barrier and to efficiently extract the desired compounds. A mild
cell disruption method is necessary to make cell components available without
losses. Cell disruption technologies can be divided into two main categories:
mechanical and nonmechanical methods (see Fig. 1). Mechanical methods include:
bead milling, homogenization, sonication, microwaving, thermolysis, freezing, use
of chemicals, electroporation, supersonic flow, among others. Detailed information
about principles, advantages, and disadvantages can be found elsewhere (Halim
et al. 2012b; Günerken et al. 2015; Postma et al. 2016; Toledo-Cervantes and
Morales 2014). The cell disruption method depends on the cell wall characteristics
(Eppink et al. 2017) and must be carefully selected because some cell components
can be denatured (Günerken et al. 2015; Pei et al. 2010). The cell wall is a barrier
that separates the cellular content from the surrounding aqueous medium. Its
composition is strain-dependent, but is usually composed of polysaccharides (cellulose, hemicellulose, etc.), lipids, and membrane proteins, which can adopt different structures (Baudelet et al. 2017). For instance, Chlorella has two or three
layers with different structures, such as a transparent microfibrillar layer and
5 Microalgae Biorefineries for Energy …
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biomass with low energy to minimize separation costs. The main technologies and
emerging options for microalgae recovery are shown in Fig. 1 and Table 2.
Harvesting includes physical (centrifugation, sedimentation, and filtration), chemical (flocculation, flotation, auto-flocculation, and bioflocculation), and electric
(electro-coagulation–filtration or electrochemical harvesting) alternatives. Japar
et al. (2017) established that filtration, flocculation, bioflocculation, and
electro-coagulation-filtration and further drying using solar heat to process the algal
cake are the most feasible solutions to remove water due to their high harvesting
efficiencies, moderate operational and logistic costs, no negative impacts on the
environment, and the shortest harvesting time. However, in a biorefinery, the
combination of separation processes is recommended: a first step where the biomass
is concentrated with a mechanical or chemical process to obtain a final concentration around 2–7% of total suspended solids, and a second, dewatering step to
produce a microalgal cake (Barros et al. 2015; Gerardo et al. 2015). The
pre-concentration step reduces the energy necessary to separate biomass from
water. However, the dewatering process must be established depending on the
strain and the final product requirements, so more research is required to reduce the
energy requirement and lower microalgal harvesting costs (Barros et al. 2015). New
emerging technologies include ultrasound, magnetophoretic procedures, the use of
polymers to absorb water, and co-culture with fungi to form flocs (Xia et al. 2011;
Zhou et al. 2013) that favor the removal of solids; however, additional research is
still required.
3.1.2 Cell Disruption
Microalgae store most of their valuable components inside the cell, behind a thick
and resistant cell wall. Therefore, energy - or solvent- consuming steps are needed
to alter this physical barrier and to efficiently extract the desired compounds. A mild
cell disruption method is necessary to make cell components available without
losses. Cell disruption technologies can be divided into two main categories:
mechanical and nonmechanical methods (see Fig. 1). Mechanical methods include:
bead milling, homogenization, sonication, microwaving, thermolysis, freezing, use
of chemicals, electroporation, supersonic flow, among others. Detailed information
about principles, advantages, and disadvantages can be found elsewhere (Halim
et al. 2012b; Günerken et al. 2015; Postma et al. 2016; Toledo-Cervantes and
Morales 2014). The cell disruption method depends on the cell wall characteristics
(Eppink et al. 2017) and must be carefully selected because some cell components
can be denatured (Günerken et al. 2015; Pei et al. 2010). The cell wall is a barrier
that separates the cellular content from the surrounding aqueous medium. Its
composition is strain-dependent, but is usually composed of polysaccharides (cellulose, hemicellulose, etc.), lipids, and membrane proteins, which can adopt different structures (Baudelet et al. 2017). For instance, Chlorella has two or three
layers with different structures, such as a transparent microfibrillar layer and
5 Microalgae Biorefineries for Energy …
99