Pretreatment techniques may be classified into four main categories: mechanical,
thermal, chemical and biological methods. These methods are based on different
mechanisms and, therefore, support different disruption efficiencies. For instance,
mechanical techniques, such as microwave, ultrasound and ball-milling, act by
reducing the particle size and increasing the superficial contact area; while biological pretreatments act by inducing an enzymatic breakdown of complex molecules. In a study comparing different techniques, physical pretreatments (i.e.
thermal and ultrasound) showed the highest effectiveness in protein solubilisation,
which was mediated by the release of alogenic organic matter and cell wall
breakage, while enzymatic pretreatments increased carbohydrate solubilisation,
which was mediated by the biodegradation of cell wall compounds rather than by
cell disruption (Ometto et al. 2014). In this experiment, the highest biogas increase
in batch tests was obtained for enzymatic pretreated microalgae (270% increase).
Most studies up-to-date were conducted using batch experiments. These tests are
mainly used for comparing pretreatments and/or pretreatment conditions. However,
continuous experiments with acclimated microorganisms are needed for validating
and quantifying the potential methane yield and for estimating the energy balance
of the process. Among the studies published so far, most of those dealing with
continuous AD of microalgae evaluated the effect of thermal pretreatment. The
results reported showed increases from 32 to 108% compared to non-pretreated
microalgae (ranging from 0.12 to 0.27 L CH 4 /g VS) (Table 3). The best results
were obtained during microalgae thermal pretreatment at 75–95 °C for 10 h (70%
increase) (Passos and Ferrer 2014) and 120 °C for 2 h (108% increase) (Schwede
et al. 2013). Moreover, the energy balance calculations showed that after applying a
low-temperature pretreatment at 75 °C, the energy balance shifted from neutral to
positive with a 2.7 GJ net energy production per day (Passos and Ferrer 2014). In
fact, most recent reviews in microalgae pretreatment concluded that thermal pretreatment is the optimal method, by combining the highest methane improvement
and the lowest energy input (Jankowska et al. 2017; Passos et al. 2014a, b;
Rodriguez et al. 2015).
Additionally, enzymatic pretreatment has recently been the focus of research on
microalgae pretreatment. Studies in continuous mode showed increases of 260% in
methane yield compared to non-pretreated microalgae, although biomass was
highly recalcitrant in this experiment, i.e. 0.05 L CH 4 / g COD (Mahdy et al. 2015).
The enzymatic pretreatment of Scenedesmus sp. in a first step anaerobic membrane
bioreactor (AnMBR) with rumen microorganisms also showed promising results in
terms of methane yield (0.203 L CH 4 /g COD) and COD removal (70%) (Giménez
et al. 2017).
Although many novel pretreatment methods are being investigated, such as pulse
electric field, ozonation or solvent addition, the energy and economic aspects for
pilot and full-scale viability must be analysed. The main pros and cons of
microalgae pretreatment techniques are summarised in Table 4. Thus, energy
demand and scalability are major issues when evaluating pretreatment viability.
Although thermal pretreatment seems advantageous, biomass thickening or dewatering is crucial. On the other hand, despite thermochemical pretreatments have
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F. Passos et al.
thermal, chemical and biological methods. These methods are based on different
mechanisms and, therefore, support different disruption efficiencies. For instance,
mechanical techniques, such as microwave, ultrasound and ball-milling, act by
reducing the particle size and increasing the superficial contact area; while biological pretreatments act by inducing an enzymatic breakdown of complex molecules. In a study comparing different techniques, physical pretreatments (i.e.
thermal and ultrasound) showed the highest effectiveness in protein solubilisation,
which was mediated by the release of alogenic organic matter and cell wall
breakage, while enzymatic pretreatments increased carbohydrate solubilisation,
which was mediated by the biodegradation of cell wall compounds rather than by
cell disruption (Ometto et al. 2014). In this experiment, the highest biogas increase
in batch tests was obtained for enzymatic pretreated microalgae (270% increase).
Most studies up-to-date were conducted using batch experiments. These tests are
mainly used for comparing pretreatments and/or pretreatment conditions. However,
continuous experiments with acclimated microorganisms are needed for validating
and quantifying the potential methane yield and for estimating the energy balance
of the process. Among the studies published so far, most of those dealing with
continuous AD of microalgae evaluated the effect of thermal pretreatment. The
results reported showed increases from 32 to 108% compared to non-pretreated
microalgae (ranging from 0.12 to 0.27 L CH 4 /g VS) (Table 3). The best results
were obtained during microalgae thermal pretreatment at 75–95 °C for 10 h (70%
increase) (Passos and Ferrer 2014) and 120 °C for 2 h (108% increase) (Schwede
et al. 2013). Moreover, the energy balance calculations showed that after applying a
low-temperature pretreatment at 75 °C, the energy balance shifted from neutral to
positive with a 2.7 GJ net energy production per day (Passos and Ferrer 2014). In
fact, most recent reviews in microalgae pretreatment concluded that thermal pretreatment is the optimal method, by combining the highest methane improvement
and the lowest energy input (Jankowska et al. 2017; Passos et al. 2014a, b;
Rodriguez et al. 2015).
Additionally, enzymatic pretreatment has recently been the focus of research on
microalgae pretreatment. Studies in continuous mode showed increases of 260% in
methane yield compared to non-pretreated microalgae, although biomass was
highly recalcitrant in this experiment, i.e. 0.05 L CH 4 / g COD (Mahdy et al. 2015).
The enzymatic pretreatment of Scenedesmus sp. in a first step anaerobic membrane
bioreactor (AnMBR) with rumen microorganisms also showed promising results in
terms of methane yield (0.203 L CH 4 /g COD) and COD removal (70%) (Giménez
et al. 2017).
Although many novel pretreatment methods are being investigated, such as pulse
electric field, ozonation or solvent addition, the energy and economic aspects for
pilot and full-scale viability must be analysed. The main pros and cons of
microalgae pretreatment techniques are summarised in Table 4. Thus, energy
demand and scalability are major issues when evaluating pretreatment viability.
Although thermal pretreatment seems advantageous, biomass thickening or dewatering is crucial. On the other hand, despite thermochemical pretreatments have
252
F. Passos et al.