is highly permeable through cell membranes and can affect methane yields due to
ammonia inhibition. The acclimation period, substrate composition and operating
conditions typically determine the inhibitory concentrations of ammonia, which can
vary from 0.05 to 2 g/L (Rajagopal et al. 2013). Thermophilic conditions enhance
the inhibition effect (Sialve et al. 2009). In this context, methanogenic communities
can acclimate to high concentrations of ammonia, increasing the inhibition
threshold level, even if methanogenic productivity remains low.
3 Pretreatments for Increasing the Anaerobic
Biodegradability
The conversion of microalgae into biogas is often limited by the hydrolysis step of
the AD process. In the 1950s, researchers already noticed that microalgae remained
intact after AD in a reactor operating at 30 days of hydraulic retention time
(HRT) (Golueke et al. 1957). This phenomenon also occurs when biodegrading
other complex organic substrates, such as activated sludge and lignocellulosic
biomass, in which organic compounds have low bioavailability and/or low
biodegradability. This bottleneck may be overcome by applying a previous pretreatment step, which is already the case in full-scale WWTPs treating sewage
sludge or in the agroindustrial field. Overall, biomass pretreatment methods aim at
increasing organic matter solubilisation and, therefore, making those compounds
more readily available to the anaerobic bacteria present in the digester, which would
ultimately increase the process rate and the methane yield (Passos et al. 2014a).
Particularly, the main reason why microalgae have slow and/or low
biodegradability is due to the nature of their cell wall structure and composition.
Most species have a complex cell wall composed of recalcitrant components,
especially those grown in open ponds treating wastewater. Nonetheless, the characteristics of these cell walls may vary depending on the strain and environmental/
operational conditions. Species with a glycoprotein-based, frustule-covered, or a
bacterial-like peptidoglycan cell walls, are more sensitive to disruption with pretreatment techniques than those with silica- or polysaccharide-based cell walls
(Bohytskyi et al. 2014). The main constituents of microalgae biomass are carbohydrates, proteins, lipids, carotenoids and lignin. Nonetheless, most of them are
polysaccharides, e.g. cellulose, hemicellulose, chitin/chitosan-like molecules, pectin
and alginate. A recent study found that, although proteins, lipids and a considerable
amount of carbohydrates were present in the cell walls of refractory microalgae
species, microalgae resistance was not correlated to the presence of a unique
monomer. The authors concluded that the responsible compounds were most likely
to be sporopollenin, lignin-like materials and heteropolysaccharides (Montingelli
et al. 2015). However, it is hypothesised that the cross-link of these compounds into
a complex network building layers around the cell could eventually work as a
barrier to anaerobic microbial community (Klassen et al. 2016).
12 Biofuels from Microalgae: Biomethane
251
ammonia inhibition. The acclimation period, substrate composition and operating
conditions typically determine the inhibitory concentrations of ammonia, which can
vary from 0.05 to 2 g/L (Rajagopal et al. 2013). Thermophilic conditions enhance
the inhibition effect (Sialve et al. 2009). In this context, methanogenic communities
can acclimate to high concentrations of ammonia, increasing the inhibition
threshold level, even if methanogenic productivity remains low.
3 Pretreatments for Increasing the Anaerobic
Biodegradability
The conversion of microalgae into biogas is often limited by the hydrolysis step of
the AD process. In the 1950s, researchers already noticed that microalgae remained
intact after AD in a reactor operating at 30 days of hydraulic retention time
(HRT) (Golueke et al. 1957). This phenomenon also occurs when biodegrading
other complex organic substrates, such as activated sludge and lignocellulosic
biomass, in which organic compounds have low bioavailability and/or low
biodegradability. This bottleneck may be overcome by applying a previous pretreatment step, which is already the case in full-scale WWTPs treating sewage
sludge or in the agroindustrial field. Overall, biomass pretreatment methods aim at
increasing organic matter solubilisation and, therefore, making those compounds
more readily available to the anaerobic bacteria present in the digester, which would
ultimately increase the process rate and the methane yield (Passos et al. 2014a).
Particularly, the main reason why microalgae have slow and/or low
biodegradability is due to the nature of their cell wall structure and composition.
Most species have a complex cell wall composed of recalcitrant components,
especially those grown in open ponds treating wastewater. Nonetheless, the characteristics of these cell walls may vary depending on the strain and environmental/
operational conditions. Species with a glycoprotein-based, frustule-covered, or a
bacterial-like peptidoglycan cell walls, are more sensitive to disruption with pretreatment techniques than those with silica- or polysaccharide-based cell walls
(Bohytskyi et al. 2014). The main constituents of microalgae biomass are carbohydrates, proteins, lipids, carotenoids and lignin. Nonetheless, most of them are
polysaccharides, e.g. cellulose, hemicellulose, chitin/chitosan-like molecules, pectin
and alginate. A recent study found that, although proteins, lipids and a considerable
amount of carbohydrates were present in the cell walls of refractory microalgae
species, microalgae resistance was not correlated to the presence of a unique
monomer. The authors concluded that the responsible compounds were most likely
to be sporopollenin, lignin-like materials and heteropolysaccharides (Montingelli
et al. 2015). However, it is hypothesised that the cross-link of these compounds into
a complex network building layers around the cell could eventually work as a
barrier to anaerobic microbial community (Klassen et al. 2016).
12 Biofuels from Microalgae: Biomethane
251