addition, purified enzymes may be expensive and jeopardise the economic viability
of the process. However, this limitation may be overcome via enzyme production
through other microorganisms, via enzyme expression through the microalgae cells
to be digested and via in situ production of hydrolytic enzymes by inoculated living
bacteria or fungi (Klassen et al. 2016).
Finally, future research should focus on investigating the mechanisms underlying microalgae cell wall damage and/or disruption with pretreatments, since the
analysis of organic matter solubilisation has been shown insufficient to predict the
increase in methane yields. The determination of soluble macromolecules, microscopic images and microbiology analyses is important for better understanding
how, where and in which scale pretreatments affect microalgae cell structure and
which compounds become more readily available. Moreover, it is crucial to conduct
experiments in continuous mode and in pilot and full-scale reactors for evaluating
the process performance.
4 Anaerobic Co-digestion of Microalgae
AD of raw microalgae or microalgae residues after the generation/extraction of
value-added products (i.e. lipids, ethanol and hydrogen) is typically characterised
by low methane yields and the occurrence of ammonia inhibition. Despite these
limitations, AD is still regarded as a key technology to maximise resource recovery
from microalgae and make algae industry economically feasible. AD also aids the
mobilisation the nutrients (N and P) needed for algae cultivation (Ward et al. 2014).
Anaerobic co-digestion, the simultaneous digestion of two or more substrates, is an
established and cost-effective option to overcome the drawbacks of mono-digestion
and boost the biogas production of AD plants (Mata-Alvarez et al. 2014). Besides
improving the feasibility of AD plants, co-digestion also allows treating several
wastes in a single facility and “share/reduce” treatment costs (Neumann et al. 2015).
Algae have been successfully co-digested with a large range of co-substrates
such as sewage sludge, animal manures, food waste, energy crops, glycerol, paper
waste and fat, oil and grease (FOG). Although the improvement of the methane
production is mainly a consequence of the increased organic loading rate
(OLR) rather than to the occurrence of synergisms during AD, microalgae have
been primarily co-digested with carbon-rich co-substrates, which allows increasing
the digester OLR while controlling ammonia concentration. Several studies have
optimised the co-substrate dose by balancing the feedstock C/N ratio with optimum
values for algae co-digestion ranging between 12 and 27 (Ehimen et al. 2011;
Fernández-Rodríguez et al. 2014). However, optimising co-substrate selection and
dosage based on the C/N ratio is an oversimplification since this approach does not
take into account the characteristics of each co-substrate (Astals et al. 2014;
Herrmann et al. 2016). The maximum dose of some co-substrates such as glycerol
and FOG is limited by secondary inhibitory mechanisms, while the deficiency of
alkalinity or essential nutrients limits the dosage of energy crops and paper waste
12 Biofuels from Microalgae: Biomethane
255
of the process. However, this limitation may be overcome via enzyme production
through other microorganisms, via enzyme expression through the microalgae cells
to be digested and via in situ production of hydrolytic enzymes by inoculated living
bacteria or fungi (Klassen et al. 2016).
Finally, future research should focus on investigating the mechanisms underlying microalgae cell wall damage and/or disruption with pretreatments, since the
analysis of organic matter solubilisation has been shown insufficient to predict the
increase in methane yields. The determination of soluble macromolecules, microscopic images and microbiology analyses is important for better understanding
how, where and in which scale pretreatments affect microalgae cell structure and
which compounds become more readily available. Moreover, it is crucial to conduct
experiments in continuous mode and in pilot and full-scale reactors for evaluating
the process performance.
4 Anaerobic Co-digestion of Microalgae
AD of raw microalgae or microalgae residues after the generation/extraction of
value-added products (i.e. lipids, ethanol and hydrogen) is typically characterised
by low methane yields and the occurrence of ammonia inhibition. Despite these
limitations, AD is still regarded as a key technology to maximise resource recovery
from microalgae and make algae industry economically feasible. AD also aids the
mobilisation the nutrients (N and P) needed for algae cultivation (Ward et al. 2014).
Anaerobic co-digestion, the simultaneous digestion of two or more substrates, is an
established and cost-effective option to overcome the drawbacks of mono-digestion
and boost the biogas production of AD plants (Mata-Alvarez et al. 2014). Besides
improving the feasibility of AD plants, co-digestion also allows treating several
wastes in a single facility and “share/reduce” treatment costs (Neumann et al. 2015).
Algae have been successfully co-digested with a large range of co-substrates
such as sewage sludge, animal manures, food waste, energy crops, glycerol, paper
waste and fat, oil and grease (FOG). Although the improvement of the methane
production is mainly a consequence of the increased organic loading rate
(OLR) rather than to the occurrence of synergisms during AD, microalgae have
been primarily co-digested with carbon-rich co-substrates, which allows increasing
the digester OLR while controlling ammonia concentration. Several studies have
optimised the co-substrate dose by balancing the feedstock C/N ratio with optimum
values for algae co-digestion ranging between 12 and 27 (Ehimen et al. 2011;
Fernández-Rodríguez et al. 2014). However, optimising co-substrate selection and
dosage based on the C/N ratio is an oversimplification since this approach does not
take into account the characteristics of each co-substrate (Astals et al. 2014;
Herrmann et al. 2016). The maximum dose of some co-substrates such as glycerol
and FOG is limited by secondary inhibitory mechanisms, while the deficiency of
alkalinity or essential nutrients limits the dosage of energy crops and paper waste
12 Biofuels from Microalgae: Biomethane
255