using microalgae to transform light into chemical energy and anaerobic digestion to
transform such biomass into biomethane.
When considering biogas production from microalgae, two scenarios should be
considered. The first one relies on coupling biogas production to a
microalgae-based biodiesel production process. Microalgae have received great
attention as a potential source of oil for biodiesel production due to the ability of
certain types of microalgae to accumulate lipids and to the higher biomass productivities achieved when compared with land-based crops (Chisti 2007; Mata et al.
2010; Weyer et al. 2010). When the primary use of microalgae is biodiesel production, the lipids extraction processes employed (usually involving solvents) will
generate a “residual” biomass suitable for biogas production. However, recent
concerns have been raised by life cycle analyses when considering biodiesel production from microalgae due to potentially low energetic yield when based on
traditional technology (Scott et al. 2010; Sialve et al. 2009; Stephens et al. 2010).
Indeed, a negative energy balance has been estimated for biodiesel process from
microalgae as a result of harvesting and drying steps, which are highly energy
intensive (Lardon et al. 2009; Scott et al. 2010). In this context, the production of
biogas as a sole fuel using whole microalgae has been proposed. This option would
entail a much simpler process, with less and simpler unit operations. However,
energy in the form of methane possesses nowadays a low economic value.
Hydrolysis is known to be the rate-limiting step of anaerobic digestion of solid
substrates, which is specially the case when using microalgae as a substrate.
Thermophilic digestion has been proposed as a way to enhance microalgae biomass
hydrolysis and the overall anaerobic digestion performance. The high temperatures
applied during thermophilic anaerobic digestion (50–57 °C) accelerate biochemical
reactions, increasing both the efficiency of organic matter degradation and the
potentially applicable organic loading rates. However, higher degradation and
loading rates will increase the concentration of ammonia nitrogen in the digester.
Contradictory results have been reported when addressing the thermophilic anaerobic digestion of microalgal biomass (Capson-Tojo et al. 2017; Cea-Barcia et al.
2015; Zamalloa et al. 2012a). Indeed, the benefits of the thermophilic digestion of
microalgae still need to be confirmed and most likely, the optimum temperature for
anaerobic digestion might be dependent on the microalgae species.
The nitrogen content of microalgae biomass is relevant since ammonia release
during anaerobic digestion is expected to be an issue of concern as a result of the
above-discussed inhibition of AD. This will be especially critical when oil-extracted
microalgae are used as substrate, since lipids extraction increases the proportion of
nitrogen per gram of biomass. If anaerobic digestion is performed at solids concentrations over 4–5%, ammonia concentration in digester could reach inhibitory
levels for the anaerobic microbial community (Torres et al. 2013). Even though the
use of ammonia tolerant inocula may provide conditions for successful operation
(Mahdy et al. 2017), measurements need to be taken in order to ensure a stable
process performance. In this context, co-digestion of microalgae biomass with
carbon-rich substrates or wastes could be an alternative. As previously discussed,
indeed, the benefits derived from the co-digestion of microalgae biomass with
12 Biofuels from Microalgae: Biomethane
257
transform such biomass into biomethane.
When considering biogas production from microalgae, two scenarios should be
considered. The first one relies on coupling biogas production to a
microalgae-based biodiesel production process. Microalgae have received great
attention as a potential source of oil for biodiesel production due to the ability of
certain types of microalgae to accumulate lipids and to the higher biomass productivities achieved when compared with land-based crops (Chisti 2007; Mata et al.
2010; Weyer et al. 2010). When the primary use of microalgae is biodiesel production, the lipids extraction processes employed (usually involving solvents) will
generate a “residual” biomass suitable for biogas production. However, recent
concerns have been raised by life cycle analyses when considering biodiesel production from microalgae due to potentially low energetic yield when based on
traditional technology (Scott et al. 2010; Sialve et al. 2009; Stephens et al. 2010).
Indeed, a negative energy balance has been estimated for biodiesel process from
microalgae as a result of harvesting and drying steps, which are highly energy
intensive (Lardon et al. 2009; Scott et al. 2010). In this context, the production of
biogas as a sole fuel using whole microalgae has been proposed. This option would
entail a much simpler process, with less and simpler unit operations. However,
energy in the form of methane possesses nowadays a low economic value.
Hydrolysis is known to be the rate-limiting step of anaerobic digestion of solid
substrates, which is specially the case when using microalgae as a substrate.
Thermophilic digestion has been proposed as a way to enhance microalgae biomass
hydrolysis and the overall anaerobic digestion performance. The high temperatures
applied during thermophilic anaerobic digestion (50–57 °C) accelerate biochemical
reactions, increasing both the efficiency of organic matter degradation and the
potentially applicable organic loading rates. However, higher degradation and
loading rates will increase the concentration of ammonia nitrogen in the digester.
Contradictory results have been reported when addressing the thermophilic anaerobic digestion of microalgal biomass (Capson-Tojo et al. 2017; Cea-Barcia et al.
2015; Zamalloa et al. 2012a). Indeed, the benefits of the thermophilic digestion of
microalgae still need to be confirmed and most likely, the optimum temperature for
anaerobic digestion might be dependent on the microalgae species.
The nitrogen content of microalgae biomass is relevant since ammonia release
during anaerobic digestion is expected to be an issue of concern as a result of the
above-discussed inhibition of AD. This will be especially critical when oil-extracted
microalgae are used as substrate, since lipids extraction increases the proportion of
nitrogen per gram of biomass. If anaerobic digestion is performed at solids concentrations over 4–5%, ammonia concentration in digester could reach inhibitory
levels for the anaerobic microbial community (Torres et al. 2013). Even though the
use of ammonia tolerant inocula may provide conditions for successful operation
(Mahdy et al. 2017), measurements need to be taken in order to ensure a stable
process performance. In this context, co-digestion of microalgae biomass with
carbon-rich substrates or wastes could be an alternative. As previously discussed,
indeed, the benefits derived from the co-digestion of microalgae biomass with
12 Biofuels from Microalgae: Biomethane
257