international regulations for injection into natural gas grids or use as autogas. This
book chapter will critically review the most recent advances in biogas production
from microalgae, with a special focus on pretreatment technologies, co-digestion
opportunities, modelling strategies, biogas upgrading and process microbiology.
Keywords Anaerobic co-digestion Á Biogas upgrading Á Microbiology
Modelling Á Pretreatments
1 Introduction
During the last decade, microalgae production and bioconversion have been widely
investigated for bioenergy generation purposes. Nonetheless, energy and life cycle
assessments of theoretical and pilot-scale studies have consistently shown that such
technology is only feasible if microalgae are grown in open ponds fed with
wastewater (Sialve et al. 2009). In this context, high rate algal ponds (HRAPs) have
been proved efficient in removing organic matter and nutrients from contaminated
effluents (Park et al. 2011), and cost-effective alternatives when compared to activated sludge processes (no external input of aeration is required due to the natural
occurrence of photosynthesis).
The microalgae-bacteria biomass produced in such systems may be valorised
through anaerobic digestion (AD) with the concomitant production of biogas. This
process is already well known and has long been used to produce bioenergy from
organic residues such as sewage sludge, agricultural and industrial by-products. In
fact, AD may convert microalgae-based wastewater treatment plants (WWTPs) into
net energy producers by converting methane into heat and electricity that may be
subsequently used in biomass pretreatment and wastewater biodegradation (Passos
and Ferrer 2014). Additionally, the mineralisation of microalgae containing organic
nitrogen and phosphorus may convert microalgae into a stabilised biosolid fertilizer
(Solé-Bundó et al. 2017).
Nonetheless, this technology platform has some bottlenecks that hinder its
viability at full-scale. The main issues are: (i) low microalgae production rates due
to carbon or light limitation, (ii) costly biomass concentration and (iii) slow
biodegradability in anaerobic digesters. Some of these challenges may be overcome
by applying pretreatment or co-digestion technologies. Pretreatment can be used to
enhance microalgae anaerobic biodegradability by weakening or disrupting
microalgae cell wall structure; co-digestion improves the process biogas yield by
improving the organic loading rate while controlling ammonia concentration. On
the other hand, mathematical models and reactor design and operation strategies
need to be carefully reviewed for a better understanding and optimisation of process
performance. Finally, the biogas produced during the AD of microalgae should be
upgraded prior to its combustion on-site, injection into natural gas grids or used as
autogas.
248
F. Passos et al.
book chapter will critically review the most recent advances in biogas production
from microalgae, with a special focus on pretreatment technologies, co-digestion
opportunities, modelling strategies, biogas upgrading and process microbiology.
Keywords Anaerobic co-digestion Á Biogas upgrading Á Microbiology
Modelling Á Pretreatments
1 Introduction
During the last decade, microalgae production and bioconversion have been widely
investigated for bioenergy generation purposes. Nonetheless, energy and life cycle
assessments of theoretical and pilot-scale studies have consistently shown that such
technology is only feasible if microalgae are grown in open ponds fed with
wastewater (Sialve et al. 2009). In this context, high rate algal ponds (HRAPs) have
been proved efficient in removing organic matter and nutrients from contaminated
effluents (Park et al. 2011), and cost-effective alternatives when compared to activated sludge processes (no external input of aeration is required due to the natural
occurrence of photosynthesis).
The microalgae-bacteria biomass produced in such systems may be valorised
through anaerobic digestion (AD) with the concomitant production of biogas. This
process is already well known and has long been used to produce bioenergy from
organic residues such as sewage sludge, agricultural and industrial by-products. In
fact, AD may convert microalgae-based wastewater treatment plants (WWTPs) into
net energy producers by converting methane into heat and electricity that may be
subsequently used in biomass pretreatment and wastewater biodegradation (Passos
and Ferrer 2014). Additionally, the mineralisation of microalgae containing organic
nitrogen and phosphorus may convert microalgae into a stabilised biosolid fertilizer
(Solé-Bundó et al. 2017).
Nonetheless, this technology platform has some bottlenecks that hinder its
viability at full-scale. The main issues are: (i) low microalgae production rates due
to carbon or light limitation, (ii) costly biomass concentration and (iii) slow
biodegradability in anaerobic digesters. Some of these challenges may be overcome
by applying pretreatment or co-digestion technologies. Pretreatment can be used to
enhance microalgae anaerobic biodegradability by weakening or disrupting
microalgae cell wall structure; co-digestion improves the process biogas yield by
improving the organic loading rate while controlling ammonia concentration. On
the other hand, mathematical models and reactor design and operation strategies
need to be carefully reviewed for a better understanding and optimisation of process
performance. Finally, the biogas produced during the AD of microalgae should be
upgraded prior to its combustion on-site, injection into natural gas grids or used as
autogas.
248
F. Passos et al.