This chapter aims at presenting and discussing the main topics involved in
microalgae AD, i.e. the microbiology involved, pretreatment technologies,
co-digestion with other substrates, design and operational considerations, process
modelling and biogas upgrading to biomethane.
2 The Role of Microbiology in the Anaerobic Digestion
of Microalgae
AD of microalgae is a spontaneous process in which organic matter from
microalgal cells is converted to biogas through reactions catalysed by naturally
occurring microorganisms. Like most biological processes, AD is affected by a
variety of factors such as “substrate type”, environmental, physical, biological and
chemical conditions. Microalgal biomass is composed mainly of organic compounds (mostly lipids, carbohydrates and protein), as well as nitrogen, phosphorus
and oligonutrients such as zinc, cobalt and iron. The average composition of
microalgae can be expressed as CO 0.48 H 1.83 N 0.11 P 0.01 (Grobbelaar 2004). The
content of proteins, lipids and carbohydrates in microalgae is strongly species
dependent (Table 1) and varies from 6 to 52%, from 7 to 23% and from 5 to 23%,
respectively (Brown et al. 1997).
Two of the most important factors determining the methane yield in anaerobic
digestion of microalgal biomass are the composition of microalgae cell wall and its
contribution to the total cell mass. Cell wall composition is recognised as the
limiting factor in hydrolysis of microalgae (Chen and Oswald 1998. Microalgae cell
wall comprises 12–36% of total cell mass (w/w) (Table 2) and may contain
biopolymers (e.g. algaenan, cellulose, sporopollenin, glucosamine, proline and
carotenoids) and/or structures (such as trilaminar outer wall or trilaminar sheath—
TLS) that are resistant to anaerobic degradation (Kadouri et al. 1988; Brown 1997;
Derenne et al. 1992; Gelin et al. 1997; Okuda 2002; Simpson et al. 2003). Cell
walls recalcitrant to microbial attack may prevent microalgal intracellular organic
Table 1 Gross composition of several microalgae species
Microalgae species
Proteins (%)
Lipids (%)
Carbohydrates (%)
Euglena gracilis
39–61
14–20
14–18
Chlamydomonas reinhardtii
48
21
17
Chlorella pyrenoidosa
57
2
26
Chlorella vulgaris
51–58
14–22
12–17
Dunaliella salina
57
6
32
Spirulina maxima
60–71
6–7
1 3 –16
Spirulina platensis
46–63
4–9
8 –14
Scenedesmus obliquus
50–56
12–14
10–17
Adapted from Sialve et al. (2009)
12 Biofuels from Microalgae: Biomethane
249
microalgae AD, i.e. the microbiology involved, pretreatment technologies,
co-digestion with other substrates, design and operational considerations, process
modelling and biogas upgrading to biomethane.
2 The Role of Microbiology in the Anaerobic Digestion
of Microalgae
AD of microalgae is a spontaneous process in which organic matter from
microalgal cells is converted to biogas through reactions catalysed by naturally
occurring microorganisms. Like most biological processes, AD is affected by a
variety of factors such as “substrate type”, environmental, physical, biological and
chemical conditions. Microalgal biomass is composed mainly of organic compounds (mostly lipids, carbohydrates and protein), as well as nitrogen, phosphorus
and oligonutrients such as zinc, cobalt and iron. The average composition of
microalgae can be expressed as CO 0.48 H 1.83 N 0.11 P 0.01 (Grobbelaar 2004). The
content of proteins, lipids and carbohydrates in microalgae is strongly species
dependent (Table 1) and varies from 6 to 52%, from 7 to 23% and from 5 to 23%,
respectively (Brown et al. 1997).
Two of the most important factors determining the methane yield in anaerobic
digestion of microalgal biomass are the composition of microalgae cell wall and its
contribution to the total cell mass. Cell wall composition is recognised as the
limiting factor in hydrolysis of microalgae (Chen and Oswald 1998. Microalgae cell
wall comprises 12–36% of total cell mass (w/w) (Table 2) and may contain
biopolymers (e.g. algaenan, cellulose, sporopollenin, glucosamine, proline and
carotenoids) and/or structures (such as trilaminar outer wall or trilaminar sheath—
TLS) that are resistant to anaerobic degradation (Kadouri et al. 1988; Brown 1997;
Derenne et al. 1992; Gelin et al. 1997; Okuda 2002; Simpson et al. 2003). Cell
walls recalcitrant to microbial attack may prevent microalgal intracellular organic
Table 1 Gross composition of several microalgae species
Microalgae species
Proteins (%)
Lipids (%)
Carbohydrates (%)
Euglena gracilis
39–61
14–20
14–18
Chlamydomonas reinhardtii
48
21
17
Chlorella pyrenoidosa
57
2
26
Chlorella vulgaris
51–58
14–22
12–17
Dunaliella salina
57
6
32
Spirulina maxima
60–71
6–7
1 3 –16
Spirulina platensis
46–63
4–9
8 –14
Scenedesmus obliquus
50–56
12–14
10–17
Adapted from Sialve et al. (2009)
12 Biofuels from Microalgae: Biomethane
249