Biomethane (CH4) production from microalgal biomass is of interest because the efficiency of algal biomass production per hectare is estimated to be 5—30 times greater than that of the terrestrial crop plants (Sheehan et al., 1998). Golueke and Oswald (1959) pub­lished one of the first feasibility studies using microalgae for CH4 production and concluded that the process was feasible (Golueke and Oswald, 1959). There are two well-established methods of CH4 production: (1) harvest of an algal polyculture from a wastewater treatment pond, or (2) axenic growth of specific algae at a bench scale (Asinari Di San Marzano et al., 1982; Yen and Brune, 2007). The digestion process is described in

Figure 10.9. It begins with bacterial hydrolysis of the algal biomass. Organic polymers, such as lipids, carbo­hydrates, and proteins, are first broken down to soluble derivatives, which are further fractionated into carbon dioxide, hydrogen, ammonia, and organic acids by acidogenic bacteria. Acetogenic bacteria then convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon diox­ide. Finally, methanogens convert these products to methane and carbon dioxide. Regardless of operating conditions and species, the proportion of methane in the biogas produced for the majority of studies falls in the range 69—75%. Anaerobic digestion is an effective process for biological oxygen demand removal, but it is not effective for nutrient removal. Thus, there is a need for further treatment of effluent from anaerobic di­gesters before it can be discharged into the environment. The nutrient-rich digestate also produced can be used as fertilizer. This process of converting microalgae to CH4 is dependent on several key metrics, namely (1) pH, (2) retention time, (3) mixing, (4) composition of the biomass and (5) composition of the surrounding milieu.

One of the most important factors influencing CH4 biogas production from algal biomass has been reported to be pH. At high pH, due to high alkalinity from NH3 release, the gas production will shift toward CH4. The oxidation state of the biomass also affects biogas quality, which in turn drives the proportion of methane released (Sialve et al., 2009). Due to lowered content of sulfated amino acids, the microalgal biomass digestion releases a lower amount of hydrogen sulfide than do other types of organic substrates (Becker, 1988). The composition of the microalgal feedstock also affects biomethane yields. The relatively high lipid, starch, and protein contents and the absence of lignin make microalgae an ideal candidate for efficient biomethane production via fermentation in biogas plants. Theoretically, higher

cellular lipid contents will result in higher methane yields. Thus lipid-rich microalgae make attractive sub­strates for anaerobic digestion, as they have a higher gas production potential when compared to carbohy­drates and proteins (Li et al., 2002; Cirne et al., 2007). The hydraulic and solid retention time is another key metric in the anaerobic process. The hydraulic and solid retention time is a measure of the average length of the time that a soluble compound remains in a constructed bioreactor. Retention times should be sufficiently high to allow active bacterial populations (e. g. methanogens) to remain in the reactor yet not limit hydrolysis, which is considered to be the rate-limiting step in the overall con­version of complex substrates to methane. Moreover, optimal loading rates and hydraulic retention times must be enhanced to ensure efficient conversion of organic matter, and will depend on algal substrate composition and accessibility.