12. Biofuels from Microalgae: Biomethane

The high cost of axenic microalgae cultivation in photobioreactors limits nowadays the potential uses of microalgal biomass as a feedstock for the production of biodiesel or bioethanol. In this context, microalgae-based wastewater treatment (WWT) has emerged as the leading method of cultivation for supplying microalgae at low cost and low environmental impacts, while achieving sewage treatment. Nonetheless, the year-round dynamics in microalgae population and cell composition when grown in WWTPs restrict the use of this low-quality biomass to biogas production via anaerobic digestion. Although the macromolecular composition of the microalgae produced during wastewater treatment is similar to that of sewage sludge, the recalcitrant nature of microalgae cell walls requires an optimisation of pretreatment technologies for enhancing microalgae biodegradability. In addition, the low C/N ratio, the high water content and the suspended nature of microalgae suggest that microalgal biomass will also benefit from anaerobic co-digestion with carbon-rich substrates, which constitutes a field for further research. Photosynthetic microalgae growth can also support an effective CO₂ capture and H₂S oxidation from biogas, which would generate a high-quality biomethane complying with most 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.