Biogas upgrading by microalgae: Strategies and future perspectives

Microalgae are being increasingly considered as a potential biomass feedstock for various biofuels, biodiesel in particular. Microalgal biomass for biofuel production purposes can be derived by cultivation using several waste resources, such as wastewater or flue gases, due mainly to the absence of the stringent regulations usually applied for food grade health supplements from microalgae. Anaerobic digestion and dark fermentation, the two highly used biomass digestion processes, generate biogas (a mixture of CH₄, CO₂ and other gases) and a COD (chemical oxygen demand)-rich effluent with leftover organic acids from the fermentation process. Microalgae can utilize the CO₂ present in the biogas stream, thus increasing the methane content and improving the fuel properties of biogas. Several reports indicate that certain microalgae are highly tolerant to the high concentrations of methane present in the biogas stream and can effectively utilize the CO₂ in photoautotrophic/mixotrophic mode of cultivation to obtain microalgal biomass. The organic acids of the effluent can also be used as a carbon source for mixotrophic/heterotrophic mode of microalgal cultivation, thus providing a cleanup of both the liquid and gaseous effluents of the fermentation process. This chapter describes in detail the capability of microalgae for carbon capture from biogas and their efficiency in the utilization of organic acids from various effluent streams. A biorefinery concept, integrating anaerobic digestion and microalgal cultivation is proposed, and the future perspectives are discussed.