Integration of Anaerobic Digestion and Algal Cultivation—Prospects and Challenges

Biogas, generated through anaerobic digestion (AD) of organic feed-stocks like bio-wastes, is a sustainable and clean source of energy, capable of generating electricity, and direct thermal use. However, biogas is a mixture of methane and CO2 and the latter does not contribute towards the overall heating value of the gaseous fuel. The AD process also generates digestate slurry which is generally rich in ammonia-based nitrogen, phosphorous, and potassium (NPK). While the solid part of the effluent is traditionally utilized as fertilizers, the liquid digestate requires proper treatment and management to prevent leach out and contamination of the ground water systems. Algal cultivation (AC), on the other hand, is gaining attention for CO2 capture and for the generation of algal lipids and biochemicals, like proteins, carbohydrates, and pigments. Algal cultivation systems can be integrated as photo bioreactors for CO2 scrubbing in a two stage system or as anaerobic membrane reactors combining AD and algal processes along with membrane filtration technology for enhancing nutrient recovery, Therefore, algal cultivation can act as a potential CO2 trap for upgradation of the quality of biogas by enhancing methane concentration while utilizing the nutrient (NPK) rich effluent from AD process. The algal biomass can be further processed for the production of value-added biochemicals and bioenergy applications, like biodiesel production. The present article focuses on the prospects and challenges of the integration of AD and AC processes which can have potential implications on bioenergy sector.