Evaluating Anaerobic Digestion as Means to Produce Energy in Algae Farming and Integrated Wastewater Treatment Systems

The production of algae-derived energy via anaerobic digestion has been a topic of growing interest. In the past few years, there have been many studies investigating the digestibility and methane yield of various algae substrates; however, these have not yet been integrated into life cycle assessment (LCA)-based evaluation of hypothetical algae-to-energy systems. This study seeks to revise previous LCA assessments of energy return on investment (EROI) for two energy-producing algae systems: 1) “pure” digestion at large-scale algae farms, and 2) co-digestion of algae and wastewater biosolids and municipal wastewater treatment plants (WWTPs). This is achieved via literature review of recent studies, followed by experimental measurement of digestion parameters for pure cultures of Scenedesmus dimorphus and mixed WWTP-grown algae. Resulting estimates for methane yield are then incorporated into LCA frameworks for both systems of interest. Results for the “pure” algae digestion evaluation indicate that revised digestion parameters increase the estimate of EROI from 1.75 to 3.29. Similarly, incorporation of algae co-digestion at a WWTP increases the EROI of a WWTP from 0.30 (without algae cultivation) to 0.35 (with algae cultivation). Together, these results underscore the potential for large-scale algae-based energy production.