Biogas from microalgae: an overview emphasizing pretreatment methods and their energy return on investment (EROI)

Studies have reported enhancements in methane yield from pretreatment methods that benefit the anaerobic digestion (AD) of microalgae. However, energy return on investment (EROI), i.e., methane production enhancement achieved over energy input, may be unfavorable. Aiming to quantify EROI of AD microalgae pretreatment, about 180 experiments applied to 30 microalgae biomasses were compiled through an extensive literature survey, classified into 4 pretreatments (physical, enzymatic, chemical, and hybrid), and analyzed. Most of these pretreatments enhanced methane yield, especially the enzymatic alternative. EROI was evaluated for the most efficient pretreatments. Only in one thermal pretreatment the energy resulting from the increase in methane production exceeded the energy demanded by the biomass pretreatment (EROI 6.8) and other two thermal pretreatments presented EROI 1. The other pretreatments presented EROI < 1, concluding that none of the evaluated methods was energy-efficient. Feasibility of pretreatment requires advancements in low energy-demanding strategies and outstanding biomass densification.