Freshwater Macroalgae as a Biofuels Feedstock: Mini-Review and Assessment of Their Bioenergy Potential

Abstract

Extensive efforts have been made to evaluate the potential of microalgae as a biofuel feedstock during the past 4–5 decades. However, filamentous freshwater macroalgae have numerous characteristics that favor their potential use as an alternative algal feedstock for biofuels production. Freshwater macroalgae exhibit high rates of areal productivity, and their tendency to form dense floating mats on the water surface imply significant reductions in harvesting and dewatering costs compared to microalgae. A review of the published literature on the elemental composition and energy content of five genera of freshwater macroalgae suggests that they compare very favorably with traditional biobased energy sources, including terrestrial residues, wood, and coal. In addition, we performed a semi-continuous culture experiment using the common Chlorophyte genus Oedogonium to investigate whether nutrient availability can influence its higher heating value (HHV), productivity, and proximate analysis. Our findings show that the most nutrient-limited growth conditions resulted in a significant increase in the HHV of the Oedogonium biomass (14.4–16.1 MJ/kg). Although there was no significant difference in productivity between the treatments, the average dry weight productivity of Oedogonium (3.37 g/m2/day) was higher than for common terrestrial plant crops. These results suggest that filamentous freshwater macroalgae have great potential as a feedstock for both liquid and solid fuels, especially if nutrient-rich wastewater can be used as the supply of water and mineral nutrients. In addition, this study demonstrates the importance of evaluating the algal cultivation conditions that influence trade-offs between biomass productivity and energy content.