Wastewater and waste CO2 for sustainable biofuels from microalgae

In the new frontier of energy, biofuels will play an important role in overcoming our dependence on fossil fuels and its associated environmental impacts. In the biofuels sector, the exploitation of microalgal biomass has the potential to be beneficial, as they do not compete on land use with food crops and their cultivation systems can be designed to have a lower water footprint. Even though a number of LCAs (Life Cycle Assessment) involving biofuel production from microalgae have been reported, few of them have focused on the use of by-product streams in algal biomass production, such as wastewater or waste CO2 recovered from flue gas, which could further reduce the environmental impact of the recovered biofuels, and none have considered a combination of different by-product streams. In this paper, an LCA is applied to compare 6 alternative scenarios, where the potential environmental benefits achievable using CO2 from different sources (commercial liquid CO2, CO2 recovered from flue gas and flue gas as is) and wastewater in the cultivation of microalgae for the production of biofuels are investigated. The analysis is based on a virtual, but realistic case, using an open microalgal cultivation pond facility located in Kingston (Canada). The results indicate that the source of CO2 is the most relevant factor affecting environmental impacts, and that the direct injection of flue gas into the algal pond and the use of wastewater represent the most environmentally friendly alternative.