Environmental Impacts of Growing Algae for Bioenergy.

Algae have been widely discussed as a seemingly desirable source of bioenergy since they have higher lipid yields per acre than terrestrial crops. From a systems perspective, however, the cultivation of algae has considerable impacts over the entire life cycle, particularly when compared with conventional terrestrial bioenergy crops such as corn, switchgrass, and canola. The higher energy, greenhouse gas, and water burdens associated with algae production are roughly ten times higher than conventional crops. Many of these impacts are associated with the production of fertilizers, which must be delivered to the algae in large quantities. While terrestrial crops can be rotated with nitrogen fixing species, algae must have a large fraction of their biologically available nitrogen delivered via anthropogenic sources. A detailed sensitivity analysis reveals a number of strategies for reducing the burden of algae cultivation. One particularly attractive option is to avoid nutrient production by leveraging widely available and inexpensive nutrient sources such as wastewater streams. Toward realizing these improvements, the viability of growing algae in wastewaters was assessed. It is found that the use of nutrient rich municipal and agricultural wastewaters greatly enhances the environmental sustainability of algae bioenergy production, in some cases making it better than terrestrial crops. Complementary modeling work reveals that the engineering decisions associated with algae growth scenarios have an important impact on the overall environmental and economic burdens of bioenergy production. These results show clearly that open ponds are significantly more viable than photobioreactors or other engineered methods for cultivation. The results of these analyses suggest that algae cultivation should be approached from an agricultural framework with careful consideration of the environmental pitfalls. Thoughtful deployment of this technology is pending technological innovations in several key areas. It is possible that these advances could transform algae into a highly attractive fuel source in spite of conventional practices.