FIGURE S-1 Pathways for cultivating and processing algae to fuels and their products. Heterotrophic routes are outside the scope of this analysis.
consumptive use of fresh water for producing 1 liter of gasoline equivalent of algal biofuel is 3.15 to 3,650 liters, depending on whether the algae or cyanobacteria need to be harvested to be processed to fuels or if they secrete fuel products; whether fresh water, inland saline water, marine water, or wastewater is used as a culture medium; the climatic condition of the region if open ponds are used; and whether the harvest water from algae cultivation is recycled. In other words, at least 123 billion liters of water would be needed to produce 39 billion liters of algal biofuels or an equivalent of 5 percent of U.S. demand for transportation fuels. The estimated requirement for nitrogen and phosphorus needed to produce that amount of algal biofuels ranges from 6 million to 15 million metric tons of nitrogen and from 1 million to 2 million metric tons of phosphorus if the nutrients are not recycled or included and used in coproducts. Those estimated requirements represent 44 to 107 percent of the total nitrogen use and 20 to 51 percent of total phosphorus use in the United States.
Sustainable development of algal biofuels would require research, development, and demonstration of the following:
• Algal strain selection and improvement to enhance desired characteristics and biofuel productivity.
• An EROI that is comparable to other transportation fuels, or at least improving and approaching the EROIs of other transportation fuels.
• The use of wastewater for cultivating algae for fuels or the recycling of harvest water, particularly if freshwater algae are used.
• Recycling of nutrients in algal biofuel pathways that require harvesting unless coproducts that meet an equivalent nutrient need are produced.