pathway is that the fuel product, ethanol, is not compatible with the fuel distribution infrastructure for petroleum-based fuels. Although ethanol can be used in flex-fuel vehicles (FFV) that accommodate a blend of 85 percent ethanol and 15 percent gasoline (E85), most vehicles in the United States have internal combustion engines that use E10, which contains 90 percent gasoline and 10 percent ethanol. As of January 2011, the U.S. Environmental Protection Agency (EPA) allows the use of E15 in vehicle models of 2001 or newer. If every drop of the 520 billion liters of gasoline consumed in the United States in 2010 was blended with ethanol for E10, the maximum ethanol that could be used is 52 billion liters. The United States produced 50 billion liters of corn-grain ethanol that year. Therefore, the U.S. transportation sector would not be able to incorporate much more ethanol into the fuel system unless the market for flex-fuel vehicles expands.

6.1.5 Comparing the Sustainability of Different Pathways

The summaries of resource use and environmental effects of different pathways illustrate that each pathway has its strengths and weaknesses in meeting different sustainability goals. For example, the use of open ponds and closed photobioreactors illustrate tradeoffs between aspects of economic and environmental sustainability. Open-pond systems could raise more environmental concerns than closed-photobioreactor systems, but the cost differential between the two systems could be a key determinant of economic viability. The direct synthesis and secretion of ethanol by cyanobacteria without cell destruction would reduce nitrogen and phosphorus input during cultivation (particularly if nitrogen and phosphorus recycling are not fully implemented in algal biofuel production systems that require biomass harvesting) and energy use from downstream processing and could result in synergistic cost savings for a closed photobioreactor system. The question arises as to how to make a holistic assessment of the relative sustainability of different algal biofuel production systems, given the multiple indicators and LCAs that represent various sustainability goals and objectives. As discussed in Chapter 2, indicators and LCAs are tools that can be used to assess a particular aspect of sustainability. Other tools are needed to integrate across disciplines to assess overall sustainability, which includes energy security, and environmental, social, and economic sustainability. As outlined in the statement of task, the committee was not asked to perform any technoeconomic analyses. Environmental sustainability has been considered more extensively than social sustainability in the literature because some aspects of social sustainability will be local and social acceptability in part depends on public opinion, transparency, stakeholder participation, and risk of catastrophe, all of which are largely unexplored for algal biofuels. Therefore, this chapter focuses on environmental sustainability.

6.2 TOOLS FOR ASSESSING OVERALL SUSTAINABILITY

The holistic assessment of sustainability is complicated by the fact that some sustainability objectives can be assessed and compared across systems while others are region-specific and cannot be compared across systems. For example, resource use and environmental effects such as nutrient budgets, energy balances, and GHG emissions can be compared directly across systems. Methods for assessing these variables are strictly quantitative. Other environmental effects such as land-use change and biodiversity are region specific and scale specific.

Some resource use and environmental effects can be assessed quantitatively, but whether they contribute to moving toward or away from the sustainability objectives could



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