analyses that examine the cumulative effects of a resource use or an environmental effect of algal biofuel production in addition to the existing activities in the production area, and cost-benefit analyses that integrate the monetized environmental costs of algal biofuel production with the monetized environmental benefits.

Determining the sustainability of algal biofuel production requires comparisons with fuels being used today to assess whether substituting algal biofuels for an existing option contributes to improving sustainability. The framework starts with assessing two of the primary goals for developing alternative liquid fuels—improving energy security and reducing GHG emissions. To be a sustainable source, any fuel produced needs to return more energy in use than was required for its production; therefore, EROI is a logical first step for assessment. Some authors suggest an EROI of less than 3 for any fuel to be considered unsustainable. Therefore, the EROIs of algal biofuels at least have to show progress toward a value that is within the range of EROIs of other transportation fuels. Ideally, the alternative fuel that is replacing petroleum-based fuels will improve energy security and contribute to reducing GHG emissions.

If algal biofuels show promise for achieving these two goals, then a few variables that reflect commonly agreed-upon sustainability objectives and that can be estimated from mass balance and engineering principles are assessed. For example, nitrogen and phosphorus inputs and freshwater use are sustainability objectives that can be assessed using LCAs. Avoiding competition for these resources between food and fuel production is a commonly agreed-upon objective. The estimated EROI, GHG emissions, nutrient, and freshwater requirements would have to be reassessed once the likely locations of deployment are determined. Then the productivities of algal feedstocks and fuel products and any potential land-use changes can be estimated with increased certainty, and the precision of the estimated resource requirements and GHG emissions can be improved. When the industry is further along in its development, direct measurements can be made in operating algal biofuel production systems to verify estimates. In addition, progressively comprehensive and regional assessments that include other variables can be made.

Though some resource use or emissions can be estimated quantitatively, some biological effects (for example, biodiversity) or the impact of some environmental effects (for example, air-quality emissions and water use) are location specific. For example, water use (coastal or inland saline water or fresh water) can be estimated over the life cycle of biofuel, but the effect of the water use has to be put into the context of regional availability. The effect of algal biofuel production on biodiversity cannot be assessed unless the specific location of deployment and the species present there are known. Some of these effects might be easily quantifiable. Other effects might require research and data collection before the effects can be understood and quantified.

The resource requirements and environmental effects also have to be assessed in the context of existing activities at the sites where algal biofuel production systems are to be developed. As the algal biofuel industry develops, the ability of different pathways for algal biofuel production to meet and balance productivity of fuel with the other environmental, economic, and social sustainability goals has to be assessed in a holistic manner. Such assessment by itself does not inform whether algal biofuels would contribute to improving sustainability of the transportation sector.

The environmental, economic, and social effects of algal biofuel production and use have to be compared with those of petroleum-based fuels and other fuel alternatives to determine whether algal biofuels contribute to improving sustainability. Such comparison



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