The committee’s sustainability analysis focused on pathways that to date have received active attention. Most of the current development involves growing selected strains of algae in open ponds or closed photobioreactors using various water sources, collecting and extracting the oil from algae or collecting fuel precursors secreted by algae, and then processing the oil into fuel.

The committee pointed out a number of concerns of high importance for large-scale development of algal biofuel, including the relatively large quantity of water required for algae cultivation; the magnitude of nutrients, such as nitrogen, phosphorus, and CO₂, needed for cultivation; the amount of land area necessary to contain the ponds that grow the algae; uncertainties in greenhouse gas emissions over the production life cycle; and energy return on investment—i.e., algal biofuel production would have to produce sufficiently more energy than is required in cultivation and fuel conversion to be sustainable

The committee found that to produce the amount of algal biofuel equivalent to 1 liter of gasoline, between 3.15 liters to 3,650 liters of freshwater is required, depending on the production pathway. Replenishing water lost from evaporation in growing systems is a key driver for use of freshwater in production, the committee said. In addition, water use could be a serious concern in an algal biofuel production system that uses freshwater without recycling the “harvest” water.

To produce 39 billion liters of algal biofuels, 6 million to 15 million metric tons of nitrogen and 1 million to 2 million metric tons of phosphorus would be needed each year if the nutrients are not recycled, the report says. These requirements represent 44% to 107% of the total nitrogen use and 20% to 51% of the total phosphorus use in the US. However, recycling nutrients or utilizing wastewater from agricultural or municipal sources could reduce nutrient and energy use, the committee said.

Another resource that could limit the amount of algal biofuels produced is land area and the number of suitable and available sites for algae to grow. Appropriate topography, climate, proximity to water supplies—whether freshwater, inland saline water, marine water, or wastewater—and proximity to nutrient supplies would have to be matched carefully to ensure successful and sustainable fuel production and avoid costs and energy consumption for transporting those resources to cultivation facilities. If the suitable sites for growing algae are near urban or suburban centers or coastal recreation areas, the price of those lands could be prohibitive. A national assessment of land requirements for algae cultivation that takes into account various concerns is needed to inform the potential amount of algal biofuels that could be produced economically in the US.

One of the primary motivations for using alternative fuels for transportation is reducing greenhouse gas emissions. However, estimates of greenhouse gas emissions over the life cycle of algal biofuel production span a wide range; some studies suggest that algal biofuel production generates less greenhouse gas emissions than petroleum-based fuels while other studies suggest the opposite. These emissions depend on many factors in the production process, including the amount of energy needed to dewater and harvest algae and the electricity sources used.

Concerns of medium importance cited by the report included:

• Presence of waterborne toxicants in cultivation systems that use flue gas as a source of CO₂ or wastewater as a source of culture water and nutrients, particularly if fertilizers or feedstuff are to be produced as co-products.

• Effects from land-use changes if pasture and rangeland are to be converted to algae cultivation. Displacing pasture and rangeland could incur direct and indirect land-use changes that would affect the net GHG emissions of algal biofuels.

• Air-quality emissions over the life cycle of algal biofuels. Emissions from the processing facilities and tailpipe emissions will be regulated. The committee is not aware of any published studies that include measured emissions of air pollutants from open-pond cultivation.

• Potential effects on local climate. The introduction of large-scale algae cultivation systems in arid or semi-arid environments could alter the local climate of the area by increasing humidity and altering temperature extremes.

• Releases of cultivated algae to natural environments and potential alteration of species composition in receiving waters.

• Effects on terrestrial biodiversity from changing landscape pattern as a result of infrastructure development for algal biofuels.

• Potential adverse effects and unintended consequences of introduction of genetically modified algae for biofuel production.

• Waste products from processing algae to fuels.

• Potential presence of pathogens if wastewater is used for algae cultivation.

• Potential presence of unknown, unidentified, or unexpected algal toxins.

Concerns of low importance included accidental releases of culture water and infiltration of nutrients and chemicals into soil or surrounding water; seepage of culture water into the local groundwater system if clay-lined ponds are used or if plastic liners are breached through normal weathering or from extreme weather events; and potential presence of mosquitoes and mosquito-borne diseases around poorly managed open ponds.

A potential framework for assessing sustainability of algal biofuels during different stages of development. Source: NRC. Click to enlarge.

The committee emphasized that the crucial aspects to sustainable development are positioning algal growth ponds close to water and nutrient resources and recycling essential resources. With proper management and good engineering designs, other environmental effects could be avoided, the committee said. Examples include releasing harvest water in other bodies of water and creating algal blooms and allowing harvest water to seep into ground water.

For algal biofuels to contribute a significant amount of fuels for transportation in the future, the committee said, research and development would be needed to improve algal strains, test additional strains for desired characteristics, advance the materials and methods for growing and processing algae into fuels, and reduce the energy requirements for multiple stages of production. To aid the US Department of Energy (DOE) in its decision-making process regarding sustainable algal biofuel development, the committee proposed a framework that includes an assessment of sustainability throughout the supply chain, a cumulative impact analysis of resource use or environmental effects, and cost-benefit analyses.

This report identified EROI; GHG emissions; water use: supply of nitrogen, phosphorus, and carbon dioxide; and appropriate land resources as potential sustainability concerns of high importance. The committee does not consider any one of these sustainability concerns a definitive barrier to sustainable development of algal biofuels because mitigation strategies for each of those concerns have been proposed and are being developed. However, all of the key sustainability concerns have to be addressed to some extent and in an integrative manner. Therefore, research, development, and demonstration are needed to test and refine the production systems and the mitigation strategies for sustainability concerns and to evaluate the systems and strategies based on the sustainability goals if the promise of sustainable development of algal biofuels has any chance of being realized.

—“Sustainable development of algal biofuels”

The report was sponsored by the US Department of Energy. The National Academy of Sciences, National Academy of Engineering, Institute of Medicine, and National Research Council make up the National Academies.

Resources

• Sustainable development of algal biofuels