Large-scale production of algae-based biofuels poses sustainability concerns

one 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 percent to 107 percent of the total nitrogen use and 20 percent to 51 percent of the total phosphorus use in the United States. Recycling nutrients or utilizing wastewater from agricultural or municipal sources, however, 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 United States.

One of the primary motivations for using alternative fuels for transportation is reducing greenhouse gas emissions.  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.

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 U.S. Department of Energy 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. 

— Read more in Sustainable Development of Algal Biofuels