Another resource that could pose a limit on the potential amount of algal biofuel that could be produced is land area and the number of suitable and available sites for algae cultivation. A number of site-specific factors—suitable topography, climate, proximity to sustainable water supplies (whether fresh water, inland saline water, marine water, or wastewater), and proximity to sustainable and economic nutrient supplies—would have to be matched carefully with algal cultivation systems to ensure the successful and sustainable production of algal biomass for fuels. Although the use of inland saline water, marine water, or wastewater has been suggested as a mitigation strategy for reducing freshwater use, information on the depth and accessible volume of saline aquifers is lacking, and the actual land area close to wastewater sources suitable for algae cultivation has not been assessed. If the sites are near urban or suburban centers or coastal recreation areas, the price of those lands could hinder their use for algae cultivation.
A national assessment of land requirements for algae cultivation that takes into account climatic conditions; fresh water, inland and coastal saline water, and wastewater resources; sources of CO2; and land prices is needed to inform the potential amount of algal biofuels that could be produced economically in the United States.
The potential environmental effects listed in Box S-1 can be divided into three types:
• Effects that can be minimized or prevented by proper management of algal cultivation systems or mitigated by engineering designs—for example, accidental release and seepage of culture water, waste products from algal biofuel production, and mosquito-borne diseases.
• Potential effects that have not been assessed or reported extensively in the literature—for example, the effects of large-scale, open-pond algae cultivation on terrestrial wildlife, natural ecosystems, and local climate; potential adverse effects of genetically engineered algae; and presence of unknown or unidentified toxins. Large investments into researching these topics might not be necessary at this early stage of development, but some preliminary assessment now and periodic monitoring as the industry develops would be prudent.
• Effects that need to be assessed for each pathway for algal biofuel production or considered carefully before deployment of algal biofuels—for example, potential land conversion and its effects on GHG emissions; net GHG emissions; air emissions; and safety and nutritional quality of feedstuff coproducts if a pathway relies heavily on coproduct production to achieve high EROI, economic viability, or low resource use.
Algal biofuels have the potential to contribute to improving the sustainability of the transportation sector, but the potential is not yet realized. Additional innovations that require research and development are needed to realize the full potential of algal biofuels.
The use of algae offers the potential for sustainability benefits over petroleum-based fuels. The potential benefits stem from the ability to produce algal biofuels domestically, the inherently high photosynthetic productivities of algae relative to terrestrial plants, the use of alternative water sources to reduce the freshwater requirement, the ability to use nonarable lands, and the potential to remediate wastewater and use it as a nutrient and water