Domestic production of renewable fuels, including algal biofuels, has the potential to meet the dual goals of improving energy security and decreasing greenhousegas (GHG) emissions from the transportation sector in the United States. Biofuels produced from microalgae and cyanobacteria1 offer potential advantages over terrestrial plant-based biofuels, such as high biomass productivity and the ability to grow in cultivation ponds or photobioreactors on non-arable lands using saline water or wastewater sources. However, along with potential environmental and social benefits, production of algal biofuels could result in significant resource inputs and in negative environmental and other detrimental effects, as is true of all forms of energy production.
At the request of the Department of Energy, Office of Energy Efficiency and Renewable Energy’s (DOE-EERE) Office of Biomass Program, the National Research Council (NRC) convened a committee of 15 experts to examine the sustainable development of algal biofuels. (See Appendix A for biographical sketches of committee members.) The purpose of this study was to identify and anticipate potential sustainability concerns associated with a selected number of pathways for large-scale deployment of algal biofuels, discuss potential strategies for mitigating those concerns, and suggest indicators and metrics that could be used and data to be collected for assessing sustainability across the biofuel supply chain to monitor progress as the industry develops. (See Appendix B for the complete statement of task.) In addition, the committee was asked to identify indicators that are most critical to address or have the greatest potential for improvement through DOE intervention and to suggest preferred cost-benefit analyses that could best aid in the decision-making process.
“Sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs.”2 The sustainability
1 Referred to as algal biofuels hereafter. Cyanobacteria, historically known as blue-green algae, are prokaryotes whereas algae are eukaryotes.
2 Definition from United Nations. 1987. Report of the World Commission on Environment and Development: Our common future. Available online at http://www.un-documents.net/ocf-02.htm. Accessed August 21, 2012.
goals for developing algal biofuels are to contribute to energy security by providing domestically sourced fuels, to maintain and enhance the natural resource base and environmental quality, to produce fuel that is economically viable, and to enhance the quality of life for society as a whole. Although economics is an important aspect of sustainability, this report does not assess the economics or costs of algal biofuels, as specified in the statement of task. Heterotrophic approaches3 for algae cultivation are not considered in this report because DOE-EERE considers the production of biofuel using heterotrophic algae a biochemical pathway to convert another feedstock (a sugar source such as cellulosic biomass) rather than a pathway that uses algae as a feedstock for fuels.
The intent of this report is to help anticipate the major sustainability concerns associated with resource use and the potential environmental and societal consequences if commercial-scale algal biofuel production is widely deployed and to explore the opportunities for mitigating the concerns. However, the ultimate productivity of algal biofuels, some of their resource use and environmental concerns, and some strategies for mitigating the concerns might affect the economic viability of algal biofuels. This report makes reference to economics if there are synergies or trade-offs among economics, productivity, resource use, and environmental effects. This report also discusses tools for assessing the multi-attribute nature of sustainability of algal biofuels.
POTENTIAL SUSTAINABILITY CONCERNS
Assessing the sustainability of an algal biofuel requires an understanding of the individual components that make up an algal biofuel production system. An algal biofuel production system involves cultivating selected strain(s) of algae; collecting the biomass and dewatering it, if necessary; and processing the algal lipid, biomass, or secreted products into fuels and possibly other co-products. The production of fuels and energy from algae is not an established industry and a variety of production systems have been proposed. Figure S-1 is a simplified diagram that attempts to limit and group the potential steps in the algal biofuel production pathway. Each row of the diagram details a processing step or process option. Different combinations of cultivation and processing options have resulted in more than 60 different proposed pathways for producing algal biofuels.
Based on a review of literature published until the authoring of this report, the committee concluded that the scale-up of algal biofuel production sufficient to meet at least 5 percent of U.S. demand for transportation fuels4 would place unsustainable demands on energy, water, and nutrients with current technologies and knowledge. However, the potential to shift this dynamic through improvements in biological and engineering variables exists.
For some system designs analyzed, the energy outputs of algal biofuels (and co-products if they are produced) are less than the energy inputs for producing the fuel. Estimated values for energy return on investment range from 0.13 to 3.33. The estimated
3 Some algae can grow heterotrophically in the absence of light by taking up organic molecules (such as glucose) as a source of carbon.
4 U.S. consumption of fuels for transportation was about 784 billion liters in 2010. Five percent of the annual U.S. consumption of transportation fuels, which would be about 39 billion liters, is mentioned to provide a quantitative illustration of the water and nutrients required to produce algal biofuels to meet a small portion of the U.S. fuel demand.
FIGURE S-1 Pathways for cultivating and processing algae to fuels and their products. Heterotrophic routes are outside the scope of this analysis.
consumptive use of fresh water for producing 1 liter of gasoline equivalent of algal biofuel is 3.15 to 3,650 liters, depending on whether the algae or cyanobacteria need to be harvested to be processed to fuels or if they secrete fuel products; whether fresh water, inland saline water, marine water, or wastewater is used as a culture medium; the climatic condition of the region if open ponds are used; and whether the harvest water from algae cultivation is recycled. In other words, at least 123 billion liters of water would be needed to produce 39 billion liters of algal biofuels or an equivalent of 5 percent of U.S. demand for transportation fuels. The estimated requirement for nitrogen and phosphorus needed to produce that amount of algal biofuels ranges from 6 million to 15 million metric tons of nitrogen and from 1 million to 2 million metric tons of phosphorus if the nutrients are not recycled or included and used in coproducts. Those estimated requirements represent 44 to 107 percent of the total nitrogen use and 20 to 51 percent of total phosphorus use in the United States.
Sustainable development of algal biofuels would require research, development, and demonstration of the following:
• Algal strain selection and improvement to enhance desired characteristics and biofuel productivity.
• An EROI that is comparable to other transportation fuels, or at least improving and approaching the EROIs of other transportation fuels.
• The use of wastewater for cultivating algae for fuels or the recycling of harvest water, particularly if freshwater algae are used.
• Recycling of nutrients in algal biofuel pathways that require harvesting unless coproducts that meet an equivalent nutrient need are produced.
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
Potential Sustainability Concerns for Large-Scale Development of Algal Biofuels
This report identifies the following resource use and environmental effects as potential sustainability concerns for large-scale development of algal biofuels. The concerns of high importance are the ones that have to be addressed, if they are not being addressed already, for a sustainable development of algal biofuels. The concerns of medium importance generally reflect the ones that require some assessment or monitoring to ensure that they do not present serious sustainability concerns. The concerns of low importance are ones that are likely avoidable with proper management and good engineering designs.
Concerns of High Importance
• The quantity of water (whether fresh water or saline water) required for algae cultivation and the quantity of freshwater addition and water purge to maintain the appropriate water chemistry. Maintenance of water level and quality in open-pond systems or evaporative loss of cooling water if it is used to maintain temperature in photobioreactors could be a concern because of the potential for high net evaporative losses, particularly in arid regions where solar resources are most suitable for cultivation.
• Supply of the key nutrients for algal growth—nitrogen, phosphorus, and CO2. Nutrient sources can include virgin sources and waste streams such as flue gas. Preparation and transport of these waste streams for reuse, nutrient recycling, production of coproducts, and fossil inputs required to produce necessary nutrients all affect the energy return and GHG emissions.
• Appropriate land area with suitable climate and slope, near water and nutrient sources (for example, a stationary source of CO2 such as a coal-fired power plant or a wastewater source such as municipality, industry, or agriculture).
• Energy return on investment. Algal biofuel production would have to produce sufficiently more energy than is required in cultivation and fuel conversion to be sustainable.
• GHG emissions over the life cycle of algal biofuels. Algal biofuel production would have to produce a GHG benefit relative to other fuel options such as fossil fuels. Yet, estimates of life-cycle GHG emissions of algal biofuels span a wide range, and depend on many factors including the source of CO2 and the disposition of coproducts.
Concerns of Medium Importance
• 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 coproducts.
• 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 (ILUC) 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 algal 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 engineered 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
• 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.
• Potential presence of mosquitoes and mosquito-borne diseases around poorly managed open ponds.
source. If algal biofuels are to contribute a significant amount of fuels for transportation, the following are needed:
• Improvements in the algal strains used.
• Testing additional strains for desired characteristics.
• Advancements in the materials and methods used for algae cultivation and for processing algal biomass into fuels.
• Reductions in the energy requirements for cultivation, algae collection, and pro-cessing to fuels.
Algal strain development is needed to enhance traits that contribute to increasing fuel production per unit resource use, reducing the environmental effects per unit fuel produced, and enhancing economic viability. Improvements in biomass or product (lipid, alcohol, or hydrocarbons) yield, culture density, nutrient uptake, ease of harvest, and photosynthetic efficiency are some of the improvements that would improve sustainability of algal biofuels.
The strains used for large-scale algal biofuel production are being improved through selection and genetic approaches. Breakthroughs and innovations in areas such as increasing the capability of algae to use nutrients efficiently or engineering designs to reduce processing requirements have the potential to greatly improve the energy balance and enhance the overall sustainability of algal biofuels.
Engineering solutions to enhance algae cultivation, to facilitate biomass or product collection, and to improve processing of algae-derived fuels can increase the EROI and reduce the GHG emissions of algal biofuel production.
Lipid collection and conversion have dominated algal biofuel development for several decades. Processing improvements to reduce energy requirements and increase productivity continue to be proposed. Whole-cell processing of algae into fuels also has been investigated. Innovations focused on reducing energy use, nutrient requirements, water use, and land use are necessary for the sustainable development of algal biofuels. These innovations may require algal strain improvements, engineering solutions to improve hardware required for fuel production, and the interplay of the two.
A FRAMEWORK TO ASSESS SUSTAINABLE DEVELOPMENT
Given the multiple resource requirements and potential environmental effects, specific sustainability concerns cannot be viewed in isolation from others. Any one life-cycle assessment (LCA) for a single resource use or environmental effect is insufficient to determine the overall sustainability of an algal biofuel production system. Challenges arise regarding how to assess the overall environmental sustainability of algal biofuels holistically and how to balance the environmental objectives against the economic and social objectives of sustainable development. An overall and comparative assessment of sustainability is complicated by the fact that some sustainability objectives can be estimated on the basis of mass balance or engineering principles and compared across systems—for example, nutrient budgets, energy balances, and GHG emissions—while others are region specific. Other sustainability objectives are specific to region and maybe species, and the environmental effects in one region might not be directly comparable to another—for example, land-use change (LUC) and biodiversity.
The committee proposes a stepwise framework (Figure S-2) to aid DOE in its decisionmaking process that would help ensure sustainable development of algal biofuels. The framework uses a variety of tools for assessing overall sustainability including LCAs that integrate a particular aspect of sustainability through the supply chain, cumulative impact
FIGURE S-2 A potential framework for assessing sustainability of algal biofuels during different stages of development.
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
will be possible only if thorough assessments of each step in the various pathways for algal biofuel production are conducted.
Given the four aspects of sustainability and the multiple goals within each aspect, a participatory approach is necessary to develop a collective vision of the importance of various sustainability objectives relative to each other. An approach that involves different stakeholders (for example, algal biofuel producers, fuel consumers, environmental groups, and residents near areas to be developed for algae cultivation or biofuel refinery) from the beginning of a sustainability assessment would help ensure that trade-offs among sustainability goals would be acceptable to the various parties.
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.