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Water Implications of Biofuels Production in the United States 1 About Biomass, Biofuels, and Water In the United States as of 2006, about 85 percent of the total energy consumed and about 97 percent of the energy for transportation came from fossil fuels such as oil, natural gas, and coal (http://www.eia.doe.gov). Fossil fuels are nonrenewable, and almost all estimates of domestic oil production indicate that the country has already used more than remains untapped. The United States imports well over 60 percent of the oil it consumes, and this percentage has been increasing. Because of a strong national interest in greater energy independence, biofuels—fuels derived from biological materials, or biomass—have become important liquid fuels for transportation and are likely to remain so for the foreseeable future. Potential sources of biomass are plentiful. They include field crops such as soy and corn; short-rotation woody crops such as poplar and willow; animal fats, vegetable oils, and recycled greases; perennial grasses such as switchgrass; agricultural and forestry residues such as manure and cellulosic waste; aquatic products such as algae and seaweed; and municipal waste streams such as sewage sludge or solid waste. Table 1-1 shows U.S. production of biofuels in 2006. In the United States, ethanol is derived mainly from corn kernels and biodiesel is derived mainly from soybeans, although other crops can serve to produce these biofuels. Approximately 4.9 billion gallons of ethanol were produced in the United States, which represents 3.6 percent of annual gasoline demand on a volume basis and 2.4 percent on an energy basis (U.S. CRS, 2007). Ethanol is blended in gasoline at levels of up to 10 percent for use in conventional vehicles and, less commonly, as high as 85 percent for use in “flexible fuel vehicles.” Because biofuels recycle carbon (by removing carbon dioxide from the atmosphere during photosynthesis and storing the carbon in plant structures) rather than release stored subsurface carbon as fossil fuels do, they also have the potential to produce lower net greenhouse gas emissions.
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Water Implications of Biofuels Production in the United States TABLE 1-1 U.S. Production of Biofuels from Various Feedstocks in 2006 Fuel Feedstock U.S. Production in 2006 Ethanol Corn 4.9 billion gallons Sorghum Less than 100 million gallons Cane sugar No production (600 million gallons imported from Brazil and Caribbean countries) Cellulose No production (one demonstration plant in Canada) Biodiesel Soybean oil Approximately 90 million gallons Other vegetable oils Less than 10 million gallons Recycled grease Less than 10 million gallons Cellulose No production SOURCE: U.S. CRS (2007). The new technology on the horizon is the production of “cellulosic ethanol” from the fibrous material from a variety of plants such as corn stalks and wheat straw, native grasses, and forest trimmings. Cellulosic ethanol production currently exists only at pilot and commercial demonstration-scales, because the technologies for breaking down the fibers into fuel on a commercial scale are still being developed and may be five or more years in the future. A 2005 joint study of the U.S. Department of Energy and the U.S. Department of Agriculture concludes that the United States could produce 60 billion gallons of ethanol by 2030 through a combination of grain and cellulosic feedstocks, enough to replace 30% of projected U.S. gasoline demand (USDA/DOE, 2005). WATER AND BIOFUEL CROPS Biofuels production will alter both what types of crops are grown and where they are grown and may increase overall agricultural production. The effects of these changes in the agricultural mix of crops on water are complex, difficult to monitor, and will vary greatly by region. In general, crops that require less irrigation, less fertilizer and pesticides, and provide
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Water Implications of Biofuels Production in the United States better year-round erosion protection will likely produce fewer negative water impacts. Understanding water quantity impacts is dependent on understanding the agricultural water cycle depicted in Figure 1-1. Crops can be either rainfed or irrigated (see Figure 1-2). Irrigation water can come from groundwater or surface water, and groundwater can be withdrawn from either a surficial aquifer (connected directly to the surface) or a confined aquifer (overlain by a low permeability layer, or aquitard, such as clay). Some of FIGURE 1-1 The agricultural water cycle. Inputs to a crop include rainfall and irrigation from surface water and groundwater. Some water is “consumed” (that is, incorporated in the crop or evapotranspired), some returns to surface waterbodies for human or ecological use downstream, and some infiltrates into the ground. Copyright by the International Mapping Associates.
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Water Implications of Biofuels Production in the United States FIGURE 1-2 Irrigated land in the United States. Note that most of this is located in the more arid regions of the country. SOURCE: N.Gollehon, USDA ERS, written commun., July 12, 2007. Based on data from U.S. Department of Agriculture (USDA) Economic Research Service (ERS) Census of Agriculture. the applied water is incorporated into the crop, but most of it leaves the fields as (1) evaporation from the soil and transpiration from plants (called evapotranspiration or ET), (2) runoff to rivers and streams (sometimes called “return flow”), and (3) infiltration to the surficial aquifer. The water that is incorporated into the crops or lost to evapotranspiration is referred to as “consumptive use,” because it cannot be reused for another purpose in the immediate vicinity. Rates of ET vary greatly by the type of crop. During a growing season, a leaf will transpire many times more water than its own weight. An acre of corn gives off about 3,000–4,000 gallons of water each day while a large oak tree can transpire 40,000 gallons per year (USGS, 2007). Grasses that might be in cellulosic production have a slightly higher ET rate than corn, but a considerably lower ET rate than trees.
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Water Implications of Biofuels Production in the United States For most crops, it is standard agricultural practice to apply fertilizers such as nitrogen (N) and phosphorus (P), as well as herbicides, fungicides, insecticides, and other pesticides. Nitrogen in forms such as nitrate (NO3) is highly soluble, and along with some pesticides infiltrates downwards toward the water table. Surface runoff and infiltration to groundwater both have significant impacts on water quality. Nutrient pollution causes excess algae to grow, decompose, and consume the oxygen in water, creating areas where fish cannot survive such as “dead zones” in the Gulf of Mexico and the Chesapeake Bay. The amount of fertilizers applied varies greatly with the type of crop. However, there are many management practices that can improve the efficiency of fertilizer application and how they are used by plants. Water quality is also impaired by sediments that result from soil erosion associated with agriculture. It has been estimated that cropland erosion accounts for about half of the sediment that reaches the nation’s waterways each year (USDA, 1993). Sediments impair water quality and also carry pollutants including excess nutrients and pesticides. The amount of sediment eroding from agricultural areas is directly related to land use—the more intensive the use, the greater the erosion. For example, more sediment erodes from row crop fields such as corn than from pastures or woodlands. Surface cover is crucial in reducing sediment in runoff and limiting soil erosion. Farmers can employ a number of conservation tillage techniques that leave some portion of crop residues on the soil surface. In “no-till” systems, as the name implies, crops are simply planted into the previous year’s crop residues. An additional consideration for corn is that its residues called corn stover—the stalks and cobs left in the field after the grain has been harvested—can be converted into biofuels. However, leaving the corn stover on the fields can greatly reduce soil erosion. WATER AND BIOREFINERIES Ethanol is made by converting the starch in corn to sugars and then converting those sugars into ethanol, similar to the process used in a brewery. As with other industrial processes, biorefineries use water in the conversion processes and to heat things up and cool things down. To produce ethanol, feedstock such as corn, wheat, barley, or other grain is ground to the consistency of coarse flour, mixed with water and enzymes, and cooked at high temperature to break down the starch polymers into glucose (sugar) molecules. The liquefied mash and yeast are put into tanks where the sugar is fermented into ethanol and carbon dioxide.
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Water Implications of Biofuels Production in the United States The fermented mash, called beer, is put in a distillation system that separates the ethanol from the water and leftover solids, called stillage, which are processed and sold for use in other industries. A dehydration system removes the remaining water, leaving nearly pure ethanol. Consumptive use of water includes steam lost through the cooling towers and water evaporated in drying the stillage. Converting cellulose to ethanol involves breaking the long chains of cellulose molecules into glucose and other sugars, and fermenting those sugars into ethanol. In nature, these processes are performed by a variety of organisms, such as the bacteria in the stomachs of cows, which use enzymes (cellulases) to break down cellulose into sugars. Other microbes, primarily yeasts, then ferment the sugars into alcohol. The first step in this process is not yet possible on a commercial scale. Ethanol distilling plants have various waste streams. First, salts build up in cooling towers and boilers due to evaporation and scaling, and must be periodically discharged (“blowdown”). Second, the technologies used to make the pure water needed for various parts of the process result in a brine effluent. Under the National Pollutant Discharge Elimination System (NPDES), permits are required from the states to discharge this effluent. PROJECTED FUTURE GROWTH OF ETHANOL PRODUCTION Recent increases in oil prices, which reflect a narrowing gap between oil supply capacity and oil demand, combined with subsidy policies have led to a dramatic expansion in corn ethanol production and high interest in further expansion over the next decade. Expansion of ethanol production to meet President Bush’s call for 35 billion gallons annually by 2017 will drive increased corn production until marketable future alternatives are developed. Even with the addition of cellulosic crops, corn will likely comprise a significant portion of biofuel crops. Figure 1-3 illustrates one possible scenario of crop production based on ethanol from cellulose becoming commercially available by 2015. The assumption is that agricultural commodity programs remain as of 2006, the current cropland base stays within 434 million acres, and yield increases in food and feed crops is sufficient to meet domestic demand, but there is a decline in U.S. exports of such crops. Figure 1-4 shows the projected geography of production of cellulosic material in dry tons by the year 2030. It illustrates that although the types of crops may change, they will be mainly in areas that are already agriculture intensive. The trend in water use may show a decline, depending on whether
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Water Implications of Biofuels Production in the United States FIGURE 1-3 Projection of ethanol production by feedstock assuming cellulose-to-ethanol production begins in 2015. Dedicated energy crops refer to those grown solely for energy production. SOURCE: Reprinted, with permission, from D.Ugarte, University of Tennessee, written commun., July 12, 2007. the biomass crops use more or less water than those that were replaced (see Chapter 2). The water quality impacts will depend on the character of the land utilized and the extent to which the crops require nutrients and pesticides. A perennial crop of cellulosic biomass such as switchgrass would hold soil and nutrients in place and require lower fertilizer and pesticide inputs, thus reducing water quality impacts. There are, however, large uncertainties surrounding the production of cellulosic ethanol. The expected cellulosic crops have very little, if any, history of use in large-scale cultivation. Therefore, even basic information such as water or nitrogen inputs needed, herbicide use, impact on soil erosion, and even overall yields is preliminary.
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Water Implications of Biofuels Production in the United States FIGURE 1-4 Distribution of the production of cellulosic materials in dry tons by the year 2030. SOURCE: Reprinted, with permission, from D.Ugarte, University of Tennessee, written commun., July 12, 2007.
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Water Implications of Biofuels Production in the United States REFERENCES U.S. CRS (U.S. Congressional Research Service). 2007. Ethanol and Biofuels: Agriculture, Infrastructure, and Market Constraints Related to Expanded Production. (RL 33928; March 16, 2007). Report by Brent D.Yacobucci and Randy Schnepf. Text in GalleryWatch CRS Reports. Accessed August 17, 2007. USDA (U.S. Department of Agriculture) Soil Conservation Service. 1993. Water Quality Indicators Guide: Surface Waters. Report by Charles Terrell (National Water Quality Specialist, Ecological Sciences Division, Soil Conservation Service) and Patricia Bytnar Perfetti (Department of Geoscience and Environmental Sciences, University of Tennessee, Chattanooga). Washington, D.C.: USDA. USDOE (U.S. Department of Energy) and USDA. 2005. Biomass as Feedstock for a Bioenergy and Bioproducts Industry. Oak Ridge, TN: USDOE. USGS (U.S. Geological Survey). 2007. The Water Cycle: Evapotranspiration. Available online at http://ga.water.usgs.gov/edu/watercycleevapotranspiration.html.
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Water Implications of Biofuels Production in the United States Courtesy of the Natural Resources Conservation Service Center, U.S. Department of Agriculture