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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.