is through improvements in cooling tower and boiler feed operations. Some of these efficiencies may be applicable to corn ethanol plants as well.
Biodiesel, which in the United States is produced primarily from soybeans, comprises several percent of the nation’s total biofuel production. Methanol and caustic (sodium hydroxide) are used in the production of biodiesel. Glycerin is a major co-product that has a low market value currently, in large part due to biodiesel production. Because of this, it is sometimes viewed as a major waste product, but greater commercial uses for glycerol could make biodiesel production more profitable. Biodiesel itself burns much cleaner than petrochemical diesel and enjoys considerable advantage in terms of lower air pollution.
Biodiesel refining requires much less water per unit of energy produced than bioethanol. Overall, consumptive use is about 1 gallon of fresh water per gallon of biodiesel and overall water use may be up to 3 gal/gal (Pate et al., 2007). Still lower usage may be possible in the future with new technologies, which include the possibility of using recycled waste water with various degrees of treatment.
Water withdrawals by biofuel production plants are similar to those of many other industries. They should be considered in the context of the total water cycle for the watershed or aquifer unit that is being utilized. Thus, biofuel plants can present local (or regional) problems depending upon where they are located. Even within the same state, the conditions can vary greatly; for example, aquifers in the northeastern part of lowa tend to be quite productive, whereas those in the south have a much more limited yield.
Siting of some ethanol plants is occurring where the water resource is already under duress. Figure 5-3 shows, for example, that many bioethanol plants that each require 0.1–1.0 million gallons per day are located on the High Plains aquifer. This aquifer is currently being pumped at a rate of more than 1.5 billion gallons per day for agriculture, municipalities, industry, and private citizens. Thus, 15 million gallons per day for bioethanol would represent only 1 percent of total withdrawals. But it is an incremental withdrawal from an already unsustainable resource. Current water withdrawals are much greater than the aquifer’s recharge rate (about 0.02 to 0.05 foot per year in