Impact on Water Supplies
Increased cultivation of crops to produce ethanol could harm water quality and leave some regions of the country with water shortages is reported by a panel of experts. And corn, the most widely grown fuel crop in the United States, might cause more damage per unit of energy than other plants, especially switchgrass and native grasses as said by the panal. The panel, convened by the National Research Council, said improved agricultural practices, water recycling and other steps might reduce possible problems. But it added that 'fundamental knowledge gaps' made it difficult to predict what would happen as the nation's embrace of biofuel crops expanded. Meanwhile, it said, it would be 'prudent' to encourage the use of ethanol sources other than corn.
But increased production could greatly increase pressure on water supplies for drinking, industry, hydropower, fish habitat and recreation, the report said. Facilities that turn plants like corn into liquid fuel would add pressure on water supplies, though these bio-refineries are relatively modest water consumers compared with agriculture, the panel said.
The report noted that additional use of fertilizers and pesticides could pollute water supplies and contribute to the overgrowth of aquatic plant life that produces 'dead zones'like those in the Chesapeake Bay, the Gulf of Mexico and elsewhere.
Increased use of biofuels puts increasing pressure on water resources in at least two ways: water use for the irrigation of crops used as feedstock for biodiesel production; and water use in the production of biofuels in refineries, mostly for boiling and cooling.
Irrigation of feedstocks
The amount of water needed to grow feedstock depends on the amount of rainfall. In areas with sufficient rainfall feedstocks can be grown without any irrigation, such as corn or sugarcane in areas with sufficient rainfall and Jatropha, which is only cultivated under rainfed conditions.
However, in many parts of the world supplemental or full irrigation is needed to grow feedstocks. For example, if in the production of corn half the water needs of crops are met through irrigation and the other half through rainfall, about 860 liters of water are needed to produce one liter of ethanol.This is 215 times more than the amount of water per liter of ethanol used in the refining process.
Water Use in Refineries
Using the example of ethanol, a typical ethanol factory producing 50m gallons of biofuels a year needs about 500 gallons of water a minute, corresponding to about 4 gallons of water per gallon of biofuel. Ethanol plants now use about half as much water per gallon of ethanol as they did a decade ago.
In the United States, the number of ethanol factories has almost tripled from 50 in 2000 to about 140 in 2008. A further 60 or so are under construction, and many more are planned. Projects are being challenged by residents at courts in Missouri, Iowa, Nebraska, Kansas, central Illinois and Minnesota. The Energy Independence and Security Act of 2007 aims at increasing biofuels production in the U.S. to 36 billion gallons per year by 2022. Using the ratio of 4 gallons of water per gallon of ethanol mentioned above this would require about 144 billion gallons of water per year for refining, corresponding to 4 percent of current depletion of the Ogallala aquifer, the largest aquifer in the United States. While water use for refining of biofuel can have significant local impacts, its overall impact on the water balance of a larger region thus seems to be relatively limited.