lion gallons per day) that would be needed by the plant. By comparison, per capita water use from public water supply nationally is about 180 gallons per person per day (Table 5, Hutson et al., 2004), so this is the equivalent to the water supply for a town of about 5,000 people.
Compared to the water incorporated in the feedstock, water use for the biorefineries is quite small. For example, in neighboring Nebraska about 2,100 gallons of irrigation water were applied per bushel of corn in 2003 (Noel Gollehon, U.S. Department of Agriculture Economic Research Service, personal commun., July 12, 2007). Assuming the common figure of about 2.7 gallons of ethanol from one bushel of corn, 2,100 gallons of water/bushel ×1 bushel/2.7 gallons of ethanol=about 780 gallons of water per gallon of ethanol. This is about 200 times larger than the approximately 4 gal/gal given above for a corn ethanol biorefinery. This indicates that biorefineries themselves generate local, but often intense, water supply challenges, while irrigated agriculture can generate regional-scale problems. If, however, the agriculture is rainfed, water for the biorefinery may be the primary source of groundwater or surface water extraction in the area.
Ethanol 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 (e.g., reverse osmosis [RO], ion exchange, iron removal; not shown in Figure 5-1) result in a brine effluent. Under the National Pollutant Discharge Elimination System (NPDES) permits are required from the states to discharge this effluent. These permits often cover total dissolved solids (TDS), acidity, iron, residual chlorine, and total suspended solids. Table 5-1 gives chemical characteristics of waste water from the RO operation and from the cooling tower blowdown for two plants in lowa. Some violations of NPDES permits have been reported in lowa and Minnesota from ethanol facilities, primarily for TDS.
Wastewater, potentially high in biochemical oxygen demand (BOD, the oxygen used when organic matter is decomposed by microbes), emanates from the processing of by-products such as thin stillage, wet distillers’ grains, and dry distillers’ grains with solubles (DDGS). Discharge of high-BOD water to rivers and lakes is problematical because decomposition can consume all of the dissolved oxygen, suffocating aquatic animals. DDGS is a valuable by-product that is rich in protein and especially good feed for animals such