Course: BFM 101 Biofuels Fundamentals

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The Water Cycle Continued

Water Demands and Biofuel Production

Biomass Crop Production – The demand for water is increasing in many regions. Water consumption for human use and food production may conflict with water needs for bioenergy crops. Increased demand for fresh water may affect ecosystem hydrology, resulting in groundwater depletion, wetland degradation, decreased stream flows, lake level decline and/or water source fragmentation. Wise allocation of this limited resource is critical.

Rainfall meets most of the water demand for crop production (80 percent). The remaining 20 percent comes from irrigation with surface and groundwater. Water quantity demands for growing annual bioenergy crops are similar to those of other farm crops. However, the competition for water resources is a concern, especially where water is scarce. The most obvious water quantity issues are in areas where irrigation is required for crop production. Changes in evapotranspiration with changes in land use can also affect ecosystems. Different crop types have different evapotranspiration rates. Crop changes leading to resource changes may be difficult to assess and be unexpected and irreversible [11].

Agricultural regions need to be considered when thinking about selecting a crop and considering the crop water demands. Water use by corn, for instance, is greater than that of soybeans or cotton in the Pacific and Mountain regions, less than those crops in the Northern and Southern Plains regions, and about the same in the North Central and Eastern regions [12].

In most of the Corn Belt, corn for ethanol is grown under rain-fed conditions, but in the Great Plains a reliance on irrigation to grow corn makes the water use for biofuel production much greater. Regional differences are significant.

Native perennial crops used for cellulosic biomass (such as switchgrass) require the vegetation to be removed, but stubble and roots are left intact, which helps hold soil in place and reduce the amount of erosion. Currently, perennial biomass crops are expected to require less fertilization and pesticide applications than most row crops, reducing the chemical pollutant threats on the water source. Greater desired yields from cellulosic crops may require added nutrients, monocropping and the use of more pesticides. In theory, native perennial crops should create a positive effect on water quality, but little research has been done on large-scale operations, so conclusions with reference to inputs and impacts are preliminary.

A wide variety of crops, both annual and perennial, have been used or proposed for biofuels, ranging from woody (willow and poplar trees) to herbaceous (Miscanthus, switchgrass, corn, sorghum, sugar cane, alfalfa, soybeans, sunflowers and crucifers [mustard, rape, oil seed radish]) [13]. Table 1.3 shows the ethanol production, biomass and land area needs for several bioenergy crops. Miscanthus outperforms corn and switchgrass, as shown in this Midwest data [14].

Table 1.3.  Comparison of biomass production, potential ethanol production and land area needed for different potential bioenergy systems to reach the 35 billion gallon U.S. renewable fuel goal.

Side-by-side comparisons of switchgrass and Miscanthus indicated that Miscanthus has double the yield of switchgrass. Leaf level photosynthesis was 33 percent greater in Miscanthus than switchgrass. However, transpiration was greater in the Miscanthus, which averaged 25 percent greater stomatal conductance (resulting in greater evapotranspiration [15]. Consequently, it appears greater biomass production comes at a water cost.

Biofuel Production - The current water requirements for biofuel production are 70–400 times greater than that used to produce other types of energy, such as fossil fuels and wind and solar energy [16]. Biofuel demands on water resources are modest when compared to feed crops. Water quality impacts from biomass production are similar to that of conventional food crop production. Processing biomass to biofuels has potentially negative implications in the form of chemical and thermal pollution via effluent release into water resources. Also, the quantity of water needed to refine biomass into biofuels may cause high impacts to local aquifers. The water a processing facility uses to produce 100 million gallons of ethanol could otherwise supply drinking water to a community of 5,000 [17].

The ethanol distillation process, if not well managed, could be a source of surface water contamination. Salt residue is produced in the cooling towers and is blown out in a discharge process. These salts, if added to the waste stream, may end up as pollution in fresh waters. Also, water loss during distillation may be significant as water is released through steam in the cooling process and through evaporation during dehydration [18].

When wastewater from processing plants is used for irrigation, it can also cause problems. The biological activity, also known as Biochemical Oxygen Demand (BOD), associated with disposal of untreated or minimally treated wastewater with a high organic content creates anaerobic conditions in the soil. This causes a release of the naturally occurring iron, manganese and arsenic that might be adsorbed onto soil particles that are then altered into a form that is soluble in groundwater. Levels above the aesthetic and drinking water standards may then be found in the groundwater [19].

Resources by eXtension, Bioenergy & Sustainability, BIOEN3 CC BY-NC-SA 2.5
www.extension.org/pages/63365/bioenergy-curriculum

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