Course: BFM 101 Biofuels Fundamentals

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Water Quantity and Quality

Bioenergy production affects water availability and quality  through two main pathways: (1) water use during feedstock conversion, and (2) water use in biomass production.. Emission of air pollutants from biopower combustion and burning of biofuels also potentially impacts water quality mostly via precipitation.  Many bioenergy feedstocks have relatively high water requirements when grown at commercially viable levels.  In areas where there is sufficient precipitation, yields may be close to yield potential (in the case of crops).  However, in areas lacking sufficient precipitation, irrigation is required for achieving commercially viable yields.  Use of irrigation water has the potential to: (1) depress groundwater supplies, (2) divert water used to grow food crops to the growth of energy crops), and (3) contribute to soil salinization. In both irrigated and rainfed systems, crops that remain longer on the landscape (i.e., perennial) and crops that provide good ground cover increase structural filtering of precipitation promoting more rainfall interception and retention and better control of sediment run-off.  Grasslands, once established, may provide better water services than traditional annual row crops because of greater soil coverage and root density.

In addition to sediment run-off, pesticides and fertilizers associated with conventional row crop production threaten water resources.  One of the top pollutants to aquatic ecosystems, as recognized by the U.S. EPA, is nitrogen.  Nitrogen in excess can be harmful to both ecosystem and human health.  Nitrogen (N) and other agronomic pollutants like phosphorous (P), can enter water resources through runoff and leaching.  Nutrient pollution is a leading cause of water quality impairment in lakes and the Gulf of Mexico.  The major concern for surface water polluted with N and P is the promotion of algal growth (eutrification) accompanied by aquatic oxygen depletion, fish mortality, clogged pipelines, and reduced recreational values.

Water use in conversion of feedstock to bioenergy also has potential negative consequences for water availability and quality.  Biopower generation requires use of water to produce high-pressure steam to drive turbines and low-pressure steam to deliver heat/cooling in centralized heating/cooling districts.  Some biopower plants also use water for cooling, particularly in co-firing of coal or natural gas with biomass.  Water is also required during fermentation of ethanol and in post-transesterification of biodiesel. Note, however, that water use for ethanol production is lower than that of many other industrial processes (figure 3.8).  Effluents from these industrial processes must be treated before being discharged to nearby surface waters.  Water use for fermentation may deplete surrounding surface or groundwater supplies and limit their availability for other uses including drinking water, wildlife habitat, and recreation.

Figure 3.8. A comparison of water use in production of ethanol versus other industrial processes. (U.S. EPA.)

Resources provided by the eXtension Farm Energy Community of Practice at http://www.extension.org/ag+energy

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