Environmental Engineering Reference
In-Depth Information
Table 28.5 shows that reining of ethanol from corn grain requires 4-6 gal of fresh-
water per gallon of ethanol. The requirement for irrigation per gallon increases water
demand to nearly 1000 gal; the requirement for biodiesel from soy uses approximately
650 gal for cultivating the crops. By contrast, the water footprint of cellulosic ethanol
is lower, ranging from 2 to 6 gal of water consumed per gallon of fuel for reining.
Demands on water by liquefaction technologies are also challenging, including bio-
mass to liquid, which consumes 2-6 gal of water for reining requirements per gallon
of liquid fuel produced.
28.3.2 Shale Oil/Gas Mining and Extraction
Recent estimates by the USEIA [11] have shown the widespread availability of shale oil and
gas resources across the United States (Figure 28.9a [26]), providing a broad resource for
domestic hydrocarbon-based sources of energy. Advances in technologies such as hydrau-
lic fracturing (HF) will enable enough domestic production that will likely provide a path-
way to energy independence from foreign energy sources along with signiicant domestic
economic growth. In fact, projections by the USEIA [11] show that shale gas will com-
prise >20% of the total US gas supply by 2020. Natural gas and oil are extracted from low-
permeability coal beds and methane shale formations by coupling improved horizontal
drilling methods with HF technologies. In this process, a starter hole is drilled to a known
vertical depth, and then turned to drill horizontally through a target rock bed formation.
The fracturing process consists of creating issures or thin cracks with a detonation usu-
ally initiated by an electric discharge, followed by the injection of a complex mixture of
water-based luid under high pressure. The process of HF is also commonly referred to as
“fracking.” The cracked rock bed is an extended network of issures that provide pathways
for oil and gas to escape for modern societal energy use; Figure 28.9b [27] shows a sche-
matic for the typical process as described here.
While the need for increased domestic US sources of energy and the implementation of
available technologies is not in question, the greatest challenges lie in developing these
energy sources with minimal health risks and possible ecosystem impacts associated with
these extraction methods, in a resource-neutral or sustainable way. Horizontal HF tech-
niques require large quantities of water (~3-5 million gallons per well during the drill-
ing phase), sand, and chemicals for each job, and produce signiicant volumes of liquid
and solid wastes that require treatment or disposal. In fact, 80%-99% of the HF luid can
be water with the remaining chemicals falling under the category of “proppants” [28,29].
The water-based slurry that is injected for HF is commonly referred to as a “frac luid,”
the typical components of which are shown in Figure 28.10a. Furthermore, as there is no
standard frac luid composition, a variety of mixtures are used with varying water content
due to the assorted drilling needs driven by the geology of the rock formations; however,
nearly 20%-40% of the frac luid returns and needs to be treated as lowback water; a
sample composition of this lowback water is shown in Figure 28.10b. As with any large-
scale industrial activity, drilling and HF also necessitate deployment of heavy equipment
and new pipelines directly affecting local communities, local surface and groundwater
supplies, and lands. It is therefore essential to develop these domestic energy sources in a
responsible and sustainable manner with minimal impact on the ecosystem and ideally in
a resource-neutral manner.
After fracking is completed, the fracking luid is either disposed of in an underground
injection well or treated, generally at a wastewater treatment plant or commercially.
However, for wastewater treatment plants, the high TDS as well as heavy metal content
