# Sustainable Engineering: An Introduction to Sustainability

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## 1.4. Materials Use

Just as energy use is a required basis of economic activity, so is materials use; and just as energy use has increased as population and affluence have increased, so has materials use. This section will examine materials use in the United States and the world, without focusing on specific materials. The exception will be water. The amount of water used is so extensive, it would dominate any examination of materials use if it were not addressed separately.

### 1.4.1. Minerals, Metals, and Organics

Figures 1-5 and 1-6 show trends in world and U.S. materials use, over multiple decades.

#### Example 1-3. Per capita materials use

Using the data in Figure 1-6, compare per capita annual materials use in the United States and worldwide. Assume a U.S. population of 300 million and a world population of 7 billion. How much does the result change if the mass of fuel used is added? As a rough approximation of the mass of fuel used, use the per capita use of gasoline equivalents from Example 1-1 (e.g., 2700 gallons of gasoline equivalent per year for the United States) multiplied by the density of gasoline (about 6 lb/gal).

Solution: Per capita materials consumption in the United States is more than six times greater than the worldwide average.

U.S. per capita annual materials use = 6 * 1012 lb/300 * 106 people = 20,000 lb/person per year.

Global per capita annual energy use = 21 * 1012 lb/7 * 109 people = 3000 lb/person per year.

An estimate of the mass of fuel used can be made by multiplying per capita use of gasoline equivalents by the density of gasoline.

U.S. per capita annual materials use = 20,000 lb/person = 2700 gal * 6 lb/gal = 36,000 lb/person per year.

Global per capita annual materials use = 3000 lb/person = 520 gal * 6 lb/gal = 6100 lb/person per year.

If we express the mass of fuel use as the amount of carbon dioxide released by burning the fuels, accounting for oxygen use, rather than as the amount of fuel consumed, the mass attributed to fuel use more than triples.

The materials accounted for in Figures 1-5 and 1-6 do not include materials used as fuels. Including fuels doubles the materials use estimates. The data in Figures 1-5 and 1-6 also do not include material flows that do not enter the economy. For example, mining for materials often involves removing the ground above the ore seam, referred to as overburden. Agricultural operations involve the loss of soil into waterways as runoff. Collectively, these indirect flows are roughly as large as the total of all flows that enter the economies of highly developed nations (for more details, see Adriaanse et al., 1997). Considering all of these flows, a reasonable estimate of the materials used in industrialized countries is more than 100 pounds per person per day, not including water.

Many of these materials are limited natural resources. Consider metals as an example. If the total amount of materials available as ores, recoverable using current technologies, is divided by annual global consumption rates, only iron and aluminum, of the industrial metals, have economically recoverable reserves that would last more than 100 years (Graedel and Allenby, 1995). Further complicating resource availability is the issue of where the material resources are located. Many of the scarcest reserves are located in politically unstable regions of the world (USGS, 2002).

Increasing scarcity of materials and concerns about releasing waste materials into the environment will likely drive engineers to design systems that reuse and recycle materials. Lead (Pb) provides a case study of this evolution toward more recycling and reuse of materials. Figure 1-7 shows the increasing recycling and reuse of lead between 1970 and the mid-1990s. The figure is formatted in a manner analogous to Figure 1-3. Supply is documented on the left side of the diagram and uses are shown on the right. For materials, however, unlike energy, there are recycle loops. In the case of lead, the recycle loops are dominated by the flows of recycled batteries.

#### Example 1-4. Recycling rates

Using the data in Figure 1-7, compare the fraction of lead that was recycled in 1970 with that recycled in the mid-1990s.

Solution: Lead recycling doubled from approximately one-third of lead use to two-thirds of lead use from 1970 to the mid-1990s.

Fraction recycled in 1970 = 450,000 metric tons (mt)/1,230,000 mt = 0.36.

Fraction recycled in mid-1990s = 910,000 mt/1,410,000 mt = 0.65.

Metals are not the only materials that can be reused and recycled. A number of case studies have been described by Allen (2004).

### 1.4.2. Water

Water is essential to life, and water use exceeds the use of any other substance. Although water is abundant, clean freshwater suitable for agriculture, industrial uses, or satisfying thirst is becoming increasingly scarce. As shown in Figure 1-8, freshwater use in the United States totals approximately 345,000 million gallons per day. This translates to approximately 1000 gallons per person per day.

The sources include surface waters, such as lakes and rivers, and groundwater. While surface water is renewed on relatively short timescales, groundwater, in some cases, is a resource that has accumulated over very long periods of time and may or may not be replaced at the same rate at which it is withdrawn.

The largest uses of water are for agriculture, thermoelectric power (power generation using a steam cycle), and public supplies. Although not shown in Figure 1-8, uses of water are generally categorized into withdrawals and consumption. Water withdrawals involve removing water from a surface or groundwater reservoir. If that water is not returned to the same reservoir, the water is referred to as having been consumed. For example, if a power plant withdraws water from a lake, the total amount removed from the lake is the withdrawal. If the power plant uses some of that water to make steam to drive a turbine, and the steam is released to the atmosphere, the amount released to the atmosphere is considered consumption.

#### Example 1-5. Water use in electricity generation

Total electricity generation in the United States in 2008 was 12.68 quads (Figure 1-3). The water used (withdrawn) by the power sector was 136,000 Mgal/day. Calculate the amount of water used per kilowatt-hour of electricity generated.

Solution: Water used = 136,000 Mgal/day * 365 day/yr = 5 * 1013 gal/yr. Electricity produced = 12.68 * 1015 BTU/yr * 1 kWh/3413 BTU = 3.7 * 1012 kWh. Average water use = 13 gal/kWh.

This extensive use of water per unit of economic output (a kilowatt-hour retails for about 10 cents) is not unusual. Mining a million dollars’ worth of coal requires 11 million gallons of water. Making a million dollars’ worth of automobiles or semiconductors requires about 9 million gallons of water (EIOLCA, 2011). Clearly, our engineered systems are extensive users of water, and our economies depend on readily available, inexpensive, clean water.