Archeologists can infer a great deal about past societies by studying waste dumps, such as the Emeryville Shellmound. In the modern world, waste comes from many sources and is made up of countless forms of discarded material. This mixture is called the waste stream. What would some future archeologist conclude about our contemporary world from studying our waste stream? Two conclusions would be inescapable: Rich countries produce a lot more waste than poor countries, and the types of things they each throw out differs greatly.

While people everywhere generate solid waste, the amount of waste generated is particularly large in cities because urban populations are particularly concentrated. Municipal solid waste (MSW) includes all solid waste from institutions, households, and businesses, including paper, packaging, food scraps, glass, metal, textiles, and all other solid discards. Consequently, waste management is one of the most critical services that cities provide. In low-income regions, waste management is the most expensive item in municipal budgets. It’s so fundamental to life in the city that most people don’t even think about it—at least, not until the system fails. For example, in 2011, garbage collectors in Athens, Greece, went on strike to protest potential tax hikes and salary cuts, allowing trash to pile up on curbsides for 17 days. Strikes by garbage collectors have had similar consequences in many places around the world (Figure 12.3).

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Because municipal solid waste is a product of economic activity, income level is correlated with the amount and composition of waste generated by a population. As shown in Figure 12.4, the per capita production of MSW in the world’s richest countries is triple that of the poorest countries.

There are many different types of solid waste, which must be handled differently. The World Bank classifies MSW into six categories: organic, paper, plastic, glass, metal, and other (Table 12.1).

Under the World Bank’s scheme, organic waste includes food scraps, yard clippings, and wood. Paper and plastics are also chemically organic, but paper is highly processed organic material and plastics are mostly synthesized from petroleum. The “other” category includes processed organic material as well, including leather and rubber, but also appliances, electronic waste, and ash.

In poor countries, organic wastes such as food, wood, and garden waste make up a full 64% of municipal solid wastes. By contrast, such items make up only 30% of waste in cities of wealthy countries. Meanwhile, the proportions of paper, glass, and metal in solid waste increase with income level (Figure 12.5). We can assume that as poor nations develop economically, the generation of these types of wastes will increase.

Some wastes are so dangerous and toxic that they cannot be disposed of with conventional means. The U.S. Environmental Protection Agency (EPA) defines hazardous waste as having a “chemical composition or other property that makes it capable of causing illness, death, or some other harm to humans and other life forms when mismanaged or released into the environment.” More specifically, waste material that has any of the following four properties is considered hazardous:

1. Flammable: Substances that ignite and burn quickly and easily. These can spontaneously ignite from friction, absorption of moisture, or contact with other waste materials.

2. Reactive: Unstable substances that readily undergo a violent chemical change when in contact with other substances, especially with water.

3. Corrosive: Any strong acids (pH of 2 or less) or strong bases (pH of 12 or greater). These substances can permanently damage a variety of surfaces, including living tissue.

4. Toxic: Relatively low amounts of these substances are harmful to living organisms.

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Hazardous wastes cannot be handled, treated, or stored in the same way as other categories of waste. The risk of an explosive chemical reaction, harm to workers, or damage to lining of waste containers or landfills is simply too great. (Hazardous chemicals are also discussed in Chapter 11, as an aspect of environmental health, and in Chapter 13, as pollutants.)

In the United States, the “basic chemical” industry, which produces everything from nail polish remover to chlorine for swimming pools, is responsible for over half of all hazardous waste (Figure 12.6). In second place is the production of petroleum- and coal-based products, such as gasoline, plastics, and lubricants. Taken together, five industry sectors accounted for 94% of the approximately 31 million metric tons of hazardous wastes produced in the United States in 2011. The remaining 6% of hazardous wastes produced in the United States comes from economic activities by 45 commercial and industrial sources, ranging from paint manufacturing to sawmills.

While most household wastes aren’t dangerous, there are some hazardous items commonly found in the home that individuals should never simply place in a trash can or pour down a drain, on the ground, or into storm sewers. For example, the following household wastes meet at least one of the EPA’s hazardous waste criteria:

Because wealthier countries tend to have greater capacity for manufacturing chemicals than do poorer countries, they tend to produce more hazardous waste. As shown in Figure 12.7, the average per capita production of hazardous waste in high-income countries is approximately 3 times that in high middle-income countries and over 16 times the rate in low middle-income countries.

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