Solid Waste Disposal — AP® Environmental Science Notes

Every product you buy eventually becomes waste. Municipal solid waste (MSW) is the everyday trash from homes, schools and businesses. Where it goes, and what it does once it gets there, is the subject of this topic.

How much, and where it goes

The EPA reported that the United States generated 292.4 million tons of MSW in 2018, about 4.9 pounds per person per day. Paper and cardboard (about 23%) and food (about 22%) were the largest parts.

Where it went (2018, EPA)	Millions of tons	Share
Landfilled	146.1	50.0%
Recycled	69.1	23.6%
Combusted with energy recovery	34.6	11.8%
Composted	24.9	8.5%
Other food management	17.7	6.1%

Worked example: a share of the total. What percent of 2018 MSW was landfilled?

Setup: $\frac{146.1\,\text{million tons}}{292.4\,\text{million tons}} \times 100$ . Answer: $= 50.0\%$ .

The sanitary landfill

Cross-section of a sanitary landfill. At the bottom, a compacted clay layer and a plastic liner sit above groundwater, with perforated pipes that collect leachate and send it to treatment. Layers of trash are separated by daily soil cover. Vertical gas wells collect methane to a flare or generator. A clay and soil cap with grass covers the top. — **Figure 1.** Cross-section of a sanitary landfill. LevlPrep original diagram.

Liner: compacted clay and a thick plastic sheet keep liquid from reaching groundwater.
Leachate collection: rainwater that seeps through trash dissolves chemicals and becomes leachate; perforated pipes collect it for treatment.
Daily cover: a layer of soil each day limits odors, pests and windblown litter.
Gas wells: bacteria break down food, yard waste and paper without oxygen, making methane, a fuel and a strong heat-trapping gas. Wells collect it to burn or to make electricity.
Cap: clay, soil and plants seal a full landfill.

Inside a landfill, trash is packed tight and kept dry, so decomposition is very slow, which is why a closed landfill must be watched for decades. Old landfills without liners can leak leachate, and landfills take up land that nearby communities often do not want.

Reading waste data

Waste questions often give a table of tons by method or by material and ask for a share, a per-person amount, or a change over time. Write the setup with units so they cancel. For example, the EPA total of 292.4 million tons for about 327 million people works out to $\frac{292.4 \times 10^{6}\,\text{tons} \times 2{,}000\,\text{lb/ton}}{327 \times 10^{6}\,\text{people} \times 365\,\text{days}}$ $= 4.9$ pounds per person per day. Keep track of scale words: "million tons" means $\times 10^{6}$ , and a short ton is 2,000 pounds while a metric ton is 1,000 kilograms.

Other ways to dispose of waste

Incineration burns trash at high temperature. It cuts volume by about 90% and can generate electricity (waste-to-energy), but it releases air pollutants such as CO2, particulates and some toxic chemicals, and leaves ash that may contain heavy metals and still needs a landfill. Ocean dumping of garbage is banned in the United States because trash, especially plastic, harms marine life. E-waste, discarded electronics, contains lead, mercury and cadmium that can leach out, along with gold, copper and other valuable metals, so it should be recycled at special facilities. Much e-waste has been shipped to poorer countries, where workers without protection burn or break it apart to recover metals, exposing themselves to the toxic metals inside. The next topic covers ways to make less waste in the first place.