8.9 Solid Waste Disposal

Municipal solid waste (MSW) is the everyday trash from homes, schools and businesses. In the United States it amounts to roughly 4.9 pounds per person per day. Paper and cardboard are the largest category, followed by food waste, then plastics, yard trimmings and metals. About half of US MSW is landfilled, about a third is recycled or composted, and about an eighth is burned in incinerators that recover energy.

Developed countries make far more waste per person than developing countries, and much of it is packaging and products designed to be thrown away.

A sanitary landfill is designed to keep waste and its byproducts from escaping into soil, water and air. Its main features:

• A bottom liner of compacted clay and a thick plastic sheet to stop liquids from seeping into groundwater.
• A storm water collection system of ditches and drains that steers rain away from the trash, so less water soaks through it and becomes leachate.
• A leachate collection system of pipes that drains leachate, the polluted liquid that forms when rainwater trickles through trash and picks up chemicals, so it can be treated.
• Methane collection pipes. Buried trash decomposes without oxygen, and anaerobic bacteria produce landfill gas, which is about half methane. The gas is burned off or used to generate electricity.
• A daily layer of soil over the trash to reduce odors, pests and blowing litter, and a final clay and soil cap when the cell is full.
• Groundwater monitoring wells around the site to detect leaks.

Landfills take up large areas of land, and liners can eventually leak. Inside a landfill, trash breaks down very slowly because it is dry, compacted and short of oxygen; excavations of old landfills have found newspapers still readable after decades. Landfills are one of the largest human sources of methane in the United States. Many communities oppose new landfills nearby, and landfills have often been placed near low-income communities and communities of color, an environmental justice concern.

Some items are banned from ordinary landfills, such as tires, car batteries, motor oil, paint and other hazardous household waste, because they can leak toxic chemicals or cause fires. These items are supposed to go to special collection sites, but some are dumped illegally. Piles of discarded tires are a well-known problem: they collect rainwater and become breeding grounds for mosquitoes that can spread diseases such as West Nile virus (topic 8.15). Some countries have also dumped trash directly into the ocean, which, along with plastic from rivers and coasts, feeds floating garbage patches that entangle wildlife and are swallowed by them (topic 8.2).

Incineration burns trash at high temperatures. It reduces the volume of waste by up to about 90 percent, and waste-to-energy plants use the heat to generate electricity. But it releases air pollutants such as particulates, carbon dioxide, mercury and dioxins (requiring scrubbers and filters), and it leaves toxic ash concentrated with heavy metals that must go to special landfills.

Electronic waste (e-waste), such as old phones, computers and TVs, contains lead, mercury, cadmium and other toxic metals that can leach out of landfills into groundwater. It should go to certified e-waste recyclers, which recover valuable metals like gold and copper safely. Much e-waste has been shipped to developing countries, where workers, including children, dismantle it or burn it in the open with little protection.