Municipal solid waste (MSW) includes everyday items from homes, schools, and businesses, such as product packaging, food scraps, yard waste, clothing, and appliances. It generally excludes industrial, hazardous, and construction waste.
In the United States, each person produces an average of 4.9 pounds of trash daily, totaling nearly 300 million tons annually. Globally, the average person generates about 1.6 pounds of waste daily, contributing to an annual worldwide total of approximately 2.01 billion tons of MSW.
Landfills: The Primary Destination
Modern landfills are engineered facilities designed to contain waste and prevent environmental contamination, unlike older open dumps. These sites are constructed with multiple layers to isolate trash from the surrounding soil and groundwater. A typical design includes a composite liner system, often consisting of a flexible geomembrane layered over compacted clay, to act as a barrier.
Below the waste, a leachate collection system is installed, which gathers the liquid that forms as water filters through the decomposing trash. This leachate, which can contain dissolved contaminants, is then pumped out and treated. Landfills also incorporate gas collection systems to capture gases produced by decomposition, primarily methane, a potent greenhouse gas. This captured methane can sometimes be converted into energy.
About 50% to 63.5% of municipal solid waste in the United States is sent to landfills. While some organic materials decompose over time, many common items like plastics, glass, and certain metals do not readily break down in the oxygen-poor environment of a landfill. Much of this waste remains intact for centuries, requiring long-term management of the sites and their potential emissions.
The Recycling Journey
Recycling begins when discarded materials are placed into designated collection bins. These materials are then transported to a Material Recovery Facility (MRF), where the process of sorting begins. At a MRF, manual labor and automated technologies, such as optical scanners, magnets, and eddy currents, separate different types of recyclables like plastics, paper, glass, and metals.
Once sorted, these materials are compacted into large bales. These bales are then shipped to various processing plants. At these facilities, the materials undergo further cleaning, shredding, melting, or other transformations to prepare them for reuse.
For instance, plastic bottles might be shredded, washed, and melted into pellets, which can then be molded into new plastic products. This process, known as closed-loop recycling, aims to turn old products into new ones of the same type. However, many materials are downcycled, meaning they are converted into products of lesser quality or different applications, such as turning plastic bottles into carpet fibers instead of new bottles.
Transforming Waste into Energy
Waste-to-energy (WTE) facilities convert municipal solid waste into usable energy. These plants, sometimes referred to as incinerators, burn processed municipal solid waste at extremely high temperatures. The intense heat generated from this combustion process boils water, creating high-pressure steam.
This steam then drives turbines, which are connected to generators to produce electricity. In 2022, 63 WTE power plants in the U.S. burned 26.6 million tons of MSW and generated approximately 12.8 billion kilowatt-hours of electricity. This electricity is then fed into the power grid.
To control emissions from the burning process, modern WTE facilities are equipped with sophisticated air pollution control systems. These systems include scrubbers to remove acidic gases and fabric filters (baghouses) to capture particulate matter before the cleaned gases are released into the atmosphere. After incineration, the remaining ash, which typically represents about 10% of the original volume, is usually sent to specialized landfills for disposal.
Composting: Nature’s Recycling
Composting is a natural process that transforms organic waste materials into a nutrient-rich soil amendment. This method primarily handles food scraps, yard trimmings, and other biodegradable materials. Microorganisms, such as bacteria and fungi, break down organic matter under controlled aerobic conditions (in the presence of oxygen).
The controlled environment, including proper moisture levels and aeration, encourages efficient decomposition. The end product, compost, is a dark, crumbly material that resembles soil. It enriches soil, improves its structure, and provides nutrients for plants, often used in gardening, agriculture, and landscaping.
Composting plays a significant role in diverting organic waste from landfills. When organic materials decompose in the oxygen-deprived environment of a landfill, they produce methane gas, a potent greenhouse gas. By composting, this methane generation is largely avoided, contributing to a reduction in overall greenhouse gas emissions and extending the lifespan of landfills.