Solid waste is any discarded material from homes, businesses, industries, and agricultural operations. Despite the name, it doesn’t have to be physically solid. Liquids, semi-solids, and even contained gases all count as solid waste under U.S. environmental law. The world produced 2.56 billion tonnes of it in 2022, and that number is projected to hit 3.86 billion tonnes by 2050.
Why “Solid Waste” Includes Liquids and Gases
The term is broader than it sounds. Under the Resource Conservation and Recovery Act (RCRA), solid waste covers garbage, refuse, sludge from wastewater treatment and water supply plants, air pollution control residues, and any other discarded material from industrial, commercial, mining, agricultural, or community activities. The “solid” in the name distinguishes it from emissions released into the air or wastewater discharged into waterways, not from its physical state. A barrel of used industrial solvent and a truckload of demolition rubble are both solid waste.
What Municipal Solid Waste Contains
Municipal solid waste, often just called trash or garbage, is what households and businesses throw away day to day. In the United States, the EPA tracks its composition by material type. The 2018 breakdown looked like this:
- Paper and paperboard: 23.05%
- Food waste: 21.59%
- Plastics: 12.20%
- Yard trimmings: 12.11%
- Glass: 4.19%
The remaining quarter or so includes metals, textiles, wood, rubber, leather, and miscellaneous materials. The high share of food waste is significant because organic material decomposes in landfills and generates methane, a potent greenhouse gas. Paper and plastics, meanwhile, dominate recycling conversations because they’re produced in such large volumes and break down slowly (or not at all, in the case of most plastics).
Industrial and Construction Waste
Factories, construction sites, and manufacturing plants generate solid waste on a different scale entirely. Construction and demolition projects produce asphalt debris, treated wood, used oils, and various chemical-laden materials from painting, flooring, and specialty contracting. Furniture manufacturing creates waste streams containing solvents, pigments, and petroleum-based compounds. Textile factories generate chemical residues from bleaching and finishing processes.
Much of this industrial waste is classified as non-hazardous, meaning it can go to standard disposal facilities. But some of it crosses the line into hazardous waste when it’s ignitable, toxic, or chemically reactive. A demolished building might produce ordinary concrete rubble alongside lead pipes that require specialized handling. The distinction matters because hazardous waste follows a stricter set of rules for transport, treatment, and disposal.
How Solid Waste Affects the Environment
The environmental impact starts underground. When rain filters through a landfill, it picks up chemicals from decomposing waste and forms a toxic liquid called leachate. Studies of landfill leachate have found extremely high concentrations of dissolved salts, chloride levels above 2,000 milligrams per liter, and iron concentrations dozens of times higher than what’s safe in drinking water. This contaminated liquid can seep into surrounding soil and groundwater if it isn’t contained.
Above ground, the problem is methane. Landfills currently release roughly 38 million tonnes of methane per year, accounting for about 10% of all human-caused methane emissions worldwide. Methane traps far more heat in the atmosphere than carbon dioxide over a 20-year period, making landfills a meaningful contributor to climate change. Open dumps in lower-income countries, which lack gas capture systems, are especially significant sources.
How Modern Landfills Are Built
Engineered landfills are designed to keep waste isolated from the surrounding environment. The key feature is the liner system at the bottom, which prevents leachate from reaching groundwater. A basic design uses a thick layer of compacted clay (0.6 to 3 meters deep) topped with a synthetic liner. More advanced designs use double composite systems: two synthetic liners separated by a leak detection zone, all layered over compacted low-permeability soil and a prepared sub-base.
The synthetic liners are nearly impermeable, allowing almost no liquid to pass through. The clay layers beneath them serve as a backup barrier. During construction, each layer of clay is compacted in thin lifts so the compaction equipment fully penetrates the soil, bonding each layer to the one below it. The clay is deliberately kept wetter than its optimal moisture content to minimize cracking. Together, these layers form the landfill’s main defense against contamination leaking into the surrounding environment.
On top of this liner system, landfills include drainage layers to collect leachate and pipe it to treatment facilities, along with gas collection systems that capture methane for flaring or energy generation.
Beyond Landfills: Other Management Methods
Landfilling is the most common disposal method worldwide, but it’s not the only one. Recycling diverts paper, metals, glass, and certain plastics back into manufacturing. Composting handles food scraps and yard waste by breaking them down into soil amendments. Waste-to-energy facilities burn trash at high temperatures to generate electricity or heat, reducing the volume of material that needs to be buried by roughly 90%.
Each method fits different waste streams. Organic waste is best composted or processed through anaerobic digestion, which captures methane in a controlled setting. Recyclable materials lose value once they’re contaminated or mixed together, which is why source separation (sorting your recycling at home) matters. Non-recyclable, non-compostable waste is where landfilling and incineration come in as the remaining options.
The Scale of the Problem
Global waste generation is growing faster than population. The World Bank found that 2.56 billion tonnes of waste were produced in 2022, and under current trends, that figure will increase by 50% within three decades. Rising incomes in developing countries drive part of this growth: as people consume more packaged goods and processed food, their waste output climbs. Urbanization concentrates the problem, putting pressure on cities to build collection systems and disposal infrastructure that many can’t yet afford.
In wealthier nations, the challenge is shifting away from landfill dependence. The average American generates over 4 pounds of trash per day. Countries like Germany, South Korea, and Japan have dramatically reduced their landfill rates through aggressive recycling mandates, deposit-return systems for beverage containers, and pay-as-you-throw pricing that charges households based on how much waste they produce. These policies show that waste generation isn’t fixed. It responds to the systems and incentives built around it.