5.1 Kinds of solid and hazardous waste

Kinds of waste

Waste is often categorized by its source, its makeup,  or its destination, and then may be broken down further. The US Resource Conservation and Recovery Act (RCRA defines solid waste as “any waste or  sludge from a wastewater treatment plant, water supply treatment plant, or air pollution control facility and other discarded material, including solid, liquid, semi-solid, or contained gaseous material resulting from industrial, commercial, mining, and agricultural operations, and from community activities. In general, solid waste can be categorized as either non-hazardous waste or hazardous waste.” This is a trifle confusing, as it includes things that are clearly not solid, and separates out some sources very specifically while grouping other sources very broadly. In the US, waste production is tracked by the EPA, but irregularly; In 2025, no major reports were available for conditions after 2018.

Non-hazardous wastes

Municipal waste includes non-hazardous waste generated by households and offices – what we often call “trash” or “garbage.” In the US, food and paper waste comprise almost half of all wastes   (Fig 1). In developing countries, waste related to packaging comprises a lower proportion of waste, resulting in a larger proportion of food-related waste (Fig 2).^[1] Medical wastes that are not hazardous are typically treated as part of municipal wastes. Hazardous waste from the municipal waste stream includes many categories of material from batteries of all kinds to radioactive medical waste.

Agricultural wastes include residue from crop processing and food processing such as residues from brewing and distilling of alcoholic beverages, juice production, grain processing, animal wastes and carcasses, packaging from feed and fertilizers, etc. Crop residues left in the fields are not categorized as agricultural waste. Pesticide containers and left-over pesticides are classified as hazardous wastes.

Industrial wastes vary considerably by industry and within industries. Construction wastes may include a large component of organic material from wood products or no wood waste at all, for example. Wastes may be largely unprocessed such as rock and mined water from mining operations, or may be heavily processed, such as paints, solvents, and other industrial chemicals. Many industrial wastes are classified as hazardous wastes.

Because they are classified as non-hazardous materials, many municipal, agricultural, and industrial waste components have potential for recycling and reuse.

Hazardous wastes

In the US, hazardous wastes are defined by the US EPA as having (at least) one of four characteristics.

Ignitable: a material that can ignite on its own, without any separate ignition sources such as a spark or flame. The material may combust as a result of heat, chemical changes, changes in moisture, friction, etc. (Combustible materials require an outside ignition source in order to burn).

Corrosive: corrosive materials can destroy other materials on contact. In particular, they can corrode containers such as metal containers, which makes them difficult to contain. Corrosives are commonly strong acids with pH <2 or strong bases with pH > 12.5 .

Reactive: materials that combust, explode, or emit harmful vapors or fumes under specific conditions. They may react to heat, compression, or the addition of water.

Toxic: materials that can cause harm or kill when ingested, absorbed through the skin, or inhaled.

In international law, hazardous wastes are largely defined in documents related to the Basel Convention on the Control of Transboundary Movements of Hazardous Waste. In addition to the categories defined by the US EPA, the United Nations adds radioactive, infectious, and mutagenic (causing cancer and birth defects) wastes. In the US, radioactive wastes are regulated at the federal level, but separately from hazardous wastes. Infectious wastes are regulated at the state level. Mutagenic wastes are regulated through disposal requirements in the Resource Conservation and Recovery Act (RCRA) and the Toxic Substances Control Act (TSCA).

Exclusions to solid waste and hazardous wastes

In the US, a wide variety of types of waste are excluded from the definitions of solid waste and hazardous waste.^[2] Some exclusions are to avoid redundant regulation. For example, the EPA does not address radioactive waste as a hazardous waste because it is regulated by the Atomic Energy Act, rather than by RCRA or TSCA. Similarly, domestic sewage is regulated under the Clean Water Act. Some substances are unregulated when they are emitted in small amounts by municipal users, but are regulated when emitted by industry. These exceptions recognize both the difficulty of regulating a large number of users and the practicality of not regulating users that, individually, cause little harm. Recycling streams are often excluded, to encourage recycling. Finally, in some cases, evidence is not yet available to determine the level of harm associated with a given substance.

Knowledge Check

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