What is Solid Waste? Understanding Types and Sources

Every day, billions of people eat, manufacture, build, and farm – and every one of these activities generates waste. Some of it gets recycled or composted, but a vast portion ends up discarded, buried, or burned. The world currently generates over 2 billion tonnes of municipal solid waste each year, and that figure is expected to climb to 3.4 billion tonnes by 2050. Understanding what solid waste actually is – and where it comes from – is the essential first step toward managing it responsibly.

Defining solid waste 🔗

The term “solid waste” is broader than it might first appear. It does not just mean garbage bags or landfill piles. According to the U.S. Environmental Protection Agency (EPA), a material qualifies as solid waste when it is discarded by being abandoned (disposed of, burned, or incinerated), classified as inherently waste-like, or recycled in specific ways such as burning for energy recovery. Importantly, this definition covers not just physically solid matter but also semi-solid and even some liquid materials.

The United Nations Office for Disaster Risk Reduction (UNDRR) defines solid waste as all discarded materials that are no longer required by their owner or user, explicitly including materials in both solid and liquid states – while excluding wastewater and small particulates released into the atmosphere. In simpler terms, solid waste is any unwanted material resulting from human activity that needs to be collected, treated, or disposed of.

A key legal framework governing solid waste in the United States is the Resource Conservation and Recovery Act (RCRA) of 1976. Under RCRA, solid waste means any garbage, refuse, or sludge from wastewater or water supply treatment plants, along with other discarded materials resulting from industrial, commercial, mining, and agricultural operations and from community activities. Only materials meeting this definition can be further classified as hazardous wastes subject to stricter regulation.

Types of solid waste 🔗

Solid waste is not a single uniform category. It is classified in multiple ways – by physical state, by chemical properties, and by the degree of hazard it poses. Understanding these distinctions matters because different waste types require different handling and treatment strategies.

By physical state 🔗

Despite the name, “solid waste” can exist in three physical states. Solid waste in the strictest sense includes food scraps, plastics, paper, glass, metals, textiles, and construction rubble – the material most people associate with garbage. Liquid waste within this category refers to sludge from wastewater treatment plants and certain industrial effluents that, while liquid in form, fall under solid waste regulations. Gaseous waste – such as methane produced from decomposing landfill material – is closely associated with solid waste management, though it is typically regulated separately under air quality frameworks.

By origin and composition 🔗

Based on origin, solid waste is broadly divided into household, institutional, agricultural, biomedical, municipal, and industrial waste. Based on properties, it can further be classified as:

• Organic (biodegradable) waste: Food scraps, garden trimmings, crop residues, and animal waste that break down naturally over time through microbial activity.
• Inorganic (non-biodegradable) waste: Plastics, glass, metals, and synthetic textiles that persist in the environment for decades or centuries without decomposing.
• Hazardous waste: Materials that pose immediate risks to human health or the environment due to toxicity, flammability, corrosiveness, or reactivity – such as industrial chemicals, pesticide containers, and medical sharps.
• Non-hazardous waste: Everyday materials that do not pose direct health threats under normal conditions, such as paper, cardboard, and most food waste.
• Radioactive waste: Generated from nuclear power plants, medical facilities using radiotherapy, and certain research institutions. This requires highly specialized containment and disposal.
• Electronic waste (e-waste): Discarded circuit boards, batteries, mobile phones, and computing equipment. Global e-waste generation reached 62 million tonnes in 2022, growing five times faster than documented recycling rates.

Municipal solid waste (MSW) 🔗

Municipal solid waste is what most people interact with daily. It includes the everyday items discarded by households, retailers, small businesses, and institutions – food scraps, paper, packaging, plastics, yard trimmings, and textiles. Paper and paperboard make up roughly 27% of MSW, while food and yard trimmings account for about 28%, making organic materials the single largest component. Plastics contribute around 13%, followed by metals, rubber and textiles, wood, and glass.

Sources of solid waste 🔗

The sources of solid waste include residential, commercial, institutional, and industrial activities, as well as agriculture, mining, and construction. Each source produces a distinct waste profile with different management needs.

Residential sources 🔗

Households are the most visible and direct source of solid waste. Everyday domestic activities generate food waste, packaging, newspapers, clothing, small appliances, and garden debris. Waste generation at this level is closely tied to consumption habits and income. High-income countries, representing just 16% of the global population, produce approximately 34% of the world’s waste. In the United States, the average municipal refuse generation is roughly 2 kilograms per person per day, compared to under 0.5 kg in some lower-income countries.

Industrial sources 🔗

Industry is one of the largest contributors to total global waste. Factories, power plants, refineries, chemical plants, and manufacturing units generate large quantities of process residues, packaging waste, chemical sludge, ash, and scrubber residues. Non-municipal solid waste – which includes industrial, mining, and agricultural waste – accounts for nearly 99% of all waste generated in the United States. Industrial waste composition varies widely depending on the type of industry and the processes involved, and it frequently contains hazardous or toxic components that require specialized treatment before disposal.

Agricultural sources 🔗

Agriculture generates an enormous volume of solid waste that is often overlooked in urban-focused discussions. Agricultural waste generation globally is estimated at around 1,300 million tonnes per year, encompassing crop residues such as seeds, husks, leaves, stalks, and fruit pulp, as well as animal manure, slaughterhouse waste, and plastic films used in greenhouse and mulch applications. The Food and Agriculture Organization (FAO) estimates that approximately one-third of all food produced globally is lost or wasted each year – equivalent to around 1.3 billion tonnes – much of which enters the waste stream at the farm or processing level.

Institutional and commercial sources 🔗

Schools, hospitals, government offices, restaurants, hotels, and retail businesses collectively generate substantial quantities of waste. Commercial solid waste overlaps significantly with municipal waste streams in terms of food scraps and packaging, but commercial and institutional sources also produce specialized waste such as healthcare waste (syringes, bandages, expired pharmaceuticals), large volumes of cardboard, and institutional food waste at scale. Healthcare or biomedical waste deserves special attention because improper handling can spread pathogens and create serious public health risks.

Construction and demolition sources 🔗

Building construction, renovation, and demolition generate significant quantities of concrete rubble, bricks, timber, steel, insulation materials, roofing shingles, electrical fixtures, and glass. This stream is categorized separately in many countries but may be included under industrial or municipal waste in others depending on national classification systems. Construction waste is generally bulky and heavy, making transport and disposal costly, though much of it is potentially recyclable.

Why the distinction between types and sources matters 🔗

Classifying solid waste by both type and source is not just an academic exercise. It directly shapes how waste is collected, treated, and ultimately disposed of. Biodegradable organic waste can be composted or anaerobically digested to produce biogas and nutrient-rich compost. Recyclable inorganics like metals, glass, and certain plastics can be processed and reintroduced into manufacturing. Hazardous waste demands controlled containment, chemical treatment, or high-temperature incineration under strictly regulated conditions. Mixing incompatible waste types not only complicates treatment but can generate new environmental hazards.

The environmental stakes are high. Solid waste that is poorly managed contributes to air, soil, and water pollution, creates breeding grounds for disease vectors such as mosquitoes and rodents, and generates greenhouse gases. In 2016, solid waste treatment and disposal generated an estimated 1.6 billion tonnes of CO₂-equivalent greenhouse gas emissions – around 5% of total global emissions – driven largely by open dumping and landfills without gas collection systems. As global waste volumes continue to climb, understanding the full scope of what solid waste includes and where it originates becomes ever more critical for developing effective, sustainable management strategies.

What do you think? Given that high-income countries generate a disproportionate share of global waste relative to their population size, where should the primary responsibility for reducing solid waste fall – on individual consumers, on industries, or on governments through policy? And with agricultural waste generating over a billion tonnes annually, do you think farm-generated residues receive enough attention in mainstream conversations about solid waste management?

References

1. https://datatopics.worldbank.org/what-a-waste/trends_in_solid_waste_management.html
2. https://www.epa.gov/hw/criteria-definition-solid-waste-and-solid-and-hazardous-waste-exclusions
3. https://www.undrr.org/understanding-disaster-risk/terminology/hips/tl0502
4. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/solid-waste
5. https://picvisa.com/waste-statistics/
6. https://archive.epa.gov/epawaste/nonhaz/municipal/web/html/
7. https://www.britannica.com/technology/solid-waste-management
8. https://bio.libretexts.org/Bookshelves/Ecology/AP_Environmental_Science/01:_Chapters/1.17:_Solid_Waste
9. https://www.sciencedirect.com/science/article/abs/pii/S0301420723006980
10. https://blog.idrenvironmental.com/what-is-solid-waste