Composting is a natural, aerobic process that converts organic materials into a nutrient-rich soil amendment through microbial decomposition. This process offers a significant opportunity to reduce the volume of material currently discarded as trash. The Municipal Solid Waste (MSW) stream in the United States contains a large fraction of material that could be diverted. Determining the percentage of this waste stream that could theoretically be recycled through composting is a fundamental step toward maximizing resource recovery and creating a more circular waste management system.
Key Components of Compostable Waste Streams
The primary materials in the Municipal Solid Waste stream suitable for composting are naturally occurring organic matter that provides the essential carbon and nitrogen ratios required for effective microbial breakdown. The three most significant components are food waste, yard trimmings, and certain types of paper products. Food waste is the single largest category of material generated in the United States’ MSW stream, ranging from plate scrapings to spoiled produce. Yard trimmings, including grass clippings, leaves, and brush, represent the second largest source of easily compostable organics. Soiled or uncoated paper and paperboard, such as pizza boxes and paper towels, can also be introduced into the composting process.
The Maximum Potential for Composting
The maximum percentage of Municipal Solid Waste that could theoretically be composted is substantial, representing roughly one-third of the total national stream. Food waste and yard trimmings alone constitute approximately 33.7 percent of the total MSW generated in the United States. This figure represents nearly 63.1 million tons of food waste and 35.4 million tons of yard trimmings generated annually, based on 2018 data. The potential increases further when considering other organic materials, such as wood and compostable paper products, that could be diverted from landfills. This percentage is the theoretical maximum, assuming 100 percent of these materials are successfully collected and processed through composting or other organic recycling facilities.
Current Composting Rates Versus Potential
The gap between the theoretical maximum and current practice highlights a massive missed opportunity for resource management. While roughly 33 percent of the total MSW stream consists of readily compostable materials, the actual composting rate remains far lower. For example, only about 25 million tons of MSW were composted in 2018, equating to approximately 8.5 percent of the total MSW generated nationwide. This disparity means the vast majority of compostable material is currently sent to landfills or incinerators. Food waste is the most common single material found in American landfills, and when yard trimmings and other organics are included, these materials make up over 51 percent of the municipal solid waste disposed of in landfills. The scale of the material being lost represents a substantial reservoir of nutrients and soil-building capacity that is not being recovered.
Environmental Impact of Diverting Organic Waste
Diverting organic waste from disposal sites into composting facilities creates significant environmental benefits, primarily by mitigating climate change and improving soil health. When organics are buried in landfills, they decompose anaerobically. This process generates methane, a powerful greenhouse gas with a global warming potential far greater than carbon dioxide. Landfills are one of the largest sources of human-related methane emissions in the United States, and successful organic waste diversion can drastically reduce this output. Composting, by contrast, is an aerobic process that produces carbon dioxide and water vapor, which are much less harmful to the climate than methane. Applying compost improves soil structure, enhances water retention, and closes the nutrient loop by returning organic matter to the earth.