Composting

There is strong evidence that composting food waste improves soil health^{9, 10, 11, 12, 13, 14}, increases soil carbon content and carbon sequestration^{9, 15, 16}, and reduces emissions, especially by averting methane emissions from landfills^{17, 18, 19, 20}.

Soil health. Composting improves soil fertility, microbial and enzymatic activity, nutrient content, and physical properties, including density and porosity^{9, 10, 13, 14, 17}. Composting can reduce soil contaminants, pathogens, and plant diseases^{9, 10, 11, 14, 21} and reduce soil erosion and eutrophication of waterways¹⁷. Applications of compost and other soil amendments can help reclaim salt-affected soils and return degraded soils to productive agricultural use²². High-quality compost applications can help remediate soils contaminated with heavy metals, microplastic pollution, and other hazardous pollutants^{15, 23, 24, 25}. Composting can reduce the availability, mobility, and ecological risk of hazards in the soil, increase the effectiveness of phytoremediation or using plants to clean up soil contaminants, and help dilute and break down some micropollutants^{15, 23, 24, 25}. Vermicomposting and using soil amendment additives can increase the effectiveness of compost remediation^{15, 23, 25}.

Soil carbon content. Composting increases carbon and nitrogen conservation in the soil, preventing carbon and nitrogen from accumulating in the atmosphere as greenhouse gases, especially carbon dioxide and nitrous oxides^{15, 16, 17, 20}. Composting increases soil’s water retention and decreases run-off, which improves carbon sequestration, plant health^{14, 16}, and crop productivity⁹. Composting supports plant growth and increases carbon dioxide uptake and storage within plants²⁰. Available evidence shows that composting to reduce food waste has the potential to provide large scale reductions to atmospheric carbon^{7, 26, 27}. Organic and regenerative farming practices use composting and have been shown to increase soil carbon and provide higher carbon sequestration than conventional farming^{26, 28, 29}. Farming practices including composting are also a suggested strategy for adaptation to climate change, since they are more resilient, more efficient with water use, and provide higher yields in drought years than conventional farming practices^{26, 29}.

Emissions effects. Removing food scraps from landfills and diverting them to compost is a highly effective way to avoid methane gas production^{18, 19}. By improving soil physical structure, compost makes the soil more workable, which can further reduce emissions by reducing fossil fuel use to work the soil²⁰. Applying compost to soil reduces fertilizer and herbicide use¹⁷, which further reduces greenhouse gas emissions from the production and application of those fertilizers and pesticides²⁰. Source separation of municipal solid waste requires heavy machinery to separate, grind, mix, and aerate compostable materials, which requires energy and produces greenhouse gas emissions²⁰. Sorting compostable food and yard waste before collection reduces these issues and produces a higher quality compost product without contamination from contact with other waste³⁰.

Vermicomposting. Vermicomposting can provide even greater benefits than traditional aerobic composting, with a faster process, more nutrient-rich final product, greater effects on disease suppression³¹, and increased removal of heavy metals and toxic substances³². Vermicomposting can also be adapted for use on the biodegradable portion of municipal solid waste, including food, paper, and green waste³³ and vermicomposting of municipal solid waste reduces bacteria and pathogen loads from final compost product³⁴.

Urban area effects. Composting efforts in urban areas can improve soil health and increase soil carbon storage^{12, 15}. Composting improves physical properties of degraded, compacted urban soil such as bulk density and water infiltration, which reduces run-off and erosion, and makes it easier for plants to grow¹². Composting to improve soil health in urban areas provides many co-benefits, including increasing urban green space, which can reduce heat island effects and improve mental health; improving urban gardening, which can improve local food security and food quality; supporting productive and sustainable community gardens; and remediating soil by reducing the availability of lead, arsenic, and other contaminants in the soil for plants to absorb¹⁵. Additional research is needed on compost application rates, depth of incorporation, and concentrations to achieve the greatest improvements in urban soil physical properties¹².

Best practices. Compost quality is affected by the waste source, raw materials used, and variables during the composting process³⁵. Implementing a system to sort and collect food waste for compost that is separate from municipal solid waste can prevent heavy metal, microplastic, and toxic substance contamination^{23, 25} and reduce the need for waste pretreatment and subsequent odors and bioaerosols^{30, 36}. Ensuring compost maturity and stability prior to soil application increases benefits to soil quality, improves stable carbon storage, and improves plant health benefits^{13, 35}. Compost processes that use physical, chemical, and microbial additives, increase aeration, and include pretreatment can improve nitrogen retention and reduce soil nitrous oxide emissions; these efforts are more necessary for municipal solid waste composting than pre-sorted food waste composting^{37, 38}. Composting benefits can be maximized with site specific application strategies that account for past land use, current conditions, and future uses¹⁶. Composting can be time consuming and compost practices need to monitor compost piles to ensure that food waste addition does not create overly wet conditions that can lead to anaerobic decomposition that produces methane. The use of brown leaves or yard waste, shredded paper, bulking agents, and other additives can improve structure to facilitate oxygen incorporation and can improve nitrogen and carbon balance to prevent anaerobic decomposition¹⁰.

Costs and scale. Composting can be implemented effectively at almost any scale, from home composting to large city-wide treatment facilities¹³. Experts calculate that composting costs less than landfill waste disposal for food waste. The cost of establishing new composting facilities is less than the cost of establishing new landfills, and though operating costs for large scale compost facilities are higher than those for landfills, composting also generates revenue with the sale of the finished product. Regulations, space, and logistic constraints can increase the costs of landfill expansion, which makes composting even more cost effective²⁷. Home composting is inexpensive to establish, requires minimal equipment, and can improve home or community gardens and save money that would have been spent on fertilizers and pesticides¹. In the U.S. developing a circular economy to sustainably manage food throughout its life cycle at the production, processing, transportation, consumption, and waste stages can conserve resources, save money, and improve food access⁶.

Life cycle analysis suggests composting provides more environmental benefits and climate change impact reductions than other organic waste disposal methods such as sending waste to the landfill or waste-to-energy projects¹⁷.

California³⁹, Connecticut⁴⁰, Maryland⁴¹, Massachusetts⁴², New Jersey^{43, 44}, New York⁴⁵, Rhode Island⁴⁶, Vermont⁴⁷, and Washington⁴⁸ are the nine states with laws that regulate sending organic waste to composting facilities instead of to landfills⁴⁹. Several states have grants available to support additional composting efforts; for example, the Maryland State Department of Education has grants for schools starting food waste reduction and composting programs⁵⁰ and the California Department of Resources Recycling and Recovery offers grants for small-scale composting projects for green spaces in disadvantaged communities⁵¹. The U.S. Composting Council is a national organization with 16 state chapters that support compost manufacturing and use for the benefits it provides to soil, air, and water quality, and to climate stabilization⁵². The U.S. Composting Council also offers a searchable database of state regulators and regulations that affect composting efforts⁵³.

Several cities in the U.S. have successful, mandatory composting programs, including San Francisco⁵⁴; Portland, Oregon⁵⁵; Boulder and Denver, Colorado^{56, 57}; and Seattle⁵⁸. San Francisco was the first U.S. city to establish a city-wide composting program in 1996 and currently composts and recycles roughly 80% of its waste⁵⁹. Many cities are implementing composting pilot programs, including Boston, Pittsburgh, and Jacksonville, Florida⁴⁹. New York City has also passed a new Zero Waste Act that includes mandatory composting programs with curbside pickup, universal composting for all residential buildings, increased organic waste drop off locations, and expanded compost facilities increasing equitable access to composting across the five boroughs⁶⁰.

At the federal level, grants are available that support initiatives to reduce food waste and increase composting opportunities, for example, the U.S. Department of Agriculture’s Composting and Food Waste Reduction Cooperative Agreements help local and city governments develop and implement community composting plans⁶¹. The Inflation Reduction Act of 2022 includes Greenhouse Gas Air Pollution Plans and Implementation Grants and the U.S. Environmental Protection Agency administers the Solid Waste Infrastructure for Recycling Grant Program; both grant programs can support composting initiatives⁶².

Composting initiatives are operated by many non-profit organizations across the country. For example, Conservation Works operates the Waste Wizards program in Northern California that teaches children about composting and reducing food waste to reduce climate change effects and diverts food waste from school cafeterias to create compost⁶³. Many community composting projects are equity-focused initiatives; for example, the Baltimore Compost Collective. This project is located on an acre of land in the Curtis Bay neighborhood that has experienced environmental injustice. The land was once polluted and now is home to a healthy community garden with 47 individual plots, beehives, goats, and ducks. The Baltimore Compost Collective is also building the nation’s largest municipally-managed compost facility and using composting to support environmental justice in disadvantaged neighborhoods, offer job training for youth, and reduce greenhouse gas emissions⁶⁴.