Municipal solid waste (MSW) is the vast stream of refuse generated by households and businesses, a substantial portion of which consists of organic, biodegradable material known as green waste. Diverting this material from landfills is a global priority because its anaerobic decomposition—breakdown without oxygen—releases methane, a potent greenhouse gas. Separating this stream allows for a controlled process that mitigates climate impact and turns waste into a valuable resource.
Defining Green Waste and Common Examples
Green waste is readily biodegradable organic material, typically derived from plants, which can be broken down by microorganisms. This material is distinguished by its high moisture content and nitrogen-rich composition, often referred to as “greens” in composting terminology. It commonly includes the two largest categories of residential organic waste: yard trimmings and food scraps.
Yard waste encompasses materials such as grass clippings, leaves, small branches, and garden weeds. Food waste consists of fruit and vegetable peels, coffee grounds, and other non-protein-based kitchen discards. These materials decompose relatively quickly and return their stored nutrients to the soil.
Processing Methods and Repurposing
Once collected, green waste is managed primarily through two industrial-scale processes: aerobic composting and anaerobic digestion. Large-scale composting is a controlled, aerobic process where microorganisms break down the organic matter. This process is highly regulated and includes a thermophilic phase (50°C to 70°C) required to kill pathogens and weed seeds (hygienization).
The end product of this decomposition is a stable, humus-like material used as a nutrient-rich soil amendment or mulch. Anaerobic digestion breaks down high-moisture organic waste in a sealed, oxygen-free environment. This process yields two valuable outputs: biogas, used as renewable energy, and a liquid or solid digestate that serves as a bio-fertilizer.
Common Contaminants and Excluded Materials
Preventing contamination is essential because non-organic materials can ruin the final product or halt the decomposition process. Chemically treated wood, such as lumber preserved with chromated copper arsenate, must be excluded because it contains toxic heavy metals like arsenic that contaminate the compost. Plants treated with herbicides or pesticides should also be excluded, as these residues harm the beneficial microorganisms necessary for decomposition.
Animal products, including meat, fish scraps, and dairy, are generally excluded from most municipal programs. These materials attract pests like rodents and flies, and they can harbor human pathogens such as Salmonella and E. coli. Excessive fats and cooking oils must also be excluded because they create water-resistant barriers, significantly slowing down aerobic decomposition. Conventional plastics, glass, and metals are non-biodegradable and must be removed to maintain the quality of the finished compost.