The ability of plastic to be composted depends entirely on its chemical composition and intended disposal environment. Composting is the natural, managed process of organic material breakdown by microorganisms, heat, and oxygen. When considering plastics, the ability of a material to integrate into this biological cycle is complex. It is a common misconception that all plastics behave the same way in a compost heap, but in reality, only specific, certified materials can be composted without causing environmental harm.
Traditional Plastics: Why They Don’t Decompose
The vast majority of everyday plastics, such as polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET), are derived from petroleum and are fundamentally incompatible with the composting process. These materials are polymers, which are long chains of repeating molecular units called monomers. Traditional plastics are characterized by incredibly stable, repeating carbon-carbon bonds within their backbones. Microorganisms in compost have evolved to break down natural organic matter, which contains weaker bonds, and lack the specific enzymes necessary to cleave the strong carbon-carbon bonds. Consequently, these plastics do not biodegrade in a meaningful timeframe but persist for hundreds of years. When exposed to sunlight, these durable materials break down into smaller fragments, creating persistent microplastics that contaminate soil and water.
Understanding Compostable and Biodegradable Labels
The terms “biodegradable” and “compostable” are often used interchangeably, but they represent a critical distinction in waste management. “Biodegradable” is a broad, unregulated term that simply means a material can be broken down by microorganisms over an unspecified period. A truly biodegradable item may take decades to decompose and could potentially leave behind toxic residues or microplastic fragments, as there is no mandated standard for the timeframe or the quality of the final byproducts. In contrast, “compostable” is a strictly regulated claim referring to a material’s ability to break down entirely into non-toxic components, such as carbon dioxide, water, and biomass. To be certified compostable in the United States, a product must meet standards like the ASTM D6400, ensuring it is safe for plant growth and biodegrades within 180 days under controlled conditions.
Certified Compostable Plastics: Home Versus Industrial
Certified compostable plastics, such as those made from plant starches like polylactic acid (PLA), are designed to break down, but they require very specific environmental conditions. The most significant factor is heat, which is necessary to initiate the breakdown process of the polymer chains. Industrial composting facilities are large-scale operations that precisely control conditions, maintaining high temperatures, typically between 55–70°C (130–160°F), to support thermophilic microbes. A typical home compost pile rarely maintains these high temperatures consistently, usually remaining below 40°C (104°F). Consequently, materials designed only for industrial composting will not break down adequately in a home system; consumers should look for the “OK compost HOME” label for domestic use.
The Consequences of Plastic Contamination
Mistakenly adding non-compostable or industrial-only plastics to a composting system, whether a backyard pile or a large commercial facility, has significant negative repercussions. When conventional plastics are introduced, they do not decompose and instead fragment into smaller, durable pieces. These fragments, known as microplastics, contaminate the finished compost, which is then spread onto agricultural fields and gardens, transferring the pollution directly into the soil ecosystem. For industrial facilities, this contamination reduces the quality and marketability of the final product, often requiring extensive, costly, and time-consuming mechanical filtering to remove the plastic debris. If the contamination levels are too high, the entire batch of compost may be deemed unusable and sent to a landfill, defeating the purpose of the composting process.