The confusion surrounding “compostable” bags is understandable, as the labels often imply a simple, natural breakdown process that can happen anywhere. This material is designed to offer an alternative to traditional plastic, suggesting it will return to the earth without leaving behind permanent pollution. However, the term “compostable” is highly specific, referring not just to the material but also to the precise environmental conditions required for its decomposition. The difference between a bag being truly compostable and simply trash depends entirely on the infrastructure available to process it.
Defining Compostable Versus Biodegradable
The terms “biodegradable” and “compostable” are frequently—and incorrectly—used interchangeably, but they represent very different outcomes. A product labeled “biodegradable” will break down eventually through the action of microorganisms, but this process has no set time limit and can take years or decades. Furthermore, biodegradable products may leave behind toxic residues or microplastics that contaminate the environment.
The term “compostable,” by contrast, is regulated and requires specific criteria to be met. A certified compostable product must break down into non-toxic components, such as water, carbon dioxide, and biomass, within a specific timeframe. In North America, the standard for commercial compostability is ASTM D6400, which requires the material to disintegrate within 90 days in a commercial facility and leave no residues that would harm plant life. Third-party certification bodies, such as the Biodegradable Products Institute (BPI), verify that products meet these standards.
Industrial Composting Requirements
Most certified compostable bags, often made from bioplastics like polylactic acid (PLA), are designed to break down only under the controlled conditions of an industrial composting facility. The process is engineered to accelerate microbial activity. These facilities maintain a sustained high temperature, typically between 131 and 160 degrees Fahrenheit (55–70 degrees Celsius), which is necessary to initiate the breakdown of materials like PLA.
In addition to high heat, industrial composting requires precise management of moisture levels and frequent aeration or turning of the compost piles. These actions ensure that the entire batch is exposed to oxygen and consistent temperatures. Without this combination of heat, moisture, and oxygen, the material will not disintegrate within the required 90-day timeframe and will remain largely intact.
The Reality of Home Composting
The vast majority of certified compostable bags are not suitable for a typical backyard compost pile. Home setups cannot replicate the intense conditions of commercial facilities. A home compost bin rarely reaches or maintains the sustained high temperatures—above 131°F—needed for materials like PLA to properly break down. At lower temperatures, the bags may only partially degrade, often fragmenting into smaller pieces that remain visually distinguishable.
Some products carry a separate “home compostable” certification, such as the TÜV Austria OK Compost HOME standard. This signifies they are designed to break down under lower, more variable temperature conditions. These home-certified bags use different formulations but still require consistent moisture and sufficient aeration to decompose effectively. Unless a bag explicitly features a home composting certification, consumers should assume it requires an industrial facility for proper breakdown.
Disposal When Composting is Not an Option
When certified compostable bags cannot be sent to an industrial facility, their environmental benefit is diminished. These materials should never be placed in the standard plastic recycling stream. They are made from different polymers, such as PLA, that contaminate batches of traditional plastics like PET or HDPE. Introducing bioplastics into the conventional recycling process lowers the quality and value of the resulting recycled plastic.
If a compostable bag is sent to a standard landfill, it will decompose very slowly, similar to conventional plastic. Landfills are designed to minimize oxygen and moisture. Under these anaerobic conditions, organic matter, including bioplastics, can break down to release methane, a potent greenhouse gas. The environmental value of a certified compostable bag is only fully realized when commercial composting infrastructure is in place to process it.