The terms biodegradable and compostable can be a source of confusion on product labels. They’ve become trendy keywords and catchalls used interchangeably that give consumers the sense that these products are eco-friendly and will decompose quickly at the end of their life cycles. But it’s not quite that simple—there are significant differences between the two, and it’s important to understand them in order to make educated decisions about products and dispose of them correctly.
Let’s start with a simple definition: biodegradation is the process by which microorganisms break down materials, transforming them into less complex compounds that can be used and reused by living systems. It is a naturally occurring process driven by bacteria and fungi, typically yeast and mold. This is to say that biodegradation can happen without human intervention—when a tree falls in the forest or an animal dies, they are broken down into carbon dioxide, water, and biomass. But human societies have also developed waste management systems that incorporate processes of controlled biodegradation, and it’s also used as a form of environmental remediation (bioremediation) to break down pollutants at sites of contamination.
What materials are biodegradable?
Any organic material—and some inorganic materials—that will eventually decompose can be considered biodegradable. Even some plastics biodegrade, as well as inorganic pollutants like oil, polychlorinated biphenyls (PCBs), hydrocarbon, radionuclides, pesticides, and metals. However, not all biodegradation processes are equal. For example, a banana peel thrown out of a car window might break down in less than a month, while an aluminum can could take upwards of 100 years, and a plastic water bottle might take somewhere around 450 years. Some items made of glass may actually never biodegrade. The main factors that determine the length of the biodegradation process are environmental: humidity, temperature, light, pH, and aeration. This means that in waste management, biodegradation processes need to occur in controlled settings that are conducive to the growth of microorganisms.
Composting is a kind of aerobic biodegradation that transforms organic materials into nutrient-rich conditioner for soil. All compostable materials are biodegradable, but not all biodegradable materials can be considered compostable. While biodegradation has no time limitations, and in some cases may go on for thousands of years, composting occurs within a much shorter period of time. This time frame is not standardized, but most composters expect full decomposition to occur within around 80 days—and some expect an even shorter turnaround. In order for materials to decompose at this rate, composting requires specific environmental conditions that allow microorganisms to thrive: water, oxygen, nutrients, and warm temperatures. Under the right conditions, these organisms (mostly bacteria, as well as some fungi and insects like earthworms) can reproduce quickly and process the organic matter, producing heat, water vapor, carbon dioxide, and humus.
What can be composted?
Composting is a much faster process than biodegradation in part because a much narrower subset of materials are considered compostable. Only organic materials can be composted, and they must be broken down into smaller pieces before they are disposed of. Wood should be composted as wood chips or sawdust, and paper, cardboard, wool, and cotton should all be shredded. Along with food scraps, yard trimmings, paper, wood waste, manure, and certain kinds of bioplastics, even human waste and sewage sludge can be considered compostable.
Industrial composting facilities
This is not to say that all of these materials will decompose along with the vegetables in your backyard compost pile – many of them won’t. Rather, they are only compostable in the highly controlled context of an industrial facility. Composting facilities have been introduced in many developed countries as governments place limits on the amount of organic waste that can be sent to landfills.
Industrial composting is a sustainable waste management strategy that can divert up to 72% of the materials entering landfills, reducing water pollution, methane emissions, and the carbon footprint. Composting is also less costly than landfills, and it provides economic benefits to the agriculture industry, improving soil for more efficient and sustainable crop production, and significantly reducing the need for pesticides and fertilizers.
As governments around the world move to ban single-use plastics, biodegradable and compostable plastics have emerged as a more sustainable alternative. Although there are a number of compostable bioplastics, they come with a few caveats and controversies. Most bioplastics are either plant-based hydro-biodegradable, made from plants like corn or sugarcane, or petroleum-based oxo-biodegradable. They can be defined by the standard conditions and timeframe under which they will decompose.
Most compostable plastics need to be sent to industrial composting facilities, where the environment is controlled, in order to decompose at the same rate as other compostable materials. Many countries have standards in place that require compostable plastics to decompose within a certain amount of time in a facility. In the US, the American Society for Testing and Materials (ASTM) standard requires compostable plastics to become “not visually distinguishable” at the same rate as other compostable items, and their degradation must not “diminish the value or utility of the compost.”
Because compostable plastics are made to be degraded in compost facilities alone, they don’t break down very well when disposed of in a different context. If compostable plastics are sent to a landfill, they may take as long, or even longer to degrade than regular plastics. Some compostable plastics are designed only to break down through aerobic processes in highly controlled composting environments, and landfills typically use anaerobic biodegradation processes, with little to no oxygen. For example, one of the most popular alternatives to traditional plastic is the corn-based bioplastic polylactic acid (PLA). It can decompose into water and carbon dioxide in 47 to 90 days, but only in composting facilities with high temperatures. In a landfill, it can take just as long as regular plastic, and produce toxic greenhouse gases like methane rather than reusable nutrients.
Understanding product labels
The US Federal Trade Commission (FTC) specifies that products that claim to be biodegradable or compostable need to clearly and prominently qualify both the product’s ability to degrade in the environment where it is customarily disposed, and the rate and extent of degradation. Claims are considered unqualified if the item is typically disposed of in landfills, incinerators, or recycling facilities because these environments don’t present conditions that are conducive to complete decomposition within a relatively short period of time. For example, even if trash bags are marketed as “biodegradable,” consumers generally place them into the solid waste stream where they are incinerated or sent to landfills. The bags won’t break down quickly in these conditions, so the FTC considers this claim to be deceptive.
Confusion about waste management leads to the improper disposal of materials every day. This is why clear product labels are so important. Under what specific conditions will a product biodegrade? How long will it take? It’s important that manufacturers provide this information so that at the end of a product’s use, consumers can direct them to the proper waste stream.