Biodegradation

Biodegradation is the biological process where microorganisms break down organic matter. These tiny living organisms, such as bacteria and fungi, consume materials and convert them into simpler substances. Most chemical compounds and materials are eventually subject to this process, with time being the main limiting factor. For example, vegetables may degrade within just a few days. In contrast, some plastics can take many years or even centuries to disappear.

The European Union uses a specific standard to define biodegradability. They require that more than 90% of the original material is converted into water, carbon dioxide, and minerals within six months. It is important to note that elements, such as heavy metal pollutants, do not biodegrade.

The mechanism of biodegradation occurs in three distinct stages. The first stage is biodeterioration, which is often a surface-level degradation. This stage involves abiotic factors, which are non-living environmental elements like light, temperature, and mechanical compression. These factors weaken the material's physical and chemical structure, making it easier for microbes to attack. The second stage is biofragmentation, a lytic process where the chemical bonds within a polymer are cleaved. This breaks the material down into smaller pieces called oligomers and monomers. The final stage is assimilation, where these small products are integrated into microbial cells. Some products move through the cell membrane easily, while others require biotransformation reactions to enter. Once inside, they enter catabolic pathways to produce adenosine triphosphate (ATP) or cell structures.

Microorganisms use two different methods to digest these materials depending on oxygen availability. When oxygen is present, the process is known as aerobic digestion. This type of digestion typically occurs more rapidly than other methods. When oxygen is absent, the process is called anaerobic digestion. While anaerobic digestion is slower, it is highly effective at reducing the total volume and mass of waste. A key difference is that anaerobic reactions produce methane, a natural gas, whereas aerobic reactions do not.

Because anaerobic digestion produces methane, it is widely used in waste management technology to create local, renewable energy.

Many factors influence how quickly biodegradation occurs in the real world. One critical factor is bioavailability, which is the rate at which a substance is made available for organisms to absorb. Compounds must be released into a solution before microbes can degrade them. Environmental conditions like light, water, and temperature also play major roles. Scientists often use respirometry tests to measure these rates in a lab. In aerobic tests, they monitor the amount of carbon dioxide produced by the microbes. In anaerobic tests, they measure the production of methane or other gases. However, materials that biodegrade quickly in a lab might not degrade well in a landfill. Landfills often lack the light, water, and microbial activity needed for efficient breakdown.

The timeline for decomposition varies wildly depending on the material and the environment. In marine environments, a paper towel might take only two to four weeks to break down. A plastic bottle can take approximately 100 years to decompose. Some items, such as glass bottles, may take an estimated one million years.

In terrestrial environments, vegetables can disappear in five days to one month. In contrast, leather shoes might take 25 to 40 years, and Styrofoam cups can last for 500 years or even forever. These differences highlight why managing different types of waste is so complex.

Recent scientific discoveries are changing how we approach plastic waste. In 2016, researchers found a bacterium called *Ideonella sakaiensis* that can biodegrade PET plastic. By 2020, scientists genetically modified a PET-degrading enzyme called PETase. They combined it with another enzyme, MHETase, to break down PET even faster. In 2021, another study reported that a mix of microorganisms from cow stomachs could break down three types of plastic.

These biological tools offer hope for managing synthetic polymers that usually resist natural decay.

Biodegradable technology is already being used in several important fields. The biomedical community uses biodegradable polymers for various medical applications. These materials are classified as medical, ecological, or dual-application, and can be either natural or synthetic. In the packaging industry, some companies use starch-based plastics that degrade in home compost bins within two to four months. Others use polylactic acid, which is a renewably derived plastic. However, polylactic acid usually requires higher temperatures to decompose effectively. Balancing the performance of a material with its ability to biodegrade remains a significant challenge for engineers.