Biodegradation
Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegradation occurs under a specific set of circumstances.
The process of biodegradation is threefold: first an object undergoes biodeterioration, which is the mechanical weakening of its structure; then follows biofragmentation, which is the breakdown of materials by microorganisms; and finally assimilation, which is the incorporation of the old material into new cells.
In practice, almost all chemical compounds and materials are subject to biodegradation, the key element being time. Things like vegetables may degrade within days, while glass and some plastics take many millennia to decompose. A standard for biodegradability used by the European Union is that greater than 90% of the original material must be converted into, water and minerals by biological processes within 6 months.
Mechanisms
The process of biodegradation can be divided into three stages: biodeterioration, biofragmentation, and assimilation. Biodeterioration is sometimes described as a surface-level degradation that modifies the mechanical, physical and chemical properties of the material. This stage occurs when the material is exposed to abiotic factors in the outdoor environment and allows for further degradation by weakening the material's structure. Some abiotic factors that influence these initial changes are compression, light, temperature and chemicals in the environment. While biodeterioration typically occurs as the first stage of biodegradation, it can in some cases be parallel to biofragmentation. Hueck, however, defined Biodeterioration as the undesirable action of living organisms on Man's materials, involving such things as breakdown of stone facades of buildings, corrosion of metals by microorganisms or merely the esthetic changes induced on man-made structures by the growth of living organisms.Biofragmentation of a polymer is the lytic process in which bonds within a polymer are cleaved, generating oligomers and monomers in its place. The steps taken to fragment these materials also differ based on the presence of oxygen in the system. The breakdown of materials by microorganisms when oxygen is present is aerobic digestion, and the breakdown of materials when oxygen is not present is anaerobic digestion. The main difference between these processes is that anaerobic reactions produce methane, while aerobic reactions do not. In addition, aerobic digestion typically occurs more rapidly than anaerobic digestion, while anaerobic digestion does a better job reducing the volume and mass of the material. Due to anaerobic digestion's ability to reduce the volume and mass of waste materials and produce a natural gas, anaerobic digestion technology is widely used for waste management systems and as a source of local, renewable energy.
In the assimilation stage, the resulting products from biofragmentation are then integrated into microbial cells. Some of the products from fragmentation are easily transported within the cell by membrane carriers. However, others still have to undergo biotransformation reactions to yield products that can then be transported inside the cell. Once inside the cell, the products enter catabolic pathways that either lead to the production of adenosine triphosphate or elements of the cells structure.
;Aerobic biodegradation equation
;Anaerobic biodegradation equation
Factors affecting biodegradation rate
In practice, almost all chemical compounds and materials are subject to biodegradation processes. The significance, however, is in the relative rates of such processes, such as days, weeks, years or centuries. A number of factors determine the rate at which this degradation of organic compounds occurs. Factors include light, water, oxygen and temperature. The degradation rate of many organic compounds is limited by their bioavailability, which is the rate at which a substance is absorbed into a system or made available at the site of physiological activity, as compounds must be released into solution before organisms can degrade them. The rate of biodegradation can be measured in a number of ways. Respirometry tests can be used for aerobic microbes. First one places a solid waste sample in a container with microorganisms and soil, and then aerates the mixture. Over the course of several days, microorganisms digest the sample bit by bit and produce carbon dioxide – the resulting amount of CO2 serves as an indicator of degradation. Biodegradability can also be measured by anaerobic microbes and the amount of methane or alloy that they are able to produce.It's important to note factors that affect biodegradation rates during product testing to ensure that the results produced are accurate and reliable. Several materials will test as being biodegradable under optimal conditions in a lab for approval but these results may not reflect real world outcomes where factors are more variable. For example, a material may have tested as biodegrading at a high rate in the lab may not degrade at a high rate in a landfill because landfills often lack light, water, and microbial activity that are necessary for degradation to occur. Thus, it is very important that there are standards for plastic biodegradable products, which have a large impact on the environment. The development and use of accurate standard test methods can help ensure that all plastics that are being produced and commercialized will actually biodegrade in natural environments. One test that has been developed for this purpose is DINV 54900.
Recent advances have enabled real-time monitoring of polymer biodegradation using biosensors combined with machine learning, improving the accuracy of degradation assessments under varying environmental conditions.
| Product | Time to Biodegrade |
| Paper towel | 2–4 weeks |
| Newspaper | 6 weeks |
| Apple core | 2 months |
| Cardboard box | 2 months |
| Wax coated milk carton | 3 months |
| Cotton gloves | 1–5 months |
| Wool gloves | 1 year |
| Plywood | 1–3 years |
| Painted wooden sticks | 13 years |
| Plastic bags | 10–20 years |
| Tin cans | 50 years |
| Disposable diapers | 50–100 years |
| Plastic bottle | 100 years |
| Aluminium cans | 200 years |
| Glass bottles | Undetermined |
| Vegetables | 5 days – 1 month |
| Paper | 2–5 months |
| Cotton T-shirt | 6 months |
| Orange peels | 6 months |
| Tree leaves | 1 year |
| Wool socks | 1–5 years |
| Plastic-coated paper milk cartons | 5 years |
| Leather shoes | 25–40 years |
| Nylon fabric | 30–40 years |
| Tin cans | 50–100 years |
| Aluminium cans | 80–100 years |
| Glass bottles | 1 million years |
| Styrofoam cup | 500 years to forever |
| Plastic bags | 500 years to forever |
Plastics
The term Biodegradable Plastics refers to materials that maintain their mechanical strength during practical use but break down into low-weight compounds and non-toxic byproducts after their use. This breakdown is made possible through an attack of microorganisms on the material, which is typically a non-water-soluble polymer. Such materials can be obtained through chemical synthesis, fermentation by microorganisms, and from chemically modified natural products.Plastics biodegrade at highly variable rates. PVC-based plumbing is selected for handling sewage because PVC resists biodegradation. Some packaging materials on the other hand are being developed that would degrade readily upon exposure to the environment. Examples of synthetic polymers that biodegrade quickly include polycaprolactone, other polyesters and aromatic-aliphatic esters, due to their ester bonds being susceptible to attack by water. A prominent example is poly-3-hydroxybutyrate, the renewably derived polylactic acid. Others are the cellulose-based cellulose acetate and celluloid.
Under low oxygen conditions plastics break down more slowly. The breakdown process can be accelerated in specially designed compost heap. Starch-based plastics will degrade within two to four months in a home compost bin, while polylactic acid is largely undecomposed, requiring higher temperatures. Polycaprolactone and polycaprolactone-starch composites decompose slower, but the starch content accelerates decomposition by leaving behind a porous, high surface area polycaprolactone. Nevertheless, it takes many months.
In 2016, a bacterium named Ideonella sakaiensis was found to biodegrade PET. In 2020, the PET degrading enzyme of the bacterium, PETase, has been genetically modified and combined with MHETase to break down PET faster, and also degrade PEF. In 2021, researchers reported that a mix of microorganisms from cow stomachs could break down three types of plastics.
Many plastic producers have gone so far even to say that their plastics are compostable, typically listing corn starch as an ingredient. However, these claims are questionable because the plastics industry operates under its own definition of compostable:
The term "composting" is often used informally to describe the biodegradation of packaging materials. Legal definitions exist for compostability, the process that leads to compost. Four criteria are offered by the European Union:
- Chemical composition: volatile matter and heavy metals as well as fluorine should be limited.
- Biodegradability: the conversion of >90% of the original material into, water and minerals by biological processes within 6 months.
- Disintegrability: at least 90% of the original mass should be decomposed into particles that are able to pass through a 2x2 mm sieve.
- Quality: absence of toxic substances and other substances that impede composting.