biodegradability

Biodegradability is the capability of a material to be broken down by living organisms, such as bacteria, fungi, or water molds, and reabsorbed by the natural environment. The term usually refers to the natural breakdown of waste by microorganisms. All biodegradable materials are formed of organic compounds, which means they contain carbon atoms, and they may be naturally occurring or human-made. Mineral substances (e.g., metals, glass, and conventional plastics) are not considered biodegradable, though they may physically break down over extended periods of time. The disposal of nonbiodegradable waste is a primary source of pollution. For a material to be labeled “biodegradable,” the European Union requires that at least 90 percent of the original material can be degraded into water, minerals, and carbon dioxide in a maximum of six months.

Surgical materials, such as sutures, that are made to be absorbed by the body are also called biodegradable.

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Stages and rates of biodegradation

There are four stages in the biodegradation process: biodeterioration, biofragmentation, assimilation, and mineralization.

• Biodeterioration

  In biodeterioration, the actions of different organisms along with abiotic factors, such as sunlight and temperature, begin to mechanically, physically, and chemically degrade the material in observable ways, including discoloration, scratches, cracks, holes, and roughness.

• Biofragmentation

  During biofragmentation, saprotrophic organisms excrete enzymes that progressively reduce the molecular weight of the material. This often involves breaking down polymers into smaller dimers and monomers. In preparation for assimilation, organic chemicals are further broken down either by aerobic digestion, which occurs in the presence of oxygen, or by anaerobic digestion, which occurs when oxygen is not present and results in the production of methane.

• Assimilation

  Assimilation occurs when the saprotrophic organisms finally absorb the fragmented compounds into their cells for use as food. The absorption process ultimately creates energy that individual cells store in a molecule called adenosine triphosphate (ATP) or provides precursor compounds for other biosynthetic pathways.

• Mineralization

  Mineralization, also called ultimate biodegradation or complete biodegradation, is the final stage in the biodegradation process. Having been degraded into such components as water and carbon dioxide and assimilated by living organisms, the mineral components of the original material are eventually redistributed by Earth’s various elemental cycles (e.g., the carbon, nitrogen, and water cycles).

Several environmental factors affect the efficiency and rate of biodegradation, including the types of saprotrophic organisms present; the temperature, humidity, pH levels, and amount of sunlight in the environment; and the nature of the material itself. Organic materials generally degrade more quickly when a variety of microorganisms are present, which often occurs in aerobic conditions of about 37 °C (98.6 °F) in a pH 7.4 environment. However, given that many saprotrophs can flourish in a variety of pH and temperature conditions, biodegradation can often still occur relatively quickly in other environments. The structure of the organic matter also determines the rate of biodegradation, Materials with flexible, amorphous, and more polar polymers degrade more quickly than those with higher molecular weight, more crystalline regions, and dense cross-linking. Naturally occurring plant and animal matter tends to biodegrade faster than human-made materials.

Biodegradable plastics

Conventional “commodity” plastics, including polyethylene, polypropylene, polystyrene, polyvinyl chloride, and polyethylene terephthalate, are derived from petroleum and are not biodegradable. Instead, such plastics typically break down into microplastics that are expected to persist in the environment for hundreds of years. Plastic pollution is a serious environmental concern, and microplastics have been found on high mountain peaks, in the air, in seafloor sediments, and within the living bodies of a myriad of organisms, including humans. Conventional plastics do not biodegrade because the enzymes of saprotrophic organisms do not act on the chemical bonds that hold the synthetic polymers together.

However, there are two classes of biodegradable plastics: bioplastics that have components made from renewable raw materials and plastics made from petrochemicals with biodegradable additives designed to encourage biodegradation. Bioplastics are commonly made up of chemical compounds that are derived from or synthesized by microbes such as bacteria or by genetically modified plants. However, not every bioplastic is considered biodegradable; some that are labeled “biodegradable” break down only with industrial processing. Although bioplastics have not been successful on a large scale, primarily because of high production costs and problems of stability during their processing, new biodegradable plastics continue to be developed and may prove more cost-effective.

In 2023, researchers from the University of Washington published a paper describing a new bioplastic they had developed made entirely from powdered cyanobacteria cells that degrades at the same rate as a banana peel in a home compost bin. Also in 2023, researchers at Michigan State University announced that they had developed a new bioplastic that incorporates a thermoplastic starch and is compostable in home and industrial settings.

Organic waste in landfills

Organic material buried in a landfill decomposes very slowly by anaerobic microbial action, and most items in landfills retain their original weight, volume, and form for the life of the landfill. Indeed, landfill excavations performed by researchers have uncovered legible 50-year-old newspapers; 25-year-old hotdogs, corncobs, and grapes; and 20-year-old steaks. Modern landfills are actually designed to minimize biodegradation, for the purpose of preventing groundwater pollution and reducing methane emissions. A by-product of anaerobic decomposition, methane gas is poisonous and explosive when diluted in the air, and it is a potent greenhouse gas and driver of anthropogenic climate change. In modern landfills, methane movement is controlled by impermeable barriers and by gas-venting systems and is sometimes collected and recovered for use as a fuel, either directly or as a component of biogas.

A highly contaminated liquid called leachate is another by-product of decomposition in sanitary landfills. Most leachate is the result of runoff that infiltrates the landfill and comes into contact with decomposing garbage. If leachate reaches the groundwater or seeps out onto the ground surface, serious environmental pollution problems can occur, including the possible contamination of drinking-water supplies. Methods of controlling leachate include the interception of surface water in order to prevent it from entering the landfill and the use of impermeable liners or barriers between the waste and the groundwater. New landfill sites are also frequently provided with groundwater-monitoring wells and leachate-collection and treatment systems.

In 1990 the first landfill designed to promote biodegradation, called a bioreactor landfill, was built as part of a research program. Bioreactor landfills enhance the microbial degradation process by recirculating air and liquid through the waste. While many have been built, they are very costly and have some disadvantages too, including an increase in leachate breakouts, the potential of fires and standing water, and bad odors.

Composting

Composting makes use of biodegradation to produce rich humus for soil and requires some degree of human intervention to promote aerobic microorganism growth. Home, municipal, and agricultural composting helps divert organic matter from landfills and circulate it back into the environment. The steps in composting include adding the correct materials to compost piles or containers (all compostable materials are biodegradable, but not all biodegradable materials are compostable), controlling moisture levels, and turning the contents for aeration. Compostable materials include what are called nitrogen-rich “greens,” such as fruit and vegetable scraps, coffee grounds, crushed egg shells, and grass clippings, and carbon-rich “browns,” such as dry leaves, plant twigs, shredded non-glossy uncolored paper, certain types of shredded cardboard, and untreated wood chips. The compost itself can be used as a fertilizer for gardens, as a mulch around plants, or as an amendment to potting soil.